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diff --git a/old/4346.txt b/old/4346.txt new file mode 100644 index 0000000..ba8cedf --- /dev/null +++ b/old/4346.txt @@ -0,0 +1,19440 @@ +The Project Gutenberg Etext of The Effects of Cross & Self-Fertilisation +in the Vegetable Kingdom, by Charles Darwin +#21 in our series by Charles Darwin + +Copyright laws are changing all over the world. Be sure to check the +copyright laws for your country before distributing this or any other +Project Gutenberg file. + +We encourage you to keep this file, exactly as it is, on your +own disk, thereby keeping an electronic path open for future +readers. Please do not remove this. + +This header should be the first thing seen when anyone starts to +view the etext. 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FOR PUBLIC DOMAIN ETEXTS*Ver.10/04/01*END* + + + + + + +This etext was prepared by Sue Asscher asschers@dingoblue.net.au + + + + + + +THE EFFECTS OF CROSS & SELF-FERTILISATION IN THE VEGETABLE KINGDOM. + +BY + +CHARLES DARWIN, M.A., F.R.S., ETC. + + + + +CONTENTS. + + +CHAPTER I. + +INTRODUCTORY REMARKS. + +Various means which favour or determine the cross-fertilisation of +plants.--Benefits derived from cross-fertilisation.--Self-fertilisation +favourable to the propagation of the species.--Brief history of the +subject.--Object of the experiments, and the manner in which they were +tried.--Statistical value of the measurements.--The experiments carried +on during several successive generations.--Nature of the relationship of +the plants in the later generations.--Uniformity of the conditions to +which the plants were subjected.--Some apparent and some real causes of +error.--Amount of pollen employed.--Arrangement of the work.--Importance +of the conclusions. + + +CHAPTER II. + +CONVOLVULACEAE. + +Ipomoea purpurea, comparison of the height and fertility of the crossed +and self-fertilised plants during ten successive generations.--Greater +constitutional vigour of the crossed plants.--The effects on the +offspring of crossing different flowers on the same plant, instead of +crossing distinct individuals.--The effects of a cross with a fresh +stock.--The descendants of the self-fertilised plant named +Hero.--Summary on the growth, vigour, and fertility of the successive +crossed and self-fertilised generations.--Small amount of pollen in the +anthers of the self-fertilised plants of the later generations, and the +sterility of their first-produced flowers.--Uniform colour of the +flowers produced by the self-fertilised plants.--The advantage from a +cross between two distinct plants depends on their differing in +constitution. + + +CHAPTER III. + +SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC. + +Mimulus luteus; height, vigour, and fertility of the crossed and +self-fertilised plants of the first four generations.--Appearance of a +new, tall, and highly self-fertile variety.--Offspring from a cross +between self-fertilised plants.--Effects of a cross with a fresh +stock.--Effects of crossing flowers on the same plant.--Summary on +Mimulus luteus.--Digitalis purpurea, superiority of the crossed +plants.--Effects of crossing flowers on the same +plant.--Calceolaria.--Linaria vulgaris.--Verbascum thapsus.--Vandellia +nummularifolia.--Cleistogene flowers.--Gesneria pendulina.--Salvia +coccinea.--Origanum vulgare, great increase of the crossed plants by +stolons.--Thunbergia alata. + + +CHAPTER IV. + +CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC. + +Brassica oleracea, crossed and self-fertilised plants.--Great effect of +a cross with a fresh stock on the weight of the offspring.--Iberis +umbellata.--Papaver vagum.--Eschscholtzia californica, seedlings from a +cross with a fresh stock not more vigorous, but more fertile than the +self-fertilised seedlings.--Reseda lutea and odorata, many individuals +sterile with their own pollen.--Viola tricolor, wonderful effects of a +cross.--Adonis aestivalis.--Delphinium consolida.--Viscaria oculata, +crossed plants hardly taller, but more fertile than the +self-fertilised.--Dianthus caryophyllus, crossed and self-fertilised +plants compared for four generations.--Great effects of a cross with a +fresh stock.--Uniform colour of the flowers on the self-fertilised +plants.--Hibiscus africanus. + + +CHAPTER V. + +GERANIACEAE, LEGUMINOSAE, ONAGRACEAE, ETC. + +Pelargonium zonale, a cross between plants propagated by cuttings does +no good.--Tropaeolum minus.--Limnanthes douglasii.--Lupinus luteus and +pilosus.--Phaseolus multiflorus and vulgaris.--Lathyrus odoratus, +varieties of, never naturally intercross in England.--Pisum sativum, +varieties of, rarely intercross, but a cross between them highly +beneficial.--Sarothamnus scoparius, wonderful effects of a +cross.--Ononis minutissima, cleistogene flowers of.--Summary on the +Leguminosae.--Clarkia elegans.--Bartonia aurea.--Passiflora +gracilis.--Apium petroselinum.--Scabiosa atropurpurea.--Lactuca +sativa.--Specularia speculum.--Lobelia ramosa, advantages of a cross +during two generations.--Lobelia fulgens.--Nemophila insignis, great +advantages of a cross.--Borago officinalis.--Nolana prostrata. + + +CHAPTER VI. + +SOLANACEAE, PRIMULACEAE, POLYGONEAE, ETC. + +Petunia violacea, crossed and self-fertilised plants compared for four +generations.--Effects of a cross with a fresh stock.--Uniform colour of +the flowers on the self-fertilised plants of the fourth +generation.--Nicotiana tabacum, crossed and self-fertilised plants of +equal height.--Great effects of a cross with a distinct sub-variety on +the height, but not on the fertility, of the offspring.--Cyclamen +persicum, crossed seedlings greatly superior to the +self-fertilised.--Anagallis collina.--Primula veris.--Equal-styled +variety of Primula veris, fertility of, greatly increased by a cross +with a fresh stock.--Fagopyrum esculentum.--Beta vulgaris.--Canna +warscewiczi, crossed and self-fertilised plants of equal height.--Zea +mays.--Phalaris canariensis. + + +CHAPTER VII. + +SUMMARY OF THE HEIGHTS AND WEIGHTS OF THE CROSSED AND SELF-FERTILISED +PLANTS. + +Number of species and plants measured.--Tables given.--Preliminary +remarks on the offspring of plants crossed by a fresh stock.--Thirteen +cases specially considered.--The effects of crossing a self-fertilised +plant either by another self-fertilised plant or by an intercrossed +plant of the old stock.--Summary of the results.--Preliminary remarks on +the crossed and self-fertilised plants of the same stock.--The +twenty-six exceptional cases considered, in which the crossed plants did +not exceed greatly in height the self-fertilised.--Most of these cases +shown not to be real exceptions to the rule that cross-fertilisation is +beneficial.--Summary of results.--Relative weights of the crossed and +self-fertilised plants. + + +CHAPTER VIII. + +DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL +VIGOUR AND IN OTHER RESPECTS. + +Greater constitutional vigour of crossed plants.--The effects of great +crowding.--Competition with other kinds of plants.--Self-fertilised +plants more liable to premature death.--Crossed plants generally flower +before the self-fertilised.--Negative effects of intercrossing flowers +on the same plant.--Cases described.--Transmission of the good effects +of a cross to later generations.--Effects of crossing plants of closely +related parentage.--Uniform colour of the flowers on plants +self-fertilised during several generations and cultivated under similar +conditions. + + +CHAPTER IX. + +THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE +PRODUCTION OF SEEDS. + +Fertility of plants of crossed and self-fertilised parentage, both lots +being fertilised in the same manner.--Fertility of the parent-plants +when first crossed and self-fertilised, and of their crossed and +self-fertilised offspring when again crossed and +self-fertilised.--Comparison of the fertility of flowers fertilised with +their own pollen and with that from other flowers on the same +plant.--Self-sterile plants.--Causes of self-sterility.--The appearance +of highly self-fertile varieties.--Self-fertilisation apparently in some +respects beneficial, independently of the assured production of +seeds.--Relative weights and rates of germination of seeds from crossed +and self-fertilised flowers. + + +CHAPTER X. + +MEANS OF FERTILISATION. + +Sterility and fertility of plants when insects are excluded.--The means +by which flowers are cross-fertilised.--Structures favourable to +self-fertilisation.--Relation between the structure and conspicuousness +of flowers, the visits of insects, and the advantages of +cross-fertilisation.--The means by which flowers are fertilised with +pollen from a distinct plant.--Greater fertilising power of such +pollen.--Anemophilous species.--Conversion of anemophilous species into +entomophilous.--Origin of nectar.--Anemophilous plants generally have +their sexes separated.--Conversion of diclinous into hermaphrodite +flowers.--Trees often have their sexes separated. + + +CHAPTER XI. + +THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS. + +Insects visit the flowers of the same species as long as they +can.--Cause of this habit.--Means by which bees recognise the flowers of +the same species.--Sudden secretion of nectar.--Nectar of certain +flowers unattractive to certain insects.--Industry of bees, and the +number of flowers visited within a short time.--Perforation of the +corolla by bees.--Skill shown in the operation.--Hive-bees profit by the +holes made by humble-bees.--Effects of habit.--The motive for +perforating flowers to save time.--Flowers growing in crowded masses +chiefly perforated. + + +CHAPTER XII. + +GENERAL RESULTS. + +Cross-fertilisation proved to be beneficial, and self-fertilisation +injurious.--Allied species differ greatly in the means by which +cross-fertilisation is favoured and self-fertilisation avoided.--The +benefits and evils of the two processes depend on the degree of +differentiation in the sexual elements.--The evil effects not due to the +combination of morbid tendencies in the parents.--Nature of the +conditions to which plants are subjected when growing near together in a +state of nature or under culture, and the effects of such +conditions.--Theoretical considerations with respect to the interaction +of differentiated sexual elements.--Practical lessons.--Genesis of the +two sexes.--Close correspondence between the effects of +cross-fertilisation and self-fertilisation, and of the legitimate and +illegitimate unions of heterostyled plants, in comparison with hybrid +unions. + + +INDEX. + + +... + + +THE EFFECTS OF CROSS AND SELF-FERTILISATION IN THE VEGETABLE KINGDOM. + + +CHAPTER I. + +INTRODUCTORY REMARKS. + +Various means which favour or determine the cross-fertilisation of plants. +Benefits derived from cross-fertilisation. +Self-fertilisation favourable to the propagation of the species. +Brief history of the subject. +Object of the experiments, and the manner in which they were tried. +Statistical value of the measurements. +The experiments carried on during several successive generations. +Nature of the relationship of the plants in the later generations. +Uniformity of the conditions to which the plants were subjected. +Some apparent and some real causes of error. +Amount of pollen employed. +Arrangement of the work. +Importance of the conclusions. + +There is weighty and abundant evidence that the flowers of most kinds of +plants are constructed so as to be occasionally or habitually +cross-fertilised by pollen from another flower, produced either by the +same plant, or generally, as we shall hereafter see reason to believe, +by a distinct plant. Cross-fertilisation is sometimes ensured by the +sexes being separated, and in a large number of cases by the pollen and +stigma of the same flower being matured at different times. Such plants +are called dichogamous, and have been divided into two sub-classes: +proterandrous species, in which the pollen is mature before the stigma, +and proterogynous species, in which the reverse occurs; this latter form +of dichogamy not being nearly so common as the other. +Cross-fertilisation is also ensured, in many cases, by mechanical +contrivances of wonderful beauty, preventing the impregnation of the +flowers by their own pollen. There is a small class of plants, which I +have called dimorphic and trimorphic, but to which Hildebrand has given +the more appropriate name of heterostyled; this class consists of plants +presenting two or three distinct forms, adapted for reciprocal +fertilisation, so that, like plants with separate sexes, they can hardly +fail to be intercrossed in each generation. The male and female organs +of some flowers are irritable, and the insects which touch them get +dusted with pollen, which is thus transported to other flowers. Again, +there is a class, in which the ovules absolutely refuse to be fertilised +by pollen from the same plant, but can be fertilised by pollen from any +other individual of the same species. There are also very many species +which are partially sterile with their own pollen. Lastly, there is a +large class in which the flowers present no apparent obstacle of any +kind to self-fertilisation, nevertheless these plants are frequently +intercrossed, owing to the prepotency of pollen from another individual +or variety over the plant's own pollen. + +As plants are adapted by such diversified and effective means for +cross-fertilisation, it might have been inferred from this fact alone +that they derived some great advantage from the process; and it is the +object of the present work to show the nature and importance of the +benefits thus derived. There are, however, some exceptions to the rule +of plants being constructed so as to allow of or to favour +cross-fertilisation, for some few plants seem to be invariably +self-fertilised; yet even these retain traces of having been formerly +adapted for cross-fertilisation. These exceptions need not make us doubt +the truth of the above rule, any more than the existence of some few +plants which produce flowers, and yet never set seed, should make us +doubt that flowers are adapted for the production of seed and the +propagation of the species. + +We should always keep in mind the obvious fact that the production of +seed is the chief end of the act of fertilisation; and that this end can +be gained by hermaphrodite plants with incomparably greater certainty by +self-fertilisation, than by the union of the sexual elements belonging +to two distinct flowers or plants. Yet it is as unmistakably plain that +innumerable flowers are adapted for cross-fertilisation, as that the +teeth and talons of a carnivorous animal are adapted for catching prey; +or that the plumes, wings, and hooks of a seed are adapted for its +dissemination. Flowers, therefore, are constructed so as to gain two +objects which are, to a certain extent, antagonistic, and this explains +many apparent anomalies in their structure. The close proximity of the +anthers to the stigma in a multitude of species favours, and often +leads, to self-fertilisation; but this end could have been gained far +more safely if the flowers had been completely closed, for then the +pollen would not have been injured by the rain or devoured by insects, +as often happens. Moreover, in this case, a very small quantity of +pollen would have been sufficient for fertilisation, instead of millions +of grains being produced. But the openness of the flower and the +production of a great and apparently wasteful amount of pollen are +necessary for cross-fertilisation. These remarks are well illustrated by +the plants called cleistogene, which bear on the same stock two kinds of +flowers. The flowers of the one kind are minute and completely closed, +so that they cannot possibly be crossed; but they are abundantly +fertile, although producing an extremely small quantity of pollen. The +flowers of the other kind produce much pollen and are open; and these +can be, and often are, cross-fertilised. Hermann Muller has also made +the remarkable discovery that there are some plants which exist under +two forms; that is, produce on distinct stocks two kinds of +hermaphrodite flowers. The one form bears small flowers constructed for +self-fertilisation; whilst the other bears larger and much more +conspicuous flowers plainly constructed for cross-fertilisation by the +aid of insects; and without their aid these produce no seed. + +The adaptation of flowers for cross-fertilisation is a subject which has +interested me for the last thirty-seven years, and I have collected a +large mass of observations, but these are now rendered superfluous by +the many excellent works which have been lately published. In the year +1857 I wrote a short paper on the fertilisation of the kidney bean (1/1. +'Gardeners' Chronicle' 1857 page 725 and 1858 pages 824 and 844. 'Annals +and Magazine of Natural History' 3rd series volume 2 1858 page 462.); +and in 1862 my work 'On the Contrivances by which British and Foreign +Orchids are Fertilised by Insects' appeared. It seemed to me a better +plan to work out one group of plants as carefully as I could, rather +than to publish many miscellaneous and imperfect observations. My +present work is the complement of that on Orchids, in which it was shown +how admirably these plants are constructed so as to permit of, or to +favour, or to necessitate cross-fertilisation. The adaptations for +cross-fertilisation are perhaps more obvious in the Orchideae than in +any other group of plants, but it is an error to speak of them, as some +authors have done, as an exceptional case. The lever-like action of the +stamens of Salvia (described by Hildebrand, Dr. W. Ogle, and others), by +which the anthers are depressed and rubbed on the backs of bees, shows +as perfect a structure as can be found in any orchid. Papilionaceous +flowers, as described by various authors--for instance, by Mr. T.H. +Farrer--offer innumerable curious adaptations for cross-fertilisation. +The case of Posoqueria fragrans (one of the Rubiaceae), is as wonderful +as that of the most wonderful orchid. The stamens, according to Fritz +Muller, are irritable, so that as soon as a moth visits a flower, the +anthers explode and cover the insect with pollen; one of the filaments +which is broader than the others then moves and closes the flower for +about twelve hours, after which time it resumes its original position. +(1/2. 'Botanische Zeitung' 1866 page 129.) Thus the stigma cannot be +fertilised by pollen from the same flower, but only by that brought by a +moth from some other flower. Endless other beautiful contrivances for +this same purpose could be specified. + +Long before I had attended to the fertilisation of flowers, a remarkable +book appeared in 1793 in Germany, 'Das Entdeckte Geheimniss der Natur,' +by C.K. Sprengel, in which he clearly proved by innumerable +observations, how essential a part insects play in the fertilisation of +many plants. But he was in advance of his age, and his discoveries were +for a long time neglected. Since the appearance of my book on Orchids, +many excellent works on the fertilisation of flowers, such as those by +Hildebrand, Delpino, Axell and Hermann Muller, and numerous shorter +papers, have been published. (1/3. Sir John Lubbock has given an +interesting summary of the whole subject in his 'British Wild Flowers +considered in relation to Insects' 1875. Hermann Muller's work 'Die +Befruchtung der Blumen durch Insekten' 1873, contains an immense number +of original observations and generalisations. It is, moreover, +invaluable as a repertory with references to almost everything which has +been published on the subject. His work differs from that of all others +in specifying what kinds of insects, as far as known, visit the flowers +of each species. He likewise enters on new ground, by showing not only +that flowers are adapted for their own good to the visits of certain +insects; but that the insects themselves are excellently adapted for +procuring nectar or pollen from certain flowers. The value of H. +Muller's work can hardly be over-estimated, and it is much to be desired +that it should be translated into English. Severin Axell's work is +written in Swedish, so that I have not been able to read it.) A list +would occupy several pages, and this is not the proper place to give +their titles, as we are not here concerned with the means, but with the +results of cross-fertilisation. No one who feels interest in the +mechanism by which nature effects her ends, can read these books and +memoirs without the most lively interest. + +From my own observations on plants, guided to a certain extent by the +experience of the breeders of animals, I became convinced many years ago +that it is a general law of nature that flowers are adapted to be +crossed, at least occasionally, by pollen from a distinct plant. +Sprengel at times foresaw this law, but only partially, for it does not +appear that he was aware that there was any difference in power between +pollen from the same plant and from a distinct plant. In the +introduction to his book (page 4) he says, as the sexes are separated in +so many flowers, and as so many other flowers are dichogamous, "it +appears that nature has not willed that any one flower should be +fertilised by its own pollen." Nevertheless, he was far from keeping +this conclusion always before his mind, or he did not see its full +importance, as may be perceived by anyone who will read his observations +carefully; and he consequently mistook the meaning of various +structures. But his discoveries are so numerous and his work so +excellent, that he can well afford to bear a small amount of blame. A +most capable judge, H. Muller, likewise says: "It is remarkable in how +very many cases Sprengel rightly perceived that pollen is necessarily +transported to the stigmas of other flowers of the same species by the +insects which visit them, and yet did not imagine that this +transportation was of any service to the plants themselves." (1/4. 'Die +Befruchtung der Blumen' 1873 page 4. His words are: "Es ist merkwurdig, +in wie zahlreichen Fallen Sprengel richtig erkannte, dass durch die +Besuchenden Insekten der Bluthenstaub mit Nothwendigkeit auf die Narben +anderer Bluthen derselben Art ubertragen wird, ohne auf die Vermuthung +zu kommen, dass in dieser Wirkung der Nutzen des Insektenbesuches fur +die Pflanzen selbst gesucht werden musse.") + +Andrew Knight saw the truth much more clearly, for he remarks, "Nature +intended that a sexual intercourse should take place between +neighbouring plants of the same species." (1/5. 'Philosophical +Transactions' 1799 page 202.) After alluding to the various means by +which pollen is transported from flower to flower, as far as was then +imperfectly known, he adds, "Nature has something more in view than that +its own proper males would fecundate each blossom." In 1811 Kolreuter +plainly hinted at the same law, as did afterwards another famous +hybridiser of plants, Herbert. (1/6. Kolreuter 'Mem. de l'Acad. de St. +Petersbourg' tome 3 1809 published 1811 page 197. After showing how well +the Malvaceae are adapted for cross-fertilisation, he asks, "An id +aliquid in recessu habeat, quod hujuscemodi flores nunquam proprio suo +pulvere, sed semper eo aliarum suae speciei impregnentur, merito +quaeritur? Certe natura nil facit frustra." Herbert 'Amaryllidaceae, +with a Treatise on Cross-bred Vegetables' 1837.) But none of these +distinguished observers appear to have been sufficiently impressed with +the truth and generality of the law, so as to insist on it and impress +their beliefs on others. + +In 1862 I summed up my observations on Orchids by saying that nature +"abhors perpetual self-fertilisation." If the word perpetual had been +omitted, the aphorism would have been false. As it stands, I believe +that it is true, though perhaps rather too strongly expressed; and I +should have added the self-evident proposition that the propagation of +the species, whether by self-fertilisation or by cross-fertilisation, or +asexually by buds, stolons, etc. is of paramount importance. Hermann +Muller has done excellent service by insisting repeatedly on this latter +point. + +It often occurred to me that it would be advisable to try whether +seedlings from cross-fertilised flowers were in any way superior to +those from self-fertilised flowers. But as no instance was known with +animals of any evil appearing in a single generation from the closest +possible interbreeding, that is between brothers and sisters, I thought +that the same rule would hold good with plants; and that it would be +necessary at the sacrifice of too much time to self-fertilise and +intercross plants during several successive generations, in order to +arrive at any result. I ought to have reflected that such elaborate +provisions favouring cross-fertilisation, as we see in innumerable +plants, would not have been acquired for the sake of gaining a distant +and slight advantage, or of avoiding a distant and slight evil. +Moreover, the fertilisation of a flower by its own pollen corresponds to +a closer form of interbreeding than is possible with ordinary bi-sexual +animals; so that an earlier result might have been expected. + +I was at last led to make the experiments recorded in the present volume +from the following circumstance. For the sake of determining certain +points with respect to inheritance, and without any thought of the +effects of close interbreeding, I raised close together two large beds +of self-fertilised and crossed seedlings from the same plant of Linaria +vulgaris. To my surprise, the crossed plants when fully grown were +plainly taller and more vigorous than the self-fertilised ones. Bees +incessantly visit the flowers of this Linaria and carry pollen from one +to the other; and if insects are excluded, the flowers produce extremely +few seeds; so that the wild plants from which my seedlings were raised +must have been intercrossed during all previous generations. It seemed +therefore quite incredible that the difference between the two beds of +seedlings could have been due to a single act of self-fertilisation; and +I attributed the result to the self-fertilised seeds not having been +well ripened, improbable as it was that all should have been in this +state, or to some other accidental and inexplicable cause. During the +next year, I raised for the same purpose as before two large beds close +together of self-fertilised and crossed seedlings from the carnation, +Dianthus caryophyllus. This plant, like the Linaria, is almost sterile +if insects are excluded; and we may draw the same inference as before, +namely, that the parent-plants must have been intercrossed during every +or almost every previous generation. Nevertheless, the self-fertilised +seedlings were plainly inferior in height and vigour to the crossed. + +My attention was now thoroughly aroused, for I could hardly doubt that +the difference between the two beds was due to the one set being the +offspring of crossed, and the other of self-fertilised flowers. +Accordingly I selected almost by hazard two other plants, which happened +to be in flower in the greenhouse, namely, Mimulus luteus and Ipomoea +purpurea, both of which, unlike the Linaria and Dianthus, are highly +self-fertile if insects are excluded. Some flowers on a single plant of +both species were fertilised with their own pollen, and others were +crossed with pollen from a distinct individual; both plants being +protected by a net from insects. The crossed and self-fertilised seeds +thus produced were sown on opposite sides of the same pots, and treated +in all respects alike; and the plants when fully grown were measured and +compared. With both species, as in the cases of the Linaria and +Dianthus, the crossed seedlings were conspicuously superior in height +and in other ways to the self-fertilised. I therefore determined to +begin a long series of experiments with various plants, and these were +continued for the following eleven years; and we shall see that in a +large majority of cases the crossed beat the self-fertilised plants. +Several of the exceptional cases, moreover, in which the crossed plants +were not victorious, can be explained. + +It should be observed that I have spoken for the sake of brevity, and +shall continue to do so, of crossed and self-fertilised seeds, +seedlings, or plants; these terms implying that they are the product of +crossed or self-fertilised flowers. Cross-fertilisation always means a +cross between distinct plants which were raised from seeds and not from +cuttings or buds. Self-fertilisation always implies that the flowers in +question were impregnated with their own pollen. + +My experiments were tried in the following manner. A single plant, if it +produced a sufficiency of flowers, or two or three plants were placed +under a net stretched on a frame, and large enough to cover the plant +(together with the pot, when one was used) without touching it. This +latter point is important, for if the flowers touch the net they may be +cross-fertilised by bees, as I have known to happen; and when the net is +wet the pollen may be injured. I used at first "white cotton net," with +very fine meshes, but afterwards a kind of net with meshes one-tenth of +an inch in diameter; and this I found by experience effectually excluded +all insects excepting Thrips, which no net will exclude. On the plants +thus protected several flowers were marked, and were fertilised with +their own pollen; and an equal number on the same plants, marked in a +different manner, were at the same time crossed with pollen from a +distinct plant. The crossed flowers were never castrated, in order to +make the experiments as like as possible to what occurs under nature +with plants fertilised by the aid of insects. Therefore, some of the +flowers which were crossed may have failed to be thus fertilised, and +afterwards have been self-fertilised. But this and some other sources of +error will presently be discussed. In some few cases of spontaneously +self-fertile species, the flowers were allowed to fertilise themselves +under the net; and in still fewer cases uncovered plants were allowed to +be freely crossed by the insects which incessantly visited them. There +are some great advantages and some disadvantages in my having +occasionally varied my method of proceeding; but when there was any +difference in the treatment, it is always so stated under the head of +each species. + +Care was taken that the seeds were thoroughly ripened before being +gathered. Afterwards the crossed and self-fertilised seeds were in most +cases placed on damp sand on opposite sides of a glass tumbler covered +by a glass plate, with a partition between the two lots; and the glass +was placed on the chimney-piece in a warm room. I could thus observe the +germination of the seeds. Sometimes a few would germinate on one side +before any on the other, and these were thrown away. But as often as a +pair germinated at the same time, they were planted on opposite sides of +a pot, with a superficial partition between the two; and I thus +proceeded until from half-a-dozen to a score or more seedlings of +exactly the same age were planted on the opposite sides of several pots. +If one of the young seedlings became sickly or was in any way injured, +it was pulled up and thrown away, as well as its antagonist on the +opposite side of the same pot. + +As a large number of seeds were placed on the sand to germinate, many +remained after the pairs had been selected, some of which were in a +state of germination and others not so; and these were sown crowded +together on the opposite sides of one or two rather larger pots, or +sometimes in two long rows out of doors. In these cases there was the +most severe struggle for life among the crossed seedlings on one side of +the pot, and the self-fertilised seedlings on the other side, and +between the two lots which grew in competition in the same pot. A vast +number soon perished, and the tallest of the survivors on both sides +when fully grown were measured. Plants treated in this manner, were +subjected to nearly the same conditions as those growing in a state of +nature, which have to struggle to maturity in the midst of a host of +competitors. + +On other occasions, from the want of time, the seeds, instead of being +allowed to germinate on damp sand, were sown on the opposite sides of +pots, and the fully grown plants measured. But this plan is less +accurate, as the seeds sometimes germinated more quickly on one side +than on the other. It was however necessary to act in this manner with +some few species, as certain kinds of seeds would not germinate well +when exposed to the light; though the glasses containing them were kept +on the chimney-piece on one side of a room, and some way from the two +windows which faced the north-east. (1/7. This occurred in the plainest +manner with the seeds of Papaver vagum and Delphinium consolida, and +less plainly with those of Adonis aestivalis and Ononis minutissima. +Rarely more than one or two of the seeds of these four species +germinated on the bare sand, though left there for some weeks; but when +these same seeds were placed on earth in pots, and covered with a thin +layer of sand, they germinated immediately in large numbers.) + +The soil in the pots in which the seedlings were planted, or the seeds +sown, was well mixed, so as to be uniform in composition. The plants on +the two sides were always watered at the same time and as equally as +possible; and even if this had not been done, the water would have +spread almost equally to both sides, as the pots were not large. The +crossed and self-fertilised plants were separated by a superficial +partition, which was always kept directed towards the chief source of +the light, so that the plants on both sides were equally illuminated. I +do not believe it possible that two sets of plants could have been +subjected to more closely similar conditions, than were my crossed and +self-fertilised seedlings, as grown in the above described manner. + +In comparing the two sets, the eye alone was never trusted. Generally +the height of every plant on both sides was carefully measured, often +more than once, namely, whilst young, sometimes again when older, and +finally when fully or almost fully grown. But in some cases, which are +always specified, owing to the want of time, only one or two of the +tallest plants on each side were measured. This plan, which is not a +good one, was never followed (except with the crowded plants raised from +the seeds remaining after the pairs had been planted) unless the tallest +plants on each side seemed fairly to represent the average difference +between those on both sides. It has, however, some great advantages, as +sickly or accidentally injured plants, or the offspring of ill-ripened +seeds, are thus eliminated. When the tallest plants alone on each side +were measured, their average height of course exceeds that of all the +plants on the same side taken together. But in the case of the much +crowded plants raised from the remaining seeds, the average height of +the tallest plants was less than that of the plants in pairs, owing to +the unfavourable conditions to which they were subjected from being +greatly crowded. For our purpose, however, of the comparison of the +crossed and self-fertilised plants, their absolute height signifies +little. + +As the plants were measured by an ordinary English standard divided into +inches and eighths of an inch, I have not thought it worth while to +change the fractions into decimals. The average or mean heights were +calculated in the ordinary rough method by adding up the measurements of +all, and dividing the product by the number of plants measured; the +result being here given in inches and decimals. As the different species +grow to various heights, I have always for the sake of easy comparison +given in addition the average height of the crossed plants of each +species taken as 100, and have calculated the average height of the +self-fertilised plant in relation to this standard. With respect to the +crowded plants raised from the seeds remaining after the pairs had been +planted, and of which only some of the tallest on each side were +measured, I have not thought it worth while to complicate the results by +giving separate averages for them and for the pairs, but have added up +all their heights, and thus obtained a single average. + +I long doubted whether it was worth while to give the measurements of +each separate plant, but have decided to do so, in order that it may be +seen that the superiority of the crossed plants over the +self-fertilised, does not commonly depend on the presence of two or +three extra fine plants on the one side, or of a few very poor plants on +the other side. Although several observers have insisted in general +terms on the offspring from intercrossed varieties being superior to +either parent-form, no precise measurements have been given (1/8. A +summary of these statements, with references, may be found in my +'Variation of Animals and Plants under Domestication' chapter 17 2nd +edition 1875 volume 2 page 109.); and I have met with no observations on +the effects of crossing and self-fertilising the individuals of the same +variety. Moreover, experiments of this kind require so much time--mine +having been continued during eleven years--that they are not likely soon +to be repeated. + +As only a moderate number of crossed and self-fertilised plants were +measured, it was of great importance to me to learn how far the averages +were trustworthy. I therefore asked Mr. Galton, who has had much +experience in statistical researches, to examine some of my tables of +measurements, seven in number, namely, those of Ipomoea, Digitalis, +Reseda lutea, Viola, Limnanthes, Petunia, and Zea. I may premise that if +we took by chance a dozen or score of men belonging to two nations and +measured them, it would I presume be very rash to form any judgment from +such small numbers on their average heights. But the case is somewhat +different with my crossed and self-fertilised plants, as they were of +exactly the same age, were subjected from first to last to the same +conditions, and were descended from the same parents. When only from two +to six pairs of plants were measured, the results are manifestly of +little or no value, except in so far as they confirm and are confirmed +by experiments made on a larger scale with other species. I will now +give the report on the seven tables of measurements, which Mr. Galton +has had the great kindness to draw up for me. + +["I have examined the measurements of the plants with care, and by many +statistical methods, to find out how far the means of the several sets +represent constant realities, such as would come out the same so long as +the general conditions of growth remained unaltered. The principal +methods that were adopted are easily explained by selecting one of the +shorter series of plants, say of Zea mays, for an example." + +TABLE 1/1. Zea mays (young plants). (Mr. Galton.) + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed, as recorded by Mr. Darwin. + +Column 3: Self-fertilised, as recorded by Mr. Darwin. + +Column 4: Crossed, in Separate Pots, arranged in order of magnitude. + +Column 5: Self-fertilised, in Separate Pots, arranged in order of magnitude. + +Column 6: Crossed, in a Single Series, arranged in order of magnitude. + +Column 7: Self-fertilised, in a Single Series, arranged in order of +magnitude. + +Column 8: Difference, in a Single Series, arranged in order of magnitude. + +Pot 1 : 23 4/8 : 17 3/8 :: 23 4/8 : 20 3/8 :: 23 4/8 : 20 3/8 : -3 1/8. +Pot 1 : 12 : 20 3/8 :: 21 : 20 :: 23 2/8 : 20 : -3 2/8. +Pot 1 : 21 : 20 :: 12 : 17 3/8 :: 23 : 20 : -3. +Pot 1 : - : - :: - : - :: 22 1/8 : 18 5/8 : -3 4/8. +Pot 1 : 22 : 20 :: 22 : 20 :: 22 1/8 : 18 5/8 : -3 4/8. + +Pot 2 : 19 1/8 : 18 3/8 :: 21 4/8 : 18 5/8 :: 22 : 18 3/8 : -3 5/8. +Pot 2 : 21 4/8 : 18 5/8 :: 19 1/8 : 18 3/8 :: 21 5/8 : 18 : -3 5/8. +Pot 2 : - : - :: - : - :: 21 4/8 : 18 : -3 4/8. +Pot 2 : 22 1/8 : 18 5/8 :: 23 2/8 : 18 5/8 :: 21 : 18 : -3. +Pot 2 : 20 3/8 : 15 2/8 :: 22 1/8 : 18 :: 21 : 17 3/8 : -3 5/8. + +Pot 3 : 18 2/8 : 16 4/8 :: 21 5/8 : 16 4/8 :: 20 3/8 : 16 4/8 : -3 7/8. +Pot 3 : 21 5/8 : 18 :: 20 3/8 : 16 2/8 :: 19 1/8 : 16 2/8 : -2 7/8. +Pot 3 : 23 2/8 : 16 2/8 :: 18 2/8 : 15 2/8 :: 18 2/8 : 15 4/8 : -2 6/8. +Pot 3 : - : - :: - : - :: 12 : 15 2/8 : +3 2/8. +Pot 3 : 21 : 18 :: 23 : 18 :: 12 : 12 6/8 : +0 6/8. + +Pot 4 : 22 1/8 : 12 6/8 :: 22 1/8 : 18. +Pot 4 : 23 : 15 4/8 :: 21 : 15 4/8. +Pot 4 : 12 : 18 :: 12 : 12 6/8. + +"The observations as I received them are shown in Table 1/1, Columns 2 +and 3, where they certainly have no prima facie appearance of +regularity. But as soon as we arrange them the in order of their +magnitudes, as in columns 4 and 5, the case is materially altered. We +now see, with few exceptions, that the largest plant on the crossed side +in each pot exceeds the largest plant on the self-fertilised side, that +the second exceeds the second, the third the third, and so on. Out of +the fifteen cases in the table, there are only two exceptions to this +rule. We may therefore confidently affirm that a crossed series will +always be found to exceed a self-fertilised series, within the range of +the conditions under which the present experiment has been made." + +TABLE 1/2. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed. + +Column 3: Self-fertilised. + +Column 4: Difference. + +Pot 1 : 18 7/8 : 19 2/8 : +0 3/8. +Pot 2 : 20 7/8 : 19 : -1 7/8. +Pot 3 : 21 1/8 : 16 7/8 : -4 2/8. +Pot 4 : 19 6/8 : 16 : -3 6/8. + +"Next as regards the numerical estimate of this excess. The mean values +of the several groups are so discordant, as is shown in Table 1/2, that +a fairly precise numerical estimate seems impossible. But the +consideration arises, whether the difference between pot and pot may not +be of much the same order of importance as that of the other conditions +upon which the growth of the plants has been modified. If so, and only +on that condition, it would follow that when all the measurements, +either of the crossed or the self-fertilised plants, were combined into +a single series, that series would be statistically regular. The +experiment is tried in Table 1/1, columns 7 and 8, where the regularity +is abundantly clear, and justifies us in considering its mean as +perfectly reliable. I have protracted these measurements, and revised +them in the usual way, by drawing a curve through them with a free hand, +but the revision barely modifies the means derived from the original +observations. In the present, and in nearly all the other cases, the +difference between the original and revised means is under 2 per cent of +their value. It is a very remarkable coincidence that in the seven kinds +of plants, whose measurements I have examined, the ratio between the +heights of the crossed and of the self-fertilised ranges in five cases +within very narrow limits. In Zea mays it is as 100 to 84, and in the +others it ranges between 100 to 76 and 100 to 86." + +"The determination of the variability (measured by what is technically +called the 'probable error') is a problem of more delicacy than that of +determining the means, and I doubt, after making many trials, whether it +is possible to derive useful conclusions from these few observations. We +ought to have measurements of at least fifty plants in each case, in +order to be in a position to deduce fair results. One fact, however, +bearing on variability, is very evident in most cases, though not in Zea +mays, namely, that the self-fertilised plants include the larger number +of exceptionally small specimens, while the crossed are more generally +full grown." + +"Those groups of cases in which measurements have been made of a few of +the tallest plants that grew in rows, each of which contained a +multitude of plants, show very clearly that the crossed plants exceed +the self-fertilised in height, but they do not tell by inference +anything about their respective mean values. If it should happen that a +series is known to follow the law of error or any other law, and if the +number of individuals in the series is known, it would be always +possible to reconstruct the whole series when a fragment of it has been +given. But I find no such method to be applicable in the present case. +The doubt as to the number of plants in each row is of minor importance; +the real difficulty lies in our ignorance of the precise law followed by +the series. The experience of the plants in pots does not help us to +determine that law, because the observations of such plants are too few +to enable us to lay down more than the middle terms of the series to +which they belong with any sort of accuracy, whereas the cases we are +now considering refer to one of its extremities. There are other special +difficulties which need not be gone into, as the one already mentioned +is a complete bar."] + +Mr. Galton sent me at the same time graphical representations which he +had made of the measurements, and they evidently form fairly regular +curves. He appends the words "very good" to those of Zea and Limnanthes. +He also calculated the average height of the crossed and self-fertilised +plants in the seven tables by a more correct method than that followed +by me, namely, by including the heights, as estimated in accordance with +statistical rules, of a few plants which died before they were measured; +whereas I merely added up the heights of the survivors, and divided the +sum by their number. The difference in our results is in one way highly +satisfactory, for the average heights of the self-fertilised plants, as +deduced by Mr. Galton, is less than mine in all the cases excepting one, +in which our averages are the same; and this shows that I have by no +means exaggerated the superiority of the crossed over the +self-fertilised plants. + +After the heights of the crossed and self-fertilised plants had been +taken, they were sometimes cut down close to the ground, and an equal +number of both weighed. This method of comparison gives very striking +results, and I wish that it had been oftener followed. Finally a record +was often kept of any marked difference in the rate of germination of +the crossed and self-fertilised seeds,--of the relative periods of +flowering of the plants raised from them,--and of their productiveness, +that is, of the number of seed-capsules which they produced and of the +average number of seeds which each capsule contained. + +When I began my experiments I did not intend to raise crossed and +self-fertilised plants for more than a single generation; but as soon as +the plants of the first generation were in flower I thought that I would +raise one more generation, and acted in the following manner. Several +flowers on one or more of the self-fertilised plants were again +self-fertilised; and several flowers on one or more of the crossed +plants were fertilised with pollen from another crossed plant of the +same lot. Having thus once begun, the same method was followed for as +many as ten successive generations with some of the species. The seeds +and seedlings were always treated in exactly the same manner as already +described. The self-fertilised plants, whether originally descended from +one or two mother-plants, were thus in each generation as closely +interbred as was possible; and I could not have improved on my plan. But +instead of crossing one of the crossed plants with another crossed +plant, I ought to have crossed the self-fertilised plants of each +generation with pollen taken from a non-related plant--that is, one +belonging to a distinct family or stock of the same species and variety. +This was done in several cases as an additional experiment, and gave +very striking results. But the plan usually followed was to put into +competition and compare intercrossed plants, which were almost always +the offspring of more or less closely related plants, with the +self-fertilised plants of each succeeding generation;--all having been +grown under closely similar conditions. I have, however, learnt more by +this method of proceeding, which was begun by an oversight and then +necessarily followed, than if I had always crossed the self-fertilised +plants of each succeeding generation with pollen from a fresh stock. + +I have said that the crossed plants of the successive generations were +almost always inter-related. When the flowers on an hermaphrodite plant +are crossed with pollen taken from a distinct plant, the seedlings thus +raised may be considered as hermaphrodite brothers or sisters; those +raised from the same capsule being as close as twins or animals of the +same litter. But in one sense the flowers on the same plant are distinct +individuals, and as several flowers on the mother-plant were crossed by +pollen taken from several flowers on the father-plant, such seedlings +would be in one sense half-brothers or sisters, but more closely related +than are the half-brothers and sisters of ordinary animals. The flowers +on the mother-plant were, however, commonly crossed by pollen taken from +two or more distinct plants; and in these cases the seedlings might be +called with more truth half-brothers or sisters. When two or three +mother-plants were crossed, as often happened, by pollen taken from two +or three father-plants (the seeds being all intermingled), some of the +seedlings of the first generation would be in no way related, whilst +many others would be whole or half-brothers and sisters. In the second +generation a large number of the seedlings would be what may be called +whole or half first-cousins, mingled with whole and half-brothers and +sisters, and with some plants not at all related. So it would be in the +succeeding generations, but there would also be many cousins of the +second and more remote degrees. The relationship will thus have become +more and more inextricably complex in the later generations; with most +of the plants in some degree and many of them closely related. + +I have only one other point to notice, but this is one of the highest +importance; namely, that the crossed and self-fertilised plants were +subjected in the same generation to as nearly similar and uniform +conditions as was possible. In the successive generations they were +exposed to slightly different conditions as the seasons varied, and they +were raised at different periods. But in other respects all were treated +alike, being grown in pots in the same artificially prepared soil, being +watered at the same time, and kept close together in the same greenhouse +or hothouse. They were therefore not exposed during successive years to +such great vicissitudes of climate as are plants growing out of doors. + +ON SOME APPARENT AND REAL CAUSES OF ERROR IN MY EXPERIMENTS. + +It has been objected to such experiments as mine, that covering plants +with a net, although only for a short time whilst in flower, may affect +their health and fertility. I have seen no such effect except in one +instance with a Myosotis, and the covering may not then have been the +real cause of injury. But even if the net were slightly injurious, and +certainly it was not so in any high degree, as I could judge by the +appearance of the plants and by comparing their fertility with that of +neighbouring uncovered plants, it would not have vitiated my +experiments; for in all the more important cases the flowers were +crossed as well as self-fertilised under a net, so that they were +treated in this respect exactly alike. + +As it is impossible to exclude such minute pollen-carrying insects as +Thrips, flowers which it was intended to fertilise with their own pollen +may sometimes have been afterwards crossed with pollen brought by these +insects from another flower on the same plant; but as we shall hereafter +see, a cross of this kind does not produce any effect, or at most only a +slight one. When two or more plants were placed near one another under +the same net, as was often done, there is some real though not great +danger of the flowers which were believed to be self-fertilised being +afterwards crossed with pollen brought by Thrips from a distinct plant. +I have said that the danger is not great because I have often found that +plants which are self-sterile, unless aided by insects, remained sterile +when several plants of the same species were placed under the same net. +If, however, the flowers which had been presumably self-fertilised by me +were in any case afterwards crossed by Thrips with pollen brought from a +distinct plant, crossed seedlings would have been included amongst the +self-fertilised; but it should be especially observed that this +occurrence would tend to diminish and not to increase any superiority in +average height, fertility, etc., of the crossed over the self-fertilised +plants. + +As the flowers which were crossed were never castrated, it is probable +or even almost certain that I sometimes failed to cross-fertilise them +effectually, and that they were afterwards spontaneously +self-fertilised. This would have been most likely to occur with +dichogamous species, for without much care it is not easy to perceive +whether their stigmas are ready to be fertilised when the anthers open. +But in all cases, as the flowers were protected from wind, rain, and the +access of insects, any pollen placed by me on the stigmatic surface +whilst it was immature, would generally have remained there until the +stigma was mature; and the flowers would then have been crossed as was +intended. Nevertheless, it is highly probable that self-fertilised +seedlings have sometimes by this means got included amongst the crossed +seedlings. The effect would be, as in the former case, not to exaggerate +but to diminish any average superiority of the crossed over the +self-fertilised plants. + +Errors arising from the two causes just named, and from others,--such as +some of the seeds not having been thoroughly ripened, though care was +taken to avoid this error--the sickness or unperceived injury of any of +the plants,--will have been to a large extent eliminated, in those cases +in which many crossed and self-fertilised plants were measured and an +average struck. Some of these causes of error will also have been +eliminated by the seeds having been allowed to germinate on bare damp +sand, and being planted in pairs; for it is not likely that ill-matured +and well-matured, or diseased and healthy seeds, would germinate at +exactly the same time. The same result will have been gained in the +several cases in which only a few of the tallest, finest, and healthiest +plants on each side of the pots were measured. + +Kolreuter and Gartner have proved that with some plants several, even as +many as from fifty to sixty, pollen-grains are necessary for the +fertilisation of all the ovules in the ovarium. (1/9. 'Kentniss der +Befruchtung' 1844 page 345. Naudin 'Nouvelles Archives du Museum' tome 1 +page 27.) Naudin also found in the case of Mirabilis that if only one or +two of its very large pollen-grains were placed on the stigma, the +plants raised from such seeds were dwarfed. I was therefore careful to +give an amply sufficient supply of pollen, and generally covered the +stigma with it; but I did not take any special pains to place exactly +the same amount on the stigmas of the self-fertilised and crossed +flowers. After having acted in this manner during two seasons, I +remembered that Gartner thought, though without any direct evidence, +that an excess of pollen was perhaps injurious; and it has been proved +by Spallanzani, Quatrefages, and Newport, that with various animals an +excess of the seminal fluid entirely prevents fertilisation. (1/10. +'Transactions of the Philosophical Society' 1853 pages 253-258.) It was +therefore necessary to ascertain whether the fertility of the flowers +was affected by applying a rather small and an extremely large quantity +of pollen to the stigma. Accordingly a very small mass of pollen-grains +was placed on one side of the large stigma in sixty-four flowers of +Ipomoea purpurea, and a great mass of pollen over the whole surface of +the stigma in sixty-four other flowers. In order to vary the experiment, +half the flowers of both lots were on plants produced from +self-fertilised seeds, and the other half on plants from crossed seeds. +The sixty-four flowers with an excess of pollen yielded sixty-one +capsules; and excluding four capsules, each of which contained only a +single poor seed, the remainder contained on an average 5.07 seeds per +capsule. The sixty-four flowers with only a little pollen placed on one +side of the stigma yielded sixty-three capsules, and excluding one from +the same cause as before, the remainder contained on an average 5.129 +seeds. So that the flowers fertilised with little pollen yielded rather +more capsules and seeds than did those fertilised with an excess; but +the difference is too slight to be of any significance. On the other +hand, the seeds produced by the flowers with an excess of pollen were a +little heavier of the two; for 170 of them weighed 79.67 grains, whilst +170 seeds from the flowers with very little pollen weighed 79.20 grains. +Both lots of seeds having been placed on damp sand presented no +difference in their rate of germination. We may therefore conclude that +my experiments were not affected by any slight difference in the amount +of pollen used; a sufficiency having been employed in all cases. + +The order in which our subject will be treated in the present volume is +as follows. A long series of experiments will first be given in Chapters +2 to 6. Tables will afterwards be appended, showing in a condensed form +the relative heights, weights, and fertility of the offspring of the +various crossed and self-fertilised species. Another table exhibits the +striking results from fertilising plants, which during several +generations had either been self-fertilised or had been crossed with +plants kept all the time under closely similar conditions, with pollen +taken from plants of a distinct stock and which had been exposed to +different conditions. In the concluding chapters various related points +and questions of general interest will be discussed. + +Anyone not specially interested in the subject need not attempt to read +all the details (marked []); though they possess, I think, some value, +and cannot be all summarised. But I would suggest to the reader to take +as an example the experiments on Ipomoea in Chapter 2; to which may be +added those on Digitalis, Origanum, Viola, or the common cabbage, as in +all these cases the crossed plants are superior to the self-fertilised +in a marked degree, but not in quite the same manner. As instances of +self-fertilised plants being equal or superior to the crossed, the +experiments on Bartonia, Canna, and the common pea ought to be read; but +in the last case, and probably in that of Canna, the want of any +superiority in the crossed plants can be explained. + +Species were selected for experiment belonging to widely distinct +families, inhabiting various countries. In some few cases several genera +belonging to the same family were tried, and these are grouped together; +but the families themselves have been arranged not in any natural order, +but in that which was the most convenient for my purpose. The +experiments have been fully given, as the results appear to me of +sufficient value to justify the details. Plants bearing hermaphrodite +flowers can be interbred more closely than is possible with bisexual +animals, and are therefore well-fitted to throw light on the nature and +extent of the good effects of crossing, and on the evil effects of close +interbreeding or self-fertilisation. The most important conclusion at +which I have arrived is that the mere act of crossing by itself does no +good. The good depends on the individuals which are crossed differing +slightly in constitution, owing to their progenitors having been +subjected during several generations to slightly different conditions, +or to what we call in our ignorance spontaneous variation. This +conclusion, as we shall hereafter see, is closely connected with various +important physiological problems, such as the benefit derived from +slight changes in the conditions of life, and this stands in the closest +connection with life itself. It throws light on the origin of the two +sexes and on their separation or union in the same individual, and +lastly on the whole subject of hybridism, which is one of the greatest +obstacles to the general acceptance and progress of the great principle +of evolution. + +In order to avoid misapprehension, I beg leave to repeat that throughout +this volume a crossed plant, seedling, or seed, means one of crossed +PARENTAGE, that is, one derived from a flower fertilised with pollen +from a distinct plant of the same species. And that a self-fertilised +plant, seedling, or seed, means one of self-fertilised PARENTAGE, that +is, one derived from a flower fertilised with pollen from the same +flower, or sometimes, when thus stated, from another flower on the same +plant. + + + +CHAPTER II. + +CONVOLVULACEAE. + +Ipomoea purpurea, comparison of the height and fertility of the crossed +and self-fertilised plants during ten successive generations. +Greater constitutional vigour of the crossed plants. +The effects on the offspring of crossing different flowers on the same +plant, instead of crossing distinct individuals. +The effects of a cross with a fresh stock. +The descendants of the self-fertilised plant named Hero. +Summary on the growth, vigour, and fertility of the successive crossed +and self-fertilised generations. +Small amount of pollen in the anthers of the self-fertilised plants of +the later generations, and the sterility of their first-produced +flowers. +Uniform colour of the flowers produced by the self-fertilised plants. +The advantage from a cross between two distinct plants depends on their +differing in constitution. + +A plant of Ipomoea purpurea, or as it is often called in England the +convolvulus major, a native of South America, grew in my greenhouse. Ten +flowers on this plant were fertilised with pollen from the same flower; +and ten other flowers on the same plant were crossed with pollen from a +distinct plant. The fertilisation of the flowers with their own pollen +was superfluous, as this convolvulus is highly self-fertile; but I acted +in this manner to make the experiments correspond in all respects. +Whilst the flowers are young the stigma projects beyond the anthers; and +it might have been thought that it could not be fertilised without the +aid of humble-bees, which often visit the flowers; but as the flower +grows older the stamens increase in length, and their anthers brush +against the stigma, which thus receives some pollen. The number of seeds +produced by the crossed and self-fertilised flowers differed very +little. + +[Crossed and self-fertilised seeds obtained in the above manner were +allowed to germinate on damp sand, and as often as pairs germinated at +the same time they were planted in the manner described in the +Introduction (Chapter 1), on the opposite sides of two pots. Five pairs +were thus planted; and all the remaining seeds, whether or not in a +state of germination, were planted on the opposite sides of a third pot, +so that the young plants on both sides were here greatly crowded and +exposed to very severe competition. Rods of iron or wood of equal +diameter were given to all the plants to twine up; and as soon as one of +each pair reached the summit both were measured. A single rod was placed +on each side of the crowded pot, Number 3, and only the tallest plant on +each side was measured. + +TABLE 2/1. Ipomoea purpurea (First Generation.). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Seedlings from Crossed Plants. + +Column 3: Seedlings from Self-fertilised Plants. + +Pot 1 : 87 4/8 : 69. +Pot 1 : 87 4/8 : 66. +Pot 1 : 89 : 73. + +Pot 2 : 88 : 68 4/8. +Pot 2 : 87 : 60 4/8. + +Pot 3 : 77 : 57. +Plants crowded; the tallest one measured on each side. + +Total : 516 : 394. + +The average height of the six crossed plants is here 86 inches, whilst +that of the six self-fertilised plants is only 65.66 inches, so that the +crossed plants are to the self-fertilised in height as 100 to 76. It +should be observed that this difference is not due to a few of the +crossed plants being extremely tall, or to a few of the self-fertilised +being extremely short, but to all the crossed plants attaining a greater +height than their antagonists. The three pairs in Pot 1 were measured at +two earlier periods, and the difference was sometimes greater and +sometimes less than that at the final measuring. But it is an +interesting fact, of which I have seen several other instances, that one +of the self-fertilised plants, when nearly a foot in height, was half an +inch taller than the crossed plant; and again, when two feet high, it +was 1 3/8 of an inch taller, but during the ten subsequent days the +crossed plant began to gain on its antagonist, and ever afterward +asserted its supremacy, until it exceeded its self-fertilised opponent +by 16 inches. + +The five crossed plants in Pots 1 and 2 were covered with a net, and +produced 121 capsules; the five self-fertilised plants produced +eighty-four capsules, so that the numbers of capsules were as 100 to 69. +Of the 121 capsules on the crossed plants sixty-five were the product of +flowers crossed with pollen from a distinct plant, and these contained +on an average 5.23 seeds per capsule; the remaining fifty-six capsules +were spontaneously self-fertilised. Of the eighty-four capsules on the +self-fertilised plants, all the product of renewed self-fertilisation, +fifty-five (which were alone examined) contained on an average 4.85 +seeds per capsule. Therefore the cross-fertilised capsules, compared +with the self-fertilised capsules, yielded seeds in the proportion of +100 to 93. The crossed seeds were relatively heavier than the +self-fertilised seeds. Combining the above data (i.e., number of +capsules and average number of contained seeds), the crossed plants, +compared with the self-fertilised, yielded seeds in the ratio of 100 to +64. + +These crossed plants produced, as already stated, fifty-six +spontaneously self-fertilised capsules, and the self-fertilised plants +produced twenty-nine such capsules. The former contained on an average, +in comparison with the latter, seeds in the proportion of 100 to 99. + +In Pot 3, on the opposite sides of which a large number of crossed and +self-fertilised seeds had been sown and the seedlings allowed to +struggle together, the crossed plants had at first no great advantage. +At one time the tallest crossed was 25 1/8 inches high, and the tallest +self-fertilised plants 21 3/8. But the difference afterwards became much +greater. The plants on both sides, from being so crowded, were poor +specimens. The flowers were allowed to fertilise themselves +spontaneously under a net; the crossed plants produced thirty-seven +capsules, the self-fertilised plants only eighteen, or as 100 to 47. The +former contained on an average 3.62 seeds per capsule; and the latter +3.38 seeds, or as 100 to 93. Combining these data (i.e., number of +capsules and average number of seeds), the crowded crossed plants +produced seeds compared with the self-fertilised as 100 to 45. These +latter seeds, however, were decidedly heavier, a hundred weighing 41.64 +grains, than those from the capsules on the crossed plants, of which a +hundred weighed 36.79 grains; and this probably was due to the fewer +capsules borne by the self-fertilised plants having been better +nourished. We thus see that the crossed plants in this the first +generation, when grown under favourable conditions, and when grown under +unfavourable conditions from being much crowded, greatly exceeded in +height, and in the number of capsules produced, and slightly in the +number of seeds per capsule, the self-fertilised plants. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION. + +Flowers on the crossed plants of the last generation (Table 2/1) were +crossed by pollen from distinct plants of the same generation; and +flowers on the self-fertilised plants were fertilised by pollen from the +same flower. The seeds thus produced were treated in every respect as +before, and we have in Table 2/2 the result. + +TABLE 2/2. Ipomoea purpurea (Second Generation.). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 87 : 67 4/8. +Pot 1 : 83 : 68 4/8. +Pot 1 : 83 : 80 4/8. + +Pot 2 : 85 4/8 : 61 4/8. +Pot 2 : 89 : 79. +Pot 2 : 77 4/8 : 41. + +Total : 505 : 398. + +Here again every single crossed plant is taller than its antagonist. The +self-fertilised plant in Pot 1, which ultimately reached the unusual +height of 80 4/8 inches, was for a long time taller than the opposed +crossed plant, though at last beaten by it. The average height of the +six crossed plants is 84.16 inches, whilst that of the six +self-fertilised plants is 66.33 inches, or as 100 to 79. + +CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION. + +Seeds from the crossed plants of the last generation (Table 2/2) again +crossed, and from the self-fertilised plants again self-fertilised, were +treated in all respects exactly as before, with the following result:-- + +TABLE 2/3. Ipomoea purpurea (Third Generation.). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 74 : 56 4/8. +Pot 1 : 72 : 51 4/8. +Pot 1 : 73 4/8 : 54. + +Pot 2 : 82 : 59. +Pot 2 : 81 : 30. +Pot 2 : 82 : 66. + +Total : 464.5 : 317. + +Again all the crossed plants are higher than their antagonists: their +average height is 77.41 inches, whereas that of the self-fertilised is +52.83 inches, or as 100 to 68. + +I attended closely to the fertility of the plants of this third +generation. Thirty flowers on the crossed plants were crossed with +pollen from other crossed plants of the same generation, and the +twenty-six capsules thus produced contained, on an average, 4.73 seeds; +whilst thirty flowers on the self-fertilised plants, fertilised with the +pollen from the same flower, produced twenty-three capsules, each +containing 4.43 seeds. Thus the average number of seeds in the crossed +capsules was to that in the self-fertilised capsules as 100 to 94. A +hundred of the crossed seeds weighed 43.27 grains, whilst a hundred of +the self-fertilised seeds weighed only 37.63 grains. Many of these +lighter self-fertilised seeds placed on damp sand germinated before the +crossed; thus thirty-six of the former germinated whilst only thirteen +of the latter or crossed seeds germinated. In Pot 1 the three crossed +plants produced spontaneously under the net (besides the twenty-six +artificially cross-fertilised capsules) seventy-seven self-fertilised +capsules containing on an average 4.41 seeds; whilst the three +self-fertilised plants produced spontaneously (besides the twenty-three +artificially self-fertilised capsules) only twenty-nine self-fertilised +capsules, containing on an average 4.14 seeds. Therefore the average +number of seeds in the two lots of spontaneously self-fertilised +capsules was as 100 to 94. Taking into consideration the number of +capsules together with the average number of seeds, the crossed plants +(spontaneously self-fertilised) produced seeds in comparison with the +self-fertilised plants (spontaneously self-fertilised) in the proportion +of 100 to 35. By whatever method the fertility of these plants is +compared, the crossed are more fertile than the self-fertilised plants. + +I tried in several ways the comparative vigour and powers of growth of +the crossed and self-fertilised plants of this third generation. Thus, +four self-fertilised seeds which had just germinated were planted on one +side of a pot, and after an interval of forty-eight hours, four crossed +seeds in the same state of germination were planted on the opposite +side; and the pot was kept in the hothouse. I thought that the advantage +thus given to the self-fertilised seedlings would have been so great +that they would never have been beaten by the crossed ones. They were +not beaten until all had grown to a height of 18 inches; and the degree +to which they were finally beaten is shown in Table 2/4. We here see +that the average height of the four crossed plants is 76.62, and of the +four self-fertilised plants 65.87 inches, or as 100 to 86; therefore +less than when both sides started fair. + +TABLE 2/4. Ipomoea purpurea (Third Generation, the self-fertilised +plants having had a start of forty-eight hours). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 3 : 78 4/8 : 73 4/8. +Pot 3 : 77 4/8 : 53. +Pot 3 : 73 : 61 4/8. +Pot 3 : 77 4/8 : 75 4/8. + +Total : 306.5 : 263.5. + +Crossed and self-fertilised seeds of the third generation were also sown +out of doors late in the summer, and therefore under unfavourable +conditions, and a single stick was given to each lot of plants to twine +up. The two lots were sufficiently separate so as not to interfere with +each other's growth, and the ground was clear of weeds. As soon as they +were killed by the first frost (and there was no difference in their +hardiness), the two tallest crossed plants were found to be 24.5 and +22.5 inches, whilst the two tallest self-fertilised plants were only 15 +and 12.5 inches in height, or as 100 to 59. + +I likewise sowed at the same time two lots of the same seeds in a part +of the garden which was shady and covered with weeds. The crossed +seedlings from the first looked the most healthy, but they twined up a +stick only to a height of 7 1/4 inches; whilst the self-fertilised were +not able to twine at all; and the tallest of them was only 3 1/2 inches +in height. + +Lastly, two lots of the same seeds were sown in the midst of a bed of +candy-tuft (Iberis) growing vigorously. The seedlings came up, but all +the self-fertilised ones soon died excepting one, which never twined and +grew to a height of only 4 inches. Many of the crossed seedlings, on the +other hand, survived; and some twined up the stems of the Iberis to the +height of 11 inches. These cases prove that the crossed seedlings have +an immense advantage over the self-fertilised, both when growing +isolated under very unfavourable conditions, and when put into +competition with each other or with other plants, as would happen in a +state of nature. + +CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION. + +Seedlings raised as before from the crossed and self-fertilised plants +of the third generation in Table 2/3, gave results as follows:-- + +TABLE 2/5. Ipomoea purpurea (Fourth Generation). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 84 : 80. +Pot 1 : 47 : 44 1/2. + +Pot 2 : 83 : 73 1/2. +Pot 2 : 59 : 51 1/2. + +Pot 3 : 82 : 56 1/2. +Pot 3 : 65 1/2 : 63. +Pot 3 : 68 : 52. + +Total : 488.5 : 421.0. + +Here the average height of the seven crossed plants is 69.78 inches, and +that of the seven self-fertilised plants 60.14; or as 100 to 86. This +smaller difference relatively to that in the former generations, may be +attributed to the plants having been raised during the depth of winter, +and consequently to their not having grown vigorously, as was shown by +their general appearance and from several of them never reaching the +summits of the rods. In Pot 2, one of the self-fertilised plants was for +a long time taller by two inches than its opponent, but was ultimately +beaten by it, so that all the crossed plants exceeded their opponents in +height. Of twenty-eight capsules produced by the crossed plants +fertilised by pollen from a distinct plant, each contained on an average +4.75 seeds; of twenty-seven self-fertilised capsules on the +self-fertilised plants, each contained on an average 4.47 seeds; so that +the proportion of seeds in the crossed and self-fertilised capsules was +as 100 to 94. + +Some of the same seeds, from which the plants in Table 2/5 had been +raised, were planted, after they had germinated on damp sand, in a +square tub, in which a large Brugmansia had long been growing. The soil +was extremely poor and full of roots; six crossed seeds were planted in +one corner, and six self-fertilised seeds in the opposite corner. All +the seedlings from the latter soon died excepting one, and this grew to +the height of only 1 1/2 inches. Of the crossed plants three survived, +and they grew to the height of 2 1/2 inches, but were not able to twine +round a stick; nevertheless, to my surprise, they produced some small +miserable flowers. The crossed plants thus had a decided advantage over +the self-fertilised plants under this extremity of bad conditions. + +CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION. + +These were raised in the same manner as before, and when measured gave +the following results:-- + +TABLE 2/6. Ipomoea purpurea (Fifth Generation). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 96 : 73. +Pot 1 : 86 : 78. +Pot 1 : 69 : 29. + +Pot 2 : 84 : 51. +Pot 2 : 84 : 84. +Pot 2 : 76 1/4 : 59. + +Total : 495.25 : 374.00. + +The average height of the six crossed plants is 82.54 inches, and that +of the six self-fertilised plants 62.33 inches, or as 100 to 75. Every +crossed plant exceeded its antagonist in height. In Pot 1 the middle +plant on the crossed side was slightly injured whilst young by a blow, +and was for a time beaten by its opponent, but ultimately recovered the +usual superiority. The crossed plants produced spontaneously a vast +number more capsules than did the self-fertilised plants; and the +capsules of the former contained on an average 3.37 seeds, whilst those +of the latter contained only 3.0 per capsule, or as 100 to 89. But +looking only to the artificially fertilised capsules, those on the +crossed plants again crossed contained on an average 4.46 seeds, whilst +those on the self-fertilised plants again self-fertilised contained 4.77 +seeds; so that the self-fertilised capsules were the more fertile of the +two, and of this unusual fact I can offer no explanation. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION. + +These were raised in the usual manner, with the following result. I +should state that there were originally eight plants on each side; but +as two of the self-fertilised became extremely unhealthy and never grew +to near their full height, these as well as their opponents have been +struck out of the list. If they had been retained, they would have made +the average height of the crossed plants unfairly greater than that of +the self-fertilised. I have acted in the same manner in a few other +instances, when one of a pair plainly became very unhealthy. + +TABLE 2/7. Ipomoea purpurea (Sixth Generation). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 93 : 50 1/2. +Pot 1 : 91 : 65. + +Pot 2 : 79 : 50. +Pot 2 : 86 1/2 : 87. +Pot 2 : 88 : 62. + +Pot 3 : 87 1/2 : 64 1/2. + +Total : 525 : 379. + +The average height of the six crossed plants is here 87.5, and of the +six self-fertilised plants 63.16, or as 100 to 72. This large difference +was chiefly due to most of the plants, especially the self-fertilised +ones, having become unhealthy towards the close of their growth, and +they were severely attacked by aphides. From this cause nothing can be +inferred with respect to their relative fertility. In this generation we +have the first instance of a self-fertilised plant in Pot 2 exceeding +(though only by half an inch) its crossed opponent. This victory was +fairly won after a long struggle. At first the self-fertilised plant was +several inches taller than its opponent, but when the latter was 4 1/2 +feet high it had grown equal; it then grew a little taller than the +self-fertilised plant, but was ultimately beaten by it to the extent of +half an inch, as shown in Table 2/7. I was so much surprised at this +case that I saved the self-fertilised seeds of this plant, which I will +call the "Hero," and experimented on its descendants, as will hereafter +be described. + +Besides the plants included in Table 2/7, nine crossed and nine +self-fertilised plants of the same lot were raised in two other pots, 4 +and 5. These pots had been kept in the hothouse, but from want of room +were, whilst the plants were young, suddenly moved during very cold +weather into the coldest part of the greenhouse. They all suffered +greatly, and never quite recovered. After a fortnight only two of the +nine self-fertilised seedlings were alive, whilst seven of the crossed +survived. The tallest of these latter plants when measured was 47 inches +in height, whilst the tallest of the two surviving self-fertilised +plants was only 32 inches. Here again we see how much more vigorous the +crossed plants are than the self-fertilised. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION. + +These were raised as heretofore with the following result:-- + +TABLE 2/8. Ipomoea purpurea (Seventh Generation). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 84 4/8 : 74 6/8. +Pot 1 : 84 6/8 : 84. +Pot 1 : 76 2/8 : 55 4/8. + +Pot 2 : 84 4/8 : 65. +Pot 2 : 90 : 51 2/8. +Pot 2 : 82 2/8 : 80 4/8. + +Pot 3 : 83 : 67 6/8. +Pot 3 : 86 : 60 2/8. + +Pot 4 : 84 2/8 : 75 2/8. + +Total : 755.50 : 614.25. + +Each of these nine crossed plants is higher than its opponent, though in +one case only by three-quarters of an inch. Their average height is +83.94 inches, and that of the self-fertilised plants 68.25, or as 100 to +81. These plants, after growing to their full height, became very +unhealthy and infested with aphides, just when the seeds were setting, +so that many of the capsules failed, and nothing can be said on their +relative fertility. + +CROSSED AND SELF-FERTILISED PLANTS OF THE EIGHTH GENERATION. + +As just stated, the plants of the last generation, from which the +present ones were raised, were very unhealthy and their seeds of +unusually small size; and this probably accounts for the two lots +behaving differently to what they did in any of the previous or +succeeding generations. Many of the self-fertilised seeds germinated +before the crossed ones, and these were of course rejected. When the +crossed seedlings in Table 2/9 had grown to a height of between 1 and 2 +feet, they were all, or almost all, shorter than their self-fertilised +opponents, but were not then measured. When they had acquired an average +height of 32.28 inches, that of the self-fertilised plants was 40.68, or +as 100 to 122. Moreover, every one of the self-fertilised plants, with a +single exception, exceeded its crossed opponent. When, however, the +crossed plants had grown to an average height of 77.56 inches, they just +exceeded (namely, by .7 of an inch) the average height of the +self-fertilised plants; but two of the latter were still taller than +their crossed opponents. I was so much astonished at this whole case, +that I tied string to the summits of the rods; the plants being thus +allowed to continue climbing upwards. When their growth was complete +they were untwined, stretched straight, and measured. The crossed plants +had now almost regained their accustomed superiority, as may be seen in +Table 2/9. + +The average height of the eight crossed plants is here 113.25 inches, +and that of the self-fertilised plants 96.65, or as 100 to 85. +Nevertheless two of the self-fertilised plants, as may be seen in Table +2/9, were still higher than their crossed opponents. The latter +manifestly had much thicker stems and many more lateral branches, and +looked altogether more vigorous than the self-fertilised plants, and +generally flowered before them. The earlier flowers produced by these +self-fertilised plants did not set any capsules, and their anthers +contained only a small amount of pollen; but to this subject I shall +return. Nevertheless capsules produced by two other self-fertilised +plants of the same lot, not included in Table 2/9, which had been highly +favoured by being grown in separate pots, contained the large average +number of 5.1 seeds per capsule. + +TABLE 2/9. Ipomoea purpurea (Eighth Generation). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 111 6/8 : 96. +Pot 1 : 127 : 54. +Pot 1 : 130 6/8 : 93 4/8. + +Pot 2 : 97 2/8 : 94. +Pot 2 : 89 4/8 : 125 6/8. + +Pot 3 : 103 6/8 : 115 4/8. +Pot 3 : 100 6/8 : 84 6/8. +Pot 3 : 147 4/8 : 109 6/8. + +Total : 908.25 : 773.25. + +CROSSED AND SELF-FERTILISED PLANTS OF THE NINTH GENERATION. + +The plants of this generation were raised in the same manner as before, +with the result shown in Table 2/10. + +The fourteen crossed plants average in height 81.39 inches and the +fourteen self-fertilised plants 64.07, or as 100 to 79. One +self-fertilised plant in Pot 3 exceeded, and one in Pot 4 equalled in +height, its opponent. The self-fertilised plants showed no sign of +inheriting the precocious growth of their parents; this having been due, +as it would appear, to the abnormal state of the seeds from the +unhealthiness of their parents. The fourteen self-fertilised plants +yielded only forty spontaneously self-fertilised capsules, to which must +be added seven, the product of ten flowers artificially self-fertilised. +On the other hand, the fourteen crossed plants yielded 152 spontaneously +self-fertilised capsules; but thirty-six flowers on these plants were +crossed (yielding thirty-three capsules), and these flowers would +probably have produced about thirty spontaneously self-fertilised +capsules. Therefore an equal number of the crossed and self-fertilised +plants would have produced capsules in the proportion of about 182 to +47, or as 100 to 26. Another phenomenon was well pronounced in this +generation, but I believe had occurred previously to a slight extent; +namely, that most of the flowers on the self-fertilised plants were +somewhat monstrous. The monstrosity consisted in the corolla being +irregularly split so that it did not open properly, with one or two of +the stamens slightly foliaceous, coloured, and firmly coherent to the +corolla. I observed this monstrosity in only one flower on the crossed +plants. The self-fertilised plants, if well nourished, would almost +certainly, in a few more generations, have produced double flowers, for +they had already become in some degree sterile. (2/1. See on this +subject 'Variation of Animals and Plants under Domestication' chapter 18 +2nd edition volume 2 page 152.) + +TABLE 2/10. Ipomoea purpurea (Ninth Generation). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 83 4/8 : 57. +Pot 1 : 85 4/8 : 71. +Pot 1 : 83 4/8 : 48 3/8. + +Pot 2 : 83 2/8 : 45. +Pot 2 : 64 2/8 : 43 6/8. +Pot 2 : 64 3/8 : 38 4/8. + +Pot 3 : 79 : 63. +Pot 3 : 88 1/8 : 71. +Pot 3 : 61 : 89 4/8. + +Pot 4 : 82 4/8 : 82 4/8. +Pot 4 : 90 : 76 1/8. + +Pot 5 : 89 4/8 : 67. +Pot 5 : 92 4/8 : 74 2/8. +Pot 5 : 92 4/8 : 70. +Crowded plants. + +Total : 1139.5 : 897.0. + +CROSSED AND SELF-FERTILISED PLANTS OF THE TENTH GENERATION. + +Six plants were raised in the usual manner from the crossed plants of +the last generation (Table 2/10) again intercrossed, and from the +self-fertilised again self-fertilised. As one of the crossed plants in +Pot 1 in Table 2/11 became much diseased, having crumpled leaves, and +producing hardly any capsules, it and its opponent have been struck out +of the table. + +TABLE 2/11. Ipomoea purpurea (Tenth Generation). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 92 3/8 : 47 2/8. +Pot 1 : 94 4/8 : 34 6/8. + +Pot 2 : 87 : 54 4/8. +Pot 2 : 89 5/8 : 49 2/8. +Pot 2 : 105 : 66 2/8. + +Total : 468.5 : 252.0. + +The five crossed plants average 93.7 inches, and the five +self-fertilised only 50.4, or as 100 to 54. This difference, however, is +so great that it must be looked at as in part accidental. The six +crossed plants (the diseased one here included) yielded spontaneously +101 capsules, and the six self-fertilised plants 88, the latter being +chiefly produced by one of the plants. But as the diseased plant, which +yielded hardly any seed, is here included, the ratio of 101 to 88 does +not fairly give the relative fertility of the two lots. The stems of the +six crossed plants looked so much finer than those of the six +self-fertilised plants, that after the capsules had been gathered and +most of the leaves had fallen off, they were weighed. Those of the +crossed plants weighed 2,693 grains, whilst those of the self-fertilised +plants weighed only 1,173 grains, or as 100 to 44; but as the diseased +and dwarfed crossed plant is here included, the superiority of the +former in weight was really greater.] + +THE EFFECTS ON THE OFFSPRING OF CROSSING DIFFERENT FLOWERS ON THE SAME +PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS. + +In all the foregoing experiments, seedlings from flowers crossed by +pollen from a distinct plant (though in the later generations more or +less closely related) were put into competition with, and almost +invariably proved markedly superior in height to the offspring from +self-fertilised flowers. I wished, therefore, to ascertain whether a +cross between two flowers on the same plant would give to the offspring +any superiority over the offspring from flowers fertilised with their +own pollen. I procured some fresh seed and raised two plants, which were +covered with a net; and several of their flowers were crossed with +pollen from a distinct flower on the same plant. Twenty-nine capsules +thus produced contained on an average 4.86 seeds per capsule; and 100 of +these seeds weighed 36.77 grains. Several other flowers were fertilised +with their own pollen, and twenty-six capsules thus produced contained +on an average 4.42 seeds per capsule; 100 of which weighed 42.61 grains. +So that a cross of this kind appears to have increased slightly the +number of seeds per capsule, in the ratio of 100 to 91; but these +crossed seeds were lighter than the self-fertilised in the ratio of 86 +to 100. I doubt, however, from other observations, whether these results +are fully trustworthy. The two lots of seeds, after germinating on sand, +were planted in pairs on the opposite sides of nine pots, and were +treated in every respect like the plants in the previous experiments. +The remaining seeds, some in a state of germination and some not so, +were sown on the opposite sides of a large pot (Number 10); and the four +tallest plants on each side of this pot were measured. The result is +shown in Table 2/12. + +TABLE 2/12. Ipomoea purpurea. + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 82 : 77 4/8. +Pot 1 : 75 : 87. +Pot 1 : 65 : 64. +Pot 1 : 76 : 87 2/8. + +Pot 2 : 78 4/8 : 84. +Pot 2 : 43 : 86 4/8. +Pot 2 : 65 4/8 : 90 4/8. + +Pot 3 : 61 2/8 : 86. +Pot 3 : 85 : 69 4/8. +Pot 3 : 89 : 87 4/8. + +Pot 4 : 83 : 80 4/8. +Pot 4 : 73 4/8 : 88 4/8. +Pot 4 : 67 : 84 4/8. + +Pot 5 : 78 : 66 4/8. +Pot 5 : 76 6/8 : 77 4/8. +Pot 5 : 57 : 81 4/8. + +Pot 6 : 70 4/8 : 80. +Pot 6 : 79 : 82 4/8. +Pot 6 : 79 6/8 : 55 4/8. + +Pot 7 : 76 : 77. +Pot 7 : 84 4/8 : 83 4/8. +Pot 7 : 79 : 73 4/8. + +Pot 8 : 73 : 76 4/8. +Pot 8 : 67 : 82. +Pot 8 : 83 : 80 4/8. + +Pot 9 : 73 2/8 : 78 4/8. +Pot 9 : 78 : 67 4/8. + +Pot 10 : 34 : 82 4/8. +Pot 10 : 82 : 36 6/8. +Pot 10 : 84 6/8 : 69 4/8. +Pot 10 : 71 : 75 2/8. +Crowded plants. + +Total : 2270.25 : 2399.75. + +The average height of the thirty-one crossed plants is 73.23 inches, and +that of the thirty-one self-fertilised plants 77.41 inches; or as 100 to +106. Looking to each pair, it may be seen that only thirteen of the +crossed plants, whilst eighteen of the self-fertilised plants exceed +their opponents. A record was kept with respect to the plant which +flowered first in each pot; and only two of the crossed flowered before +one of the self-fertilised in the same pot; whilst eight of the +self-fertilised flowered first. It thus appears that the crossed plants +are slightly inferior in height and in earliness of flowering to the +self-fertilised. But the inferiority in height is so small, namely as +100 to 106, that I should have felt very doubtful on this head, had I +not cut down all the plants (except those in the crowded pot Number 10) +close to the ground and weighed them. The twenty-seven crossed plants +weighed 16 1/2 ounces, and the twenty-seven self-fertilised plants 20 +1/2 ounces; and this gives a ratio of 100 to 124. + +A self-fertilised plant of the same parentage as those in Table 2/12 had +been raised in a separate pot for a distinct purpose; and it proved +partially sterile, the anthers containing very little pollen. Several +flowers on this plant were crossed with the little pollen which could be +obtained from the other flowers on the same plant; and other flowers +were self-fertilised. From the seeds thus produced four crossed and four +self-fertilised plants were raised, which were planted in the usual +manner on the opposite sides of two pots. All these four crossed plants +were inferior in height to their opponents; they averaged 78.18 inches, +whilst the four self-fertilised plants averaged 84.8 inches; or as 100 +to 108. (2/2. From one of these self-fertilised plants, spontaneously +self-fertilised, I gathered twenty-four capsules, and they contained on +an average only 3.2 seeds per capsule; so that this plant had apparently +inherited some of the sterility of its parent.) This case, therefore, +confirms the last. Taking all the evidence together, we must conclude +that these strictly self-fertilised plants grew a little taller, were +heavier, and generally flowered before those derived from a cross +between two flowers on the same plant. These latter plants thus present +a wonderful contrast with those derived from a cross between two +distinct individuals. + +THE EFFECTS ON THE OFFSPRING OF A CROSS WITH A DISTINCT OR FRESH STOCK +BELONGING TO THE SAME VARIETY. + +From the two foregoing series of experiments we see, firstly, the good +effects during several successive generations of a cross between +distinct plants, although these were in some degree inter-related and +had been grown under nearly the same conditions; and, secondly, the +absence of all such good effects from a cross between flowers on the +same plant; the comparison in both cases being made with the offspring +of flowers fertilised with their own pollen. The experiments now to be +given show how powerfully and beneficially plants, which have been +intercrossed during many successive generations, having been kept all +the time under nearly uniform conditions, are affected by a cross with +another plant belonging to the same variety, but to a distinct family or +stock, which had grown under different conditions. + +[Several flowers on the crossed plants of the ninth generation in Table +2/10, were crossed with pollen from another crossed plant of the same +lot. The seedlings thus raised formed the tenth intercrossed generation, +and I will call them the "INTERCROSSED PLANTS." Several other flowers on +the same crossed plants of the ninth generation were fertilised (not +having been castrated) with pollen taken from plants of the same +variety, but belonging to a distinct family, which had been grown in a +distant garden at Colchester, and therefore under somewhat different +conditions. The capsules produced by this cross contained, to my +surprise, fewer and lighter seeds than did the capsules of the +intercrossed plants; but this, I think, must have been accidental. The +seedlings raised from them I will call the "COLCHESTER-CROSSED." The two +lots of seeds, after germinating on sand, were planted in the usual +manner on the opposite sides of five pots, and the remaining seeds, +whether or not in a state of germination, were thickly sown on the +opposite sides of a very large pot, Number 6 in Table 2/13. In three of +the six pots, after the young plants had twined a short way up their +sticks, one of the Colchester-crossed plants was much taller than any +one of the intercrossed plants on the opposite side of the same pot; and +in the three other pots somewhat taller. I should state that two of the +Colchester-crossed plants in Pot 4, when about two-thirds grown, became +much diseased, and were, together with their intercrossed opponents, +rejected. The remaining nineteen plants, when almost fully grown, were +measured, with the following result: + +TABLE 2/13. Ipomoea purpurea. + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Colchester-Crossed Plants. + +Column 3: Intercrossed Plants of the Tenth Generation. + +Pot 1 : 87 : 78. +Pot 1 : 87 4/8 : 68 4/8. +Pot 1 : 85 1/8 : 94 4/8. + +Pot 2 : 93 6/8 : 60. +Pot 2 : 85 4/8 : 87 2/8. +Pot 2 : 90 5/8 : 45 4/8. + +Pot 3 : 84 2/8 : 70 1/8. +Pot 3 : 92 4/8 : 81 6/8. +Pot 3 : 85 : 86 2/8. + +Pot 4 : 95 6/8 : 65 1/8. + +Pot 5 : 90 4/8 : 85 6/8. +Pot 5 : 86 6/8 : 63. +Pot 5 : 84 : 62 6/8. + +Pot 6 : 90 4/8 : 43 4/8. +Pot 6 : 75 : 39 6/8. +Pot 6 : 71 : 30 2/8. +Pot 6 : 83 6/8 : 86. +Pot 6 : 63 : 53. +Pot 6 : 65 : 48 6/8. +Crowded plants in a very large pot. + +Total : 1596.50 : 1249.75. + +In sixteen out of these nineteen pairs, the Colchester-crossed plant +exceeded in height its intercrossed opponent. The average height of the +Colchester-crossed is 84.03 inches, and that of the intercrossed 65.78 +inches; or as 100 to 78. With respect to the fertility of the two lots, +it was too troublesome to collect and count the capsules on all the +plants; so I selected two of the best pots, 5 and 6, and in these the +Colchester-crossed produced 269 mature and half-mature capsules, whilst +an equal number of the intercrossed plants produced only 154 capsules; +or as 100 to 57. By weight the capsules from the Colchester-crossed +plants were to those from the intercrossed plants as 100 to 51; so that +the former probably contained a somewhat larger average number of +seeds.] + +We learn from this important experiment that plants in some degree +related, which had been intercrossed during the nine previous +generations, when they were fertilised with pollen from a fresh stock, +yielded seedlings as superior to the seedlings of the tenth intercrossed +generation, as these latter were to the self-fertilised plants of the +corresponding generation. For if we look to the plants of the ninth +generation in Table 2/10 (and these offer in most respects the fairest +standard of comparison) we find that the intercrossed plants were in +height to the self-fertilised as 100 to 79, and in fertility as 100 to +26; whilst the Colchester-crossed plants are in height to the +intercrossed as 100 to 78, and in fertility as 100 to 51. + +[THE DESCENDANTS OF THE SELF-FERTILISED PLANT, NAMED HERO, WHICH +APPEARED IN THE SIXTH SELF-FERTILISED GENERATION. + +In the five generations before the sixth, the crossed plant of each pair +was taller than its self-fertilised opponent; but in the sixth +generation (Table 2/7, Pot 2) the Hero appeared, which after a long and +dubious struggle conquered its crossed opponent, though by only half an +inch. I was so much surprised at this fact, that I resolved to ascertain +whether this plant would transmit its powers of growth to its seedlings. +Several flowers on Hero were therefore fertilised with their own pollen, +and the seedlings thus raised were put into competition with +self-fertilised and intercrossed plants of the corresponding generation. +The three lots of seedlings thus all belong to the seventh generation. +Their relative heights are shown in Tables 2/14 and 2/15. + +TABLE 2/14. Ipomoea purpurea. + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Self-fertilised Plants of the Seventh Generation, Children of +Hero. + +Column 3: Self-fertilised Plants of the Seventh Generation. + +Pot 1 : 74 : 89 4/8. +Pot 1 : 60 : 61. +Pot 1 : 55 2/8 : 49. + +Pot 2 : 92 : 82. +Pot 2 : 91 6/8 : 56. +Pot 2 : 74 2/8 : 38. + +Total : 447.25 : 375.50. + +The average height of the six self-fertilised children of Hero is 74.54 +inches, whilst that of the ordinary self-fertilised plants of the +corresponding generation is only 62.58 inches, or as 100 to 84. + +TABLE 2/15. Ipomoea purpurea. + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Self-fertilised Plants of the Seventh Generation, Children of +Hero. + +Column 3: Intercrossed Plants of the Seventh Generation. + +Pot 3 : 92 : 76 6/8. + +Pot 4 : 87 : 89. +Pot 4 : 87 6/8 : 86 6/8. + +Total : 266.75 : 252.50. + +Here the average height of the three self-fertilised children of Hero is +88.91 inches, whilst that of the intercrossed plants is 84.16; or as 100 +to 95. We thus see that the self-fertilised children of Hero certainly +inherit the powers of growth of their parents; for they greatly exceed +in height the self-fertilised offspring of the other self-fertilised +plants, and even exceed by a trifle the intercrossed plants,--all of the +corresponding generation. + +Several flowers on the self-fertilised children of Hero in Table 2/14 +were fertilised with pollen from the same flower; and from the seeds +thus produced, self-fertilised plants of the eighth generation +(grandchildren of Hero) were raised. Several other flowers on the same +plants were crossed with pollen from the other children of Hero. The +seedlings raised from this cross may be considered as the offspring of +the union of brothers and sisters. The result of the competition between +these two sets of seedlings (namely self-fertilised and the offspring of +brothers and sisters) is given in Table 2/16. + +TABLE 2/16. Ipomoea purpurea. + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Self-fertilised Grandchildren of Hero, from the +Self-fertilised Children. Eighth Generation. + +Column 3: Grandchildren from a cross between the self-fertilised +children of Hero. Eighth Generation. + +Pot 1 : 86 6/8 : 95 6/8. +Pot 1 : 90 3/8 : 95 3/8. + +Pot 2 : 96 : 85. +Pot 2 : 77 2/8 : 93. + +Pot 3 : 73 : 86 2/8. +Pot 3 : 66 : 82 2/8. +Pot 3 : 84 4/8 : 70 6/8. + +Pot 4 : 88 1/8 : 66 3/8. +Pot 4 : 84 : 15 4/8. +Pot 4 : 36 2/8 : 38. +Pot 4 : 74 : 78 3/8. + +Pot 5 : 90 1/8 : 82 6/8. +Pot 5 : 90 5/8 : 83 6/8. + +Total : 1037.00 : 973.16. + +The average height of the thirteen self-fertilised grandchildren of Hero +is 79.76 inches, and that of the grandchildren from a cross between the +self-fertilised children is 74.85; or as 100 to 94. But in Pot 4 one of +the crossed plants grew only to a height of 15 1/2 inches; and if this +plant and its opponent are struck out, as would be the fairest plan, the +average height of the crossed plants exceeds only by a fraction of an +inch that of the self-fertilised plants. It is therefore clear that a +cross between the self-fertilised children of Hero did not produce any +beneficial effect worth notice; and it is very doubtful whether this +negative result can be attributed merely to the fact of brothers and +sisters having been united, for the ordinary intercrossed plants of the +several successive generations must often have been derived from the +union of brothers and sisters (as shown in Chapter 1), and yet all of +them were greatly superior to the self-fertilised plants. We are +therefore driven to the suspicion, which we shall soon see strengthened, +that Hero transmitted to its offspring a peculiar constitution adapted +for self-fertilisation. + +It would appear that the self-fertilised descendants of Hero have not +only inherited from Hero a power of growth equal to that of the ordinary +intercrossed plants, but have become more fertile when self-fertilised +than is usual with the plants of the present species. The flowers on the +self-fertilised grandchildren of Hero in Table 2.16 (the eighth +generation of self-fertilised plants) were fertilised with their own +pollen and produced plenty of capsules, ten of which (though this is too +few a number for a safe average) contained 5.2 seeds per capsule,--a +higher average than was observed in any other case with the +self-fertilised plants. The anthers produced by these self-fertilised +grandchildren were also as well developed and contained as much pollen +as those on the intercrossed plants of the corresponding generation; +whereas this was not the case with the ordinary self-fertilised plants +of the later generations. Nevertheless some few of the flowers produced +by the grandchildren of Hero were slightly monstrous, like those of the +ordinary self-fertilised plants of the later generations. In order not +to recur to the subject of fertility, I may add that twenty-one +self-fertilised capsules, spontaneously produced by the +great-grandchildren of Hero (forming the ninth generation of +self-fertilised plants), contained on an average 4.47 seeds; and this is +as high an average as the self-fertilised flowers of any generation +usually yielded. + +Several flowers on the self-fertilised grandchildren of Hero in Table +2/16 were fertilised with pollen from the same flower; and the seedlings +raised from them (great-grandchildren of Hero) formed the ninth +self-fertilised generation. Several other flowers were crossed with +pollen from another grandchild, so that they may be considered as the +offspring of brothers and sisters, and the seedlings thus raised may be +called the INTERCROSSED great-grandchildren. And lastly, other flowers +were fertilised with pollen from a distinct stock, and the seedlings +thus raised may be called the COLCHESTER-CROSSED great-grandchildren. In +my anxiety to see what the result would be, I unfortunately planted the +three lots of seeds (after they had germinated on sand) in the hothouse +in the middle of winter, and in consequence of this the seedlings +(twenty in number of each kind) became very unhealthy, some growing only +a few inches in height, and very few to their proper height. The result, +therefore, cannot be fully trusted; and it would be useless to give the +measurements in detail. In order to strike as fair an average as +possible, I first excluded all the plants under 50 inches in height, +thus rejecting all the most unhealthy plants. The six self-fertilised +thus left were on an average 66.86 inches high; the eight intercrossed +plants 63.2 high; and the seven Colchester-crossed 65.37 high; so that +there was not much difference between the three sets, the +self-fertilised plants having a slight advantage. Nor was there any +great difference when only the plants under 36 inches in height were +excluded. Nor again when all the plants, however much dwarfed and +unhealthy, were included. In this latter case the Colchester-crossed +gave the lowest average of all; and if these plants had been in any +marked manner superior to the other two lots, as from my former +experience I fully expected they would have been, I cannot but think +that some vestige of such superiority would have been evident, +notwithstanding the very unhealthy condition of most of the plants. No +advantage, as far as we can judge, was derived from intercrossing two of +the grandchildren of Hero, any more than when two of the children were +crossed. It appears therefore that Hero and its descendants have varied +from the common type, not only in acquiring great power of growth, and +increased fertility when subjected to self-fertilisation, but in not +profiting from a cross with a distinct stock; and this latter fact, if +trustworthy, is a unique case, as far as I have observed in all my +experiments.] + +SUMMARY ON THE GROWTH, VIGOUR, AND FERTILITY OF THE SUCCESSIVE +GENERATIONS OF THE CROSSED AND SELF-FERTILISED PLANTS OF Ipomoea +purpurea, TOGETHER WITH SOME MISCELLANEOUS OBSERVATIONS. + +In Table 2/17, we see the average or mean heights of the ten successive +generations of the intercrossed and self-fertilised plants, grown in +competition with each other; and in the right hand column we have the +ratios of the one to the other, the height of the intercrossed plants +being taken at 100. In the bottom line the mean height of the +seventy-three intercrossed plants is shown to be 85.84 inches, and that +of the seventy-three self-fertilised plants 66.02 inches, or as 100 to +77. + +TABLE 2/17. Ipomoea purpurea. Summary of measurements of the ten +generations. + +Heights of Plants in inches: + +Column 1: Name of Generation. + +Column 2: Number of Crossed Plants. + +Column 3: Average Height of Crossed Plants. + +Column 4: Number of Self-fertilised Plants. + +Column 5: Average Height of Self-fertilised Plants. + +Column 6: n in Ratio between Average Heights of Crossed and +Self-fertilised Plants, expressed as 100 to n. + +First generation Table 2/1 : 6 : 86.00 : 6 : 65.66 : 76. + +Second generation Table 2/2 : 6 : 84.16 : 6 : 66.33 : 79. + +Third generation Table 2/3 : 6 : 77.41 : 6 : 52.83 : 68. + +Fourth generation Table 2/5 : 7 : 69.78 : 7 : 60.14 : 86. + +Fifth generation Table 2/6 : 6 : 82.54 : 6 : 62.33 : 75. + +Sixth generation Table 2/7 : 6 : 87.50 : 6 : 63.16 : 72. + +Seventh generation Table 2/8 : 9 : 83.94 : 9 : 68.25 : 81. + +Eighth generation Table 2/9 : 8 : 113.25 : 8 : 96.65 : 85. + +Ninth generation Table 2/10 : 14 : 81.39 : 14 : 64.07 : 79. + +Tenth generation Table 2/11 : 5 : 93.70 : 5 : 50.40 : 54. + +All ten generations together : 73 : 85.84 : 73 : 66.02 : 77. + +(DIAGRAM 2/1. Diagram showing the mean heights of the crossed and +self-fertilised plants of Ipomoea purpurea in the ten generations; the +mean height of the crossed plants being taken as 100. On the right hand, +the mean heights of the crossed and self-fertilised plants of all the +generations taken together are shown (as eleven pairs of unequal +vertical lines.)) + +The mean height of the self-fertilised plants in each of the ten +generations is also shown in the diagram 2/1, that of the intercrossed +plants being taken at 100, and on the right side we see the relative +heights of the seventy-three intercrossed plants, and of the +seventy-three self-fertilised plants. The difference in height between +the crossed and self-fertilised plants will perhaps be best appreciated +by an illustration: If all the men in a country were on an average 6 +feet high, and there were some families which had been long and closely +interbred, these would be almost dwarfs, their average height during ten +generations being only 4 feet 8 1/4 inches. + +It should be especially observed that the average difference between the +crossed and self-fertilised plants is not due to a few of the former +having grown to an extraordinary height, or to a few of the +self-fertilised being extremely short, but to all the crossed plants +having surpassed their self-fertilised opponents, with the few following +exceptions. The first occurred in the sixth generation, in which the +plant named "Hero" appeared; two in the eighth generation, but the +self-fertilised plants in this generation were in an anomalous +condition, as they grew at first at an unusual rate and conquered for a +time the opposed crossed plants; and two exceptions in the ninth +generation, though one of these plants only equalled its crossed +opponent. Therefore, of the seventy-three crossed plants, sixty-eight +grew to a greater height than the self-fertilised plants, to which they +were opposed. + +In the right-hand column of figures, the difference in height between +the crossed and self-fertilised plants in the successive generations is +seen to fluctuate much, as might indeed have been expected from the +small number of plants measured in each generation being insufficient to +give a fair average. It should be remembered that the absolute height of +the plants goes for nothing, as each pair was measured as soon as one of +them had twined up to the summit of its rod. The great difference in the +tenth generation, namely, 100 to 54, no doubt was partly accidental, +though, when these plants were weighed, the difference was even greater, +namely, 100 to 44. The smallest amount of difference occurred in the +fourth and the eighth generations, and this was apparently due to both +the crossed and self-fertilised plants having become unhealthy, which +prevented the former attaining their usual degree of superiority. This +was an unfortunate circumstance, but my experiments were not thus +vitiated, as both lots of plants were exposed to the same conditions, +whether favourable or unfavourable. + +There is reason to believe that the flowers of this Ipomoea, when +growing out of doors, are habitually crossed by insects, so that the +first seedlings which I raised from purchased seeds were probably the +offspring of a cross. I infer that this is the case, firstly from +humble-bees often visiting the flowers, and from the quantity of pollen +left by them on the stigmas of such flowers; and, secondly, from the +plants raised from the same lot of seed varying greatly in the colour of +their flowers, for as we shall hereafter see, this indicates much +intercrossing. (2/3. Verlot says 'Sur la Production des Variétés' 1865 +page 66, that certain varieties of a closely allied plant, the +Convolvulus tricolor, cannot be kept pure unless grown at a distance +from all other varieties.) It is, therefore, remarkable that the plants +raised by me from flowers which were, in all probability, +self-fertilised for the first time after many generations of crossing, +should have been so markedly inferior in height to the intercrossed +plants as they were, namely, as 76 to 100. As the plants which were +self-fertilised in each succeeding generation necessarily became much +more closely interbred in the later than in the earlier generations, it +might have been expected that the difference in height between them and +the crossed plants would have gone on increasing; but, so far is this +from being the case, that the difference between the two sets of plants +in the seventh, eighth, and ninth generations taken together is less +than in the first and second generations together. When, however, we +remember that the self-fertilised and crossed plants are all descended +from the same mother-plant, that many of the crossed plants in each +generation were related, often closely related, and that all were +exposed to the same conditions, which, as we shall hereafter find, is a +very important circumstance, it is not at all surprising that the +difference between them should have somewhat decreased in the later +generations. It is, on the contrary, an astonishing fact, that the +crossed plants should have been victorious, even to a slight degree, +over the self-fertilised plants of the later generations. + +The much greater constitutional vigour of the crossed than of the +self-fertilised plants, was proved on five occasions in various ways; +namely, by exposing them, while young, to a low temperature or to a +sudden change of temperature, or by growing them, under very +unfavourable conditions, in competition with full-grown plants of other +kinds. + +With respect to the productiveness of the crossed and self-fertilised +plants of the successive generations, my observations unfortunately were +not made on any uniform plan, partly from the want of time, and partly +from not having at first intended to observe more than a single +generation. A summary of the results is here given in a tabulated form, +the fertility of the crossed plants being taken as 100. + +TABLE 2/18. Ratio of productiveness of crossed and self-fertilised +plants. Ipomoea purpurea. + +FIRST GENERATION OF CROSSED AND SELF-FERTILISED PLANTS GROWING IN +COMPETITION WITH ONE ANOTHER. + +Sixty-five capsules produced from flowers on five crossed plants +fertilised by pollen from a distinct plant, and fifty-five capsules +produced from flowers on five self-fertilised plants fertilised by their +own pollen, contained seeds in the proportion of : 100 to 93. + +Fifty-six spontaneously self-fertilised capsules on the above five +crossed plants, and twenty-five spontaneously self-fertilised capsules +on the above five self-fertilised plants, yielded seeds in the +proportion of : 100 to 99. + +Combining the total number of capsules produced by these plants, and the +average number of seeds in each, the above crossed and self-fertilised +plants yielded seeds in the proportion of : 100 to 64. + +Other plants of this first generation grown under unfavourable +conditions and spontaneously self-fertilised, yielded seeds in the +proportion of : 100 to 45. + +THIRD GENERATION OF CROSSED AND SELF-FERTILISED PLANTS. + +Crossed capsules compared with self-fertilised capsules contained seeds +in the ratio of : 100 to 94. + +An equal number of crossed and self-fertilised plants, both +spontaneously self-fertilised, produced capsules in the ratio of : 100 +to 38. + +And these capsules contained seeds in the ratio of : 100 to 94. + +Combining these data, the productiveness of the crossed to the +self-fertilised plants, both spontaneously self-fertilised, was as : 100 +to 35. + +FOURTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS. + +Capsules from flowers on the crossed plants fertilised by pollen from +another plant, and capsules from flowers on the self-fertilised plants +fertilised with their own pollen, contained seeds in the proportion of : +100 to 94. + +FIFTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS. + +The crossed plants produced spontaneously a vast number more pods (not +actually counted) than the self-fertilised, and these contained seeds in +the proportion of : 100 to 89. + +NINTH GENERATION OF CROSSED AND SELF-FERTILISED PLANTS. + +Fourteen crossed plants, spontaneously self-fertilised, and fourteen +self-fertilised plants spontaneously self-fertilised, yielded capsules +(the average number of seeds per capsule not having been ascertained) in +the proportion of : 100 to 26. + +PLANTS DERIVED FROM A CROSSED WITH A FRESH STOCK COMPARED WITH +INTERCROSSED PLANTS. + +The offspring of intercrossed plants of the ninth generation, crossed by +a fresh stock, compared with plants of the same stock intercrossed +during ten generations, both sets of plants left uncovered and naturally +fertilised, produced capsules by weight as : 100 to 51. + +We see in this table that the crossed plants are always in some degree +more productive than the self-fertilised plants, by whatever standard +they are compared. The degree differs greatly; but this depends chiefly +on whether an average was taken of the seeds alone, or of the capsules +alone, or of both combined. The relative superiority of the crossed +plants is chiefly due to their producing a much greater number of +capsules, and not to each capsule containing a larger average number of +seeds. For instance, in the third generation the crossed and +self-fertilised plants produced capsules in the ratio of 100 to 38, +whilst the seeds in the capsules on the crossed plants were to those on +the self-fertilised plants only as 100 to 94. In the eighth generation +the capsules on two self-fertilised plants (not included in table 2/18), +grown in separate pots and thus not subjected to any competition, +yielded the large average of 5.1 seeds. The smaller number of capsules +produced by the self-fertilised plants may be in part, but not +altogether, attributed to their lessened size or height; this being +chiefly due to their lessened constitutional vigour, so that they were +not able to compete with the crossed plants growing in the same pots. +The seeds produced by the crossed flowers on the crossed plants were not +always heavier than the self-fertilised seeds on the self-fertilised +plants. The lighter seeds, whether produced from crossed or +self-fertilised flowers, generally germinated before the heavier seeds. +I may add that the crossed plants, with very few exceptions, flowered +before their self-fertilised opponents, as might have been expected from +their greater height and vigour. + +The impaired fertility of the self-fertilised plants was shown in +another way, namely, by their anthers being smaller than those in the +flowers on the crossed plants. This was first observed in the seventh +generation, but may have occurred earlier. Several anthers from flowers +on the crossed and self-fertilised plants of the eighth generation were +compared under the microscope; and those from the former were generally +longer and plainly broader than the anthers of the self-fertilised +plants. The quantity of pollen contained in one of the latter was, as +far as could be judged by the eye, about half of that contained in one +from a crossed plant. The impaired fertility of the self-fertilised +plants of the eighth generation was also shown in another manner, which +may often be observed in hybrids--namely, by the first-formed flowers +being sterile. For instance, the fifteen first flowers on a +self-fertilised plant of one of the later generations were carefully +fertilised with their own pollen, and eight of them dropped off; at the +same time fifteen flowers on a crossed plant growing in the same pot +were self-fertilised, and only one dropped off. On two other crossed +plants of the same generation, several of the earliest flowers were +observed to fertilise themselves and to produce capsules. In the plants +of the ninth, and I believe of some previous generations, very many of +the flowers, as already stated, were slightly monstrous; and this +probably was connected with their lessened fertility. + +All the self-fertilised plants of the seventh generation, and I believe +of one or two previous generations, produced flowers of exactly the same +tint, namely, of a rich dark purple. So did all the plants, without any +exception, in the three succeeding generations of self-fertilised +plants; and very many were raised on account of other experiments in +progress not here recorded. My attention was first called to this fact +by my gardener remarking that there was no occasion to label the +self-fertilised plants, as they could always be known by their colour. +The flowers were as uniform in tint as those of a wild species growing +in a state of nature; whether the same tint occurred, as is probable, in +the earlier generations, neither my gardener nor self could recollect. +The flowers on the plants which were first raised from purchased seed, +as well as during the first few generations, varied much in the depth of +the purple tint; many were more or less pink, and occasionally a white +variety appeared. The crossed plants continued to the tenth generation +to vary in the same manner as before, but to a much less degree, owing, +probably, to their having become more or less closely inter-related. We +must therefore attribute the extraordinary uniformity of colour in the +flowers on the plants of the seventh and succeeding self-fertilised +generations, to inheritance not having been interfered with by crosses +during several preceding generations, in combination with the conditions +of life having been very uniform. + +A plant appeared in the sixth self-fertilised generation, named the +Hero, which exceeded by a little in height its crossed antagonist, and +which transmitted its powers of growth and increased self-fertility to +its children and grandchildren. A cross between the children of Hero did +not give to the grandchildren any advantage over the self-fertilised +grandchildren raised from the self-fertilised children. And as far as my +observations can be trusted, which were made on very unhealthy plants, +the great-grandchildren raised from intercrossing the grandchildren had +no advantage over the seedlings from the grandchildren the product of +continued self-fertilisation; and what is far more remarkable, the +great-grandchildren raised by crossing the grandchildren with a fresh +stock, had no advantage over either the intercrossed or self-fertilised +great-grandchildren. It thus appears that Hero and its descendants +differed in constitution in an extraordinary manner from ordinary plants +of the present species. + +Although the plants raised during ten successive generations from +crosses between distinct yet inter-related plants almost invariably +exceeded in height, constitutional vigour, and fertility their +self-fertilised opponents, it has been proved that seedlings raised by +intercrossing flowers on the same plant are by no means superior, on the +contrary are somewhat inferior in height and weight, to seedlings raised +from flowers fertilised with their own pollen. This is a remarkable +fact, which seems to indicate that self-fertilisation is in some manner +more advantageous than crossing, unless the cross brings with it, as is +generally the case, some decided and preponderant advantage; but to this +subject I shall recur in a future chapter. + +The benefits which so generally follow from a cross between two plants +apparently depend on the two differing somewhat in constitution or +character. This is shown by the seedlings from the intercrossed plants +of the ninth generation, when crossed with pollen from a fresh stock, +being as superior in height and almost as superior in fertility to the +again intercrossed plants, as these latter were to seedlings from +self-fertilised plants of the corresponding generation. We thus learn +the important fact that the mere act of crossing two distinct plants, +which are in some degree inter-related and which have been long +subjected to nearly the same conditions, does little good as compared +with that from a cross between plants belonging to different stocks or +families, and which have been subjected to somewhat different +conditions. We may attribute the good derived from the crossing of the +intercrossed plants during the ten successive generations to their still +differing somewhat in constitution or character, as was indeed proved by +their flowers still differing somewhat in colour. But the several +conclusions which may be deduced from the experiments on Ipomoea will be +more fully considered in the final chapters, after all my other +observations have been given. + + + +CHAPTER III. + +SCROPHULARIACEAE, GESNERIACEAE, LABIATAE, ETC. + +Mimulus luteus; height, vigour, and fertility of the crossed and +self-fertilised plants of the first four generations. +Appearance of a new, tall, and highly self-fertile variety. +Offspring from a cross between self-fertilised plants. +Effects of a cross with a fresh stock. +Effects of crossing flowers on the same plant. +Summary on Mimulus luteus. +Digitalis purpurea, superiority of the crossed plants. +Effects of crossing flowers on the same plant. +Calceolaria. +Linaria vulgaris. +Verbascum thapsus. +Vandellia nummularifolia. +Cleistogene flowers. +Gesneria pendulina. +Salvia coccinea. +Origanum vulgare, great increase of the crossed plants by stolons. +Thunbergia alata. + +In the family of the Scrophulariaceae I experimented on species in the +six following genera: Mimulus, Digitalis, Calceolaria, Linaria, +Verbascum, and Vandellia. + +[3/2. SCROPHULARIACEAE.--Mimulus luteus. + +The plants which I raised from purchased seed varied greatly in the +colour of their flowers, so that hardly two individuals were quite +alike; the corolla being of all shades of yellow, with the most +diversified blotches of purple, crimson, orange, and coppery brown. But +these plants differed in no other respect. (3/1. I sent several +specimens with variously coloured flowers to Kew, and Dr. Hooker informs +me that they all consisted of Mimulus luteus. The flowers with much red +have been named by horticulturists as var. Youngiana.) The flowers are +evidently well adapted for fertilisation by the agency of insects; and +in the case of a closely allied species, Mimulus rosea, I have watched +bees entering the flowers, thus getting their backs well dusted with +pollen; and when they entered another flower the pollen was licked off +their backs by the two-lipped stigma, the lips of which are irritable +and close like a forceps on the pollen-grains. If no pollen is enclosed +between the lips, these open again after a time. Mr. Kitchener has +ingeniously explained the use of these movements, namely, to prevent the +self-fertilisation of the flower. (3/2. 'A Year's Botany' 1874 page +118.) If a bee with no pollen on its back enters a flower it touches the +stigma, which quickly closes, and when the bee retires dusted with +pollen, it can leave none on the stigma of the same flower. But as soon +as it enters any other flower, plenty of pollen is left on the stigma, +which will be thus cross-fertilised. Nevertheless, if insects are +excluded, the flowers fertilise themselves perfectly and produce plenty +of seed; but I did not ascertain whether this is effected by the stamens +increasing in length with advancing age, or by the bending down of the +pistil. The chief interest in my experiments on the present species, +lies in the appearance in the fourth self-fertilised generation of a +variety which bore large peculiarly-coloured flowers, and grew to a +greater height than the other varieties; it likewise became more highly +self-fertile, so that this variety resembles the plant named Hero, which +appeared in the sixth self-fertilised generation of Ipomoea. + +Some flowers on one of the plants raised from the purchased seeds were +fertilised with their own pollen; and others on the same plant were +crossed with pollen from a distinct plant. The seeds from twelve +capsules thus produced were placed in separate watch-glasses for +comparison; and those from the six crossed capsules appeared to the eye +hardly more numerous than those from the six self-fertilised capsules. +But when the seeds were weighed, those from the crossed capsules +amounted to 1.02 grain, whilst those from the self-fertilised capsules +were only .81 grain; so that the former were either heavier or more +numerous than the latter, in the ratio of 100 to 79. + +CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST GENERATION. + +Having ascertained, by leaving crossed and self-fertilised seed on damp +sand, that they germinated simultaneously, both kinds were thickly sown +on opposite sides of a broad and rather shallow pan; so that the two +sets of seedlings, which came up at the same time, were subjected to the +same unfavourable conditions. This was a bad method of treatment, but +this species was one of the first on which I experimented. When the +crossed seedlings were on an average half an inch high, the +self-fertilised ones were only a quarter of an inch high. When grown to +their full height under the above unfavourable conditions, the four +tallest crossed plants averaged 7.62, and the four tallest +self-fertilised 5.87 inches in height; or as 100 to 77. Ten flowers on +the crossed plants were fully expanded before one on the self-fertilised +plants. A few of these plants of both lots were transplanted into a +large pot with plenty of good earth, and the self-fertilised plants, not +now being subjected to severe competition, grew during the following +year as tall as the crossed plants; but from a case which follows it is +doubtful whether they would have long continued equal. Some flowers on +the crossed plants were crossed with pollen from another plant, and the +capsules thus produced contained a rather greater weight of seed than +those on the self-fertilised plants again self-fertilised. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION. + +Seeds from the foregoing plants, fertilised in the manner just stated, +were sown on the opposite sides of a small pot (1) and came up crowded. +The four tallest crossed seedlings, at the time of flowering, averaged 8 +inches in height, whilst the four tallest self-fertilised plants +averaged only 4 inches. Crossed seeds were sown by themselves in a +second small pot, and self-fertilised seeds were sown by themselves in a +third small pot so that there was no competition whatever between these +two lots. Nevertheless the crossed plants grew from 1 to 2 inches higher +on an average than the self-fertilised. Both lots looked equally +vigorous, but the crossed plants flowered earlier and more profusely +than the self-fertilised. In Pot 1, in which the two lots competed with +each other, the crossed plants flowered first and produced a large +number of capsules, whilst the self-fertilised produced only nineteen. +The contents of twelve capsules from the crossed flowers on the crossed +plants, and of twelve capsules from self-fertilised flowers on the +self-fertilised plants, were placed in separate watch-glasses for +comparison; and the crossed seeds seemed more numerous by half than the +self-fertilised. + +The plants on both sides of Pot 1, after they had seeded, were cut down +and transplanted into a large pot with plenty of good earth, and on the +following spring, when they had grown to a height of between 5 and 6 +inches, the two lots were equal, as occurred in a similar experiment in +the last generation. But after some weeks the crossed plants exceeded +the self-fertilised ones on the opposite side of the same pot, though +not nearly to so great a degree as before, when they were subjected to +very severe competition. + +CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION. + +Crossed seeds from the crossed plants, and self-fertilised seeds from +the self-fertilised plants of the last generation, were sown thickly on +opposite sides of a small pot, Number 1. The two tallest plants on each +side were measured after they had flowered, and the two crossed ones +were 12 and 7 1/2 inches, and the two self-fertilised ones 8 and 5 1/2 +inches in height; that is, in the ratio of 100 to 69. Twenty flowers on +the crossed plants were again crossed and produced twenty capsules; ten +of which contained 1.33 grain weight of seeds. Thirty flowers on the +self-fertilised plants were again self-fertilised and produced +twenty-six capsules; ten of the best of which (many being very poor) +contained only .87 grain weight of seeds; that is, in the ratio of 100 +to 65 by weight. + +The superiority of the crossed over the self-fertilised plants was +proved in various ways. Self-fertilised seeds were sown on one side of a +pot, and two days afterwards crossed seeds on the opposite side. The two +lots of seedlings were equal until they were above half an inch high; +but when fully grown the two tallest crossed plants attained a height of +12 1/2 and 8 3/4 inches, whilst the two tallest self-fertilised plants +were only 8 and 5 1/2 inches high. + +In a third pot, crossed seeds were sown four days after the +self-fertilised, and the seedlings from the latter had at first, as +might have been expected, an advantage; but when the two lots were +between 5 and 6 inches in height, they were equal, and ultimately the +three tallest crossed plants were 11, 10, and 8 inches, whilst the three +tallest self-fertilised were 12, 8 1/2, and 7 1/2 inches in height. So +that there was not much difference between them, the crossed plants +having an average advantage of only the third of an inch. The plants +were cut down, and without being disturbed were transplanted into a +larger pot. Thus the two lots started fair on the following spring, and +now the crossed plants showed their inherent superiority, for the two +tallest were 13 inches, whilst the two tallest self-fertilised plants +were only 11 and 8 1/2 inches in height; or as 100 to 75. The two lots +were allowed to fertilise themselves spontaneously: the crossed plants +produced a large number of capsules, whilst the self-fertilised produced +very few and poor ones. The seeds from eight of the capsules on the +crossed plants weighed .65 grain, whilst those from eight of the +capsules on the self-fertilised plants weighed only .22 grain; or as 100 +to 34. + +The crossed plants in the above three pots, as in almost all the +previous experiments, flowered before the self-fertilised. This occurred +even in the third pot in which the crossed seeds were sown four days +after the self-fertilised seeds. + +Lastly, seeds of both lots were sown on opposite sides of a large pot in +which a Fuchsia had long been growing, so that the earth was full of +roots. Both lots grew miserably; but the crossed seedlings had an +advantage at all times, and ultimately attained to a height of 3 1/2 +inches, whilst the self-fertilised seedlings never exceeded 1 inch. The +several foregoing experiments prove in a decisive manner the superiority +in constitutional vigour of the crossed over the self-fertilised plants. + +In the three generations now described and taken together, the average +height of the ten tallest crossed plants was 8.19 inches, and that of +the ten tallest self-fertilised plants 5.29 inches (the plants having +been grown in small pots), or as 100 to 65. + +In the next or fourth self-fertilised generation, several plants of a +new and tall variety appeared, which increased in the later +self-fertilised generations, owing to its great self-fertility, to the +complete exclusion of the original kinds. The same variety also appeared +amongst the crossed plants, but as it was not at first regarded with any +particular attention, I know not how far it was used for raising the +intercrossed plants; and in the later crossed generations it was rarely +present. Owing to the appearance of this tall variety, the comparison of +the crossed and self-fertilised plants of the fifth and succeeding +generations was rendered unfair, as all the self-fertilised and only a +few or none of the crossed plants consisted of it. Nevertheless, the +results of the later experiments are in some respects well worth giving. + + +CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION. + +Seeds of the two kinds, produced in the usual way from the two sets of +plants of the third generation, were sown on opposite sides of two pots +(1 and 2); but the seedlings were not thinned enough and did not grow +well. Many of the self-fertilised plants, especially in one of the pots, +consisted of the new and tall variety above referred to, which bore +large and almost white flowers marked with crimson blotches. I will call +it the WHITE VARIETY. I believe that it first appeared amongst both the +crossed and self-fertilised plants of the last generation; but neither +my gardener nor myself could remember any such variety in the seedlings +raised from the purchased seed. It must therefore have arisen either +through ordinary variation, or, judging from its appearance amongst both +the crossed and self-fertilised plants, more probably through reversion +to a formerly existing variety. + +In Pot 1 the tallest crossed plant was 8 1/2 inches, and the tallest +self-fertilised 5 inches in height. In Pot 2, the tallest crossed plant +was 6 1/2 inches, and the tallest self-fertilised plant, which consisted +of the white variety, 7 inches in height; and this was the first +instance in my experiments on Mimulus in which the tallest +self-fertilised plant exceeded the tallest crossed. Nevertheless, the +two tallest crossed plants taken together were to the two tallest +self-fertilised plants in height as 100 to 80. As yet the crossed plants +were superior to the self-fertilised in fertility; for twelve flowers on +the crossed plants were crossed and yielded ten capsules, the seeds of +which weighed 1.71 grain. Twenty flowers on the self-fertilised plants +were self-fertilised, and produced fifteen capsules, all appearing poor; +and the seeds from ten of them weighed only .68 grain, so that from an +equal number of capsules the crossed seeds were to the self-fertilised +in weight as 100 to 40. + +CROSSED AND SELF-FERTILISED PLANTS OF THE FIFTH GENERATION. + +Seeds from both lots of the fourth generation, fertilised in the usual +manner, were sown on opposite sides of three pots. When the seedlings +flowered, most of the self-fertilised plants were found to consist of +the tall white variety. Several of the crossed plants in Pot 1 likewise +belonged to this variety, as did a very few in Pots 2 and 3. The tallest +crossed plant in Pot 1 was 7 inches, and the tallest self-fertilised +plant on the opposite side 8 inches; in Pots 2 and 3 the tallest crossed +were 4 1/2 and 5 1/2, and the tallest self-fertilised 7 and 6 1/2 inches +in height; so that the average height of the tallest plants in the two +lots was as 100 for the crossed to 126 for the self-fertilised; and thus +we have a complete reversal of what occurred in the four previous +generations. Nevertheless, in all three pots the crossed plants retained +their habit of flowering before the self-fertilised. The plants were +unhealthy from being crowded and from the extreme heat of the season, +and were in consequence more or less sterile; but the crossed plants +were somewhat less sterile than the self-fertilised plants. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SIXTH GENERATION. + +Seeds from plants of the fifth generation crossed and self-fertilised in +the usual manner were sown on opposite sides of several pots. On the +self-fertilised side every single plant belonged to the tall white +variety. On the crossed side some plants belonged to this variety, but +the greater number approached in character to the old and shorter kinds +with smaller yellowish flowers blotched with coppery brown. When the +plants on both sides were from 2 to 3 inches in height they were equal, +but when fully grown the self-fertilised were decidedly the tallest and +finest plants, but, from want of time, they were not actually measured. +In half the pots the first plant which flowered was a self-fertilised +one, and in the other half a crossed one. And now another remarkable +change was clearly perceived, namely, that the self-fertilised plants +had become more self-fertile than the crossed. The pots were all put +under a net to exclude insects, and the crossed plants produced +spontaneously only fifty-five capsules, whilst the self-fertilised +plants produced eighty-one capsules, or as 100 to 147. The seeds from +nine capsules of both lots were placed in separate watch-glasses for +comparison, and the self-fertilised appeared rather the more numerous. +Besides these spontaneously self-fertilised capsules, twenty flowers on +the crossed plants again crossed yielded sixteen capsules; twenty-five +flowers on the self-fertilised plants again self-fertilised yielded +seventeen capsules, and this is a larger proportional number of capsules +than was produced by the self-fertilised flowers on the self-fertilised +plants in the previous generations. The contents of ten capsules of both +these lots were compared in separate watch-glasses, and the seeds from +the self-fertilised appeared decidedly more numerous than those from the +crossed plants. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SEVENTH GENERATION. + +Crossed and self-fertilised seeds from the crossed and self-fertilised +plants of the sixth generation were sown in the usual manner on opposite +sides of three pots, and the seedlings were well and equally thinned. +Every one of the self-fertilised plants (and many were raised) in this, +as well as in the eighth and ninth generations, belonged to the tall +white variety. Their uniformity of character, in comparison with the +seedlings first raised from the purchased seed, was quite remarkable. On +the other hand, the crossed plants differed much in the tints of their +flowers, but not, I think, to so great a degree as those first raised. I +determined this time to measure the plants on both sides carefully. The +self-fertilised seedlings came up rather before the crossed, but both +lots were for a time of equal height. When first measured, the average +height of the six tallest crossed plants in the three pots was 7.02, and +that of the six tallest self-fertilised plants 8.97 inches, or as 100 to +128. When fully grown the same plants were again measured, with the +result shown in Table 3/18. + +TABLE 3/18. Mimulus luteus (Seventh Generation). + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 11 2/8 : 19 1/8. +Pot 1 : 11 7/8 : 18. + +Pot 2 : 12 6/8 : 18 2/8. +Pot 2 : 11 2/8 : 14 6/8. + +Pot 3 : 9 6/8 : 12 6/8. +Pot 3 : 11 6/8 : 11. + +Total : 68.63 : 93.88. + +The average height of the six crossed is here 11.43, and that of the six +self-fertilised 15.64, or as 100 to 137. + +As it is now evident that the tall white variety transmitted its +characters faithfully, and as the self-fertilised plants consisted +exclusively of this variety, it was manifest that they would always +exceed in height the crossed plants which belonged chiefly to the +original shorter varieties. This line of experiment was therefore +discontinued, and I tried whether intercrossing two self-fertilised +plants of the sixth generation, growing in distinct pots, would give +their offspring any advantage over the offspring of flowers on one of +the same plants fertilised with their own pollen. These latter seedlings +formed the seventh generation of self-fertilised plants, like those in +the right hand column in Table 3/18; the crossed plants were the product +of six previous self-fertilised generations with an intercross in the +last generation. The seeds were allowed to germinate on sand, and were +planted in pairs on opposite sides of four pots, all the remaining seeds +being sown crowded on opposite sides of Pot 5 in Table 3/19; the three +tallest on each side in this latter pot being alone measured. All the +plants were twice measured--the first time whilst young, and the average +height of the crossed plants to that of the self-fertilised was then as +100 to 122. When fully grown they were again measured, as in Table 3/19. + +TABLE 3/19. Mimulus luteus. + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Intercrossed Plants from Self-fertilised Plants of the Sixth +Generation. + +Column 3: Self-fertilised Plants of the Seventh Generation. + +Pot 1 : 12 6/8 : 15 2/8. +Pot 1 : 10 4/8 : 11 5/8. +Pot 1 : 10 : 11. +Pot 1 : 14 5/8 : 11. + +Pot 2 : 10 2/8 : 11 3/8. +Pot 2 : 7 6/8 : 11 4/8. +Pot 2 : 12 1/8 : 8 5/8. +Pot 2 : 7 : 14 3/8. + +Pot 3 : 13 5/8 : 10 3/8. +Pot 3 : 12 2/8 : 11 6/8. + +Pot 4 : 7 1/8 : 14 6/8. +Pot 4 : 8 2/8 : 7. +Pot 4 : 7 2/8 : 8. + +Pot 5 : 8 5/8 : 10 2/8 +Pot 5 : 9 : 9 3/8. +Pot 5 : 8 2/8 : 9 2/8. +Crowded. + +Total : 159.38 : 175.50. + +The average height of the sixteen intercrossed plants is here 9.96 +inches, and that of the sixteen self-fertilised plants 10.96, or as 100 +to 110; so that the intercrossed plants, the progenitors of which had +been self-fertilised for the six previous generations, and had been +exposed during the whole time to remarkably uniform conditions, were +somewhat inferior in height to the plants of the seventh self-fertilised +generation. But as we shall presently see that a similar experiment made +after two additional generations of self-fertilisation gave a different +result, I know not how far to trust the present one. In three of the +five pots in Table 3/19 a self-fertilised plant flowered first, and in +the other two a crossed plant. These self-fertilised plants were +remarkably fertile, for twenty flowers fertilised with their own pollen +produced no less than nineteen very fine capsules! + +THE EFFECTS OF A CROSS WITH A DISTINCT STOCK. + +Some flowers on the self-fertilised plants in Pot 4 in Table 3/19 were +fertilised with their own pollen, and plants of the eighth +self-fertilised generation were thus raised, merely to serve as parents +in the following experiment. Several flowers on these plants were +allowed to fertilise themselves spontaneously (insects being of course +excluded), and the plants raised from these seeds formed the ninth +self-fertilised generation; they consisted wholly of the tall white +variety with crimson blotches. Other flowers on the same plants of the +eighth self-fertilised generation were crossed with pollen taken from +another plant of the same lot; so that the seedlings thus raised were +the offspring of eight previous generations of self-fertilisation with +an intercross in the last generation; these I will call the INTERCROSSED +PLANTS. Lastly, other flowers on the same plants of the eighth +self-fertilised generation were crossed with pollen taken from plants +which had been raised from seed procured from a garden at Chelsea. The +Chelsea plants bore yellow flowers blotched with red, but differed in no +other respect. They had been grown out of doors, whilst mine had been +cultivated in pots in the greenhouse for the last eight generations, and +in a different kind of soil. The seedlings raised from this cross with a +wholly different stock may be called the CHELSEA-CROSSED. The three lots +of seeds thus obtained were allowed to germinate on bare sand; and +whenever a seed in all three lots, or in only two, germinated at the +same time, they were planted in pots superficially divided into three or +two compartments. The remaining seeds, whether or not in a state of +germination, were thickly sown in three divisions in a large pot, 10, in +Table 3/20. When the plants had grown to their full height they were +measured, as shown in Table 3/20; but only the three tallest plants in +each of the three divisions in Pot 10 were measured. + +TABLE 3/20. Mimulus luteus. + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Plants from Self-fertilised Plants of the Eighth Generation +crossed by Chelsea Plants. + +Column 3: Plants from an intercross between the Plants of the Eighth +Self-fertilised Generation. + +Column 4: Self-fertilised Plants of the Ninth Generation from Plants of +the Eighth Self-fertilised Generation. + +Pot 1 : 30 7/8 : 14 : 9 4/8. +Pot 1 : 28 3/8 : 13 6/8 : 10 5/8. +Pot 1 : -- : 13 7/8 : 10. + +Pot 2 : 20 6/8 : 11 4/8 : 11 6/8. +Pot 2 : 22 2/8 : 12 : 12 3/8. +Pot 2 : -- : 9 1/8 : --. + +Pot 3 : 23 6/8 : 12 2/8 : 8 5/8. +Pot 3 : 24 1/8 : -- : 11 4/8. +Pot 3 : 25 6/8 : -- : 6 7/8. + +Pot 4 : 22 5/8 : 9 2/8 : 4. +Pot 4 : 22 : 8 1/8 : 13 3/8. +Pot 4 : 17 : -- : 11. + +Pot 5 : 22 3/8 : 9 : 4 4/8. +Pot 5 : 19 5/8 : 11 : 13. +Pot 5 : 23 4/8 : -- : 13 4/8. + +Pot 6 : 28 2/8 : 18 6/8 : 12. +Pot 6 : 22 : 7 : 16 1/8. +Pot 6 : -- : 12 4/8 : --. + +Pot 7 : 12 4/8 : 15 : --. +Pot 7 : 24 3/8 : 12 3/8 : --. +Pot 7 : 20 4/8 : 11 2/8 : --. +Pot 7 : 26 4/8 : 15 2/8 : --. + +Pot 8 : 17 2/8 : 13 3/8 : --. +Pot 8 : 22 6/8 : 14 5/8 : --. +Pot 8 : 27 : 14 3/8 : --. + +Pot 9 : 22 6/8 : 11 6/8 : --. +Pot 9 : 6 : 17 : --. +Pot 9 : 20 2/8 : 14 7/8 : --. + +Pot 10 : 18 1/8 : 9 2/8 : 10 3/8. +Pot 10 : 16 5/8 : 8 2/8 : 8 1/8. +Pot 10 : 17 4/8 : 10 : 11 2/8. +Crowded plants. + +Total : 605.38 : 329.50 : 198.50. + +In this table the average height of the twenty-eight Chelsea-crossed +plants is 21.62 inches; that of the twenty-seven intercrossed plants +12.2; and that of the nineteen self-fertilised 10.44. But with respect +to the latter it will be the fairest plan to strike out two dwarfed ones +(only 4 inches in height), so as not to exaggerate the inferiority of +the self-fertilised plants; and this will raise the average height of +the seventeen remaining self-fertilised plants to 11.2 inches. Therefore +the Chelsea-crossed are to the intercrossed in height as 100 to 56; the +Chelsea-crossed to the self-fertilised as 100 to 52; and the +intercrossed to the self-fertilised as 100 to 92. We thus see how +immensely superior in height the Chelsea-crossed are to the intercrossed +and to the self-fertilised plants. They began to show their superiority +when only one inch high. They were also, when fully grown, much more +branched with larger leaves and somewhat larger flowers than the plants +of the other two lots, so that if they had been weighed, the ratio would +certainly have been much higher than that of 100 to 56 and 52. + +The intercrossed plants are here to the self-fertilised in height as 100 +to 92; whereas in the analogous experiment given in Table 3/19 the +intercrossed plants from the self-fertilised plants of the sixth +generation were inferior in height to the self-fertilised plants in the +ratio of 100 to 110. I doubt whether this discordance in the results of +the two experiments can be explained by the self-fertilised plants in +the present case having been raised from spontaneously self-fertilised +seeds, whereas in the former case they were raised from artificially +self-fertilised seeds; nor by the present plants having been +self-fertilised during two additional generations, though this is a more +probable explanation. + +With respect to fertility, the twenty-eight Chelsea-crossed plants +produced 272 capsules; the twenty-seven intercrossed plants produced 24; +and the seventeen self-fertilised plants 17 capsules. All the plants +were left uncovered so as to be naturally fertilised, and empty capsules +were rejected. + +Therefore 20 Chelsea-crossed plants would have produced 194.29 capsules. + +Therefore 20 Intercrossed plants would have produced 17.77 capsules. + +Therefore 20 Self-fertilised plants would have produced 20.00 capsules. + +The seeds contained in 8 capsules from the Chelsea-crossed plants +weighed 1.1 grains. + +The seeds contained in 8 capsules from the Intercrossed plants weighed +0.51 grains. + +The seeds contained in 8 capsules from the Self-fertilised plants +weighed 0.33 grains. + +If we combine the number of capsules produced together with the average +weight of contained seeds, we get the following extraordinary ratios: + +Weight of seed produced by the same number of Chelsea-crossed and +intercrossed plants as 100 to 4. + +Weight of seed produced by the same number of Chelsea-crossed and +self-fertilised plants as 100 to 3. + +Weight of seeds produced by the same number of intercrossed and +self-fertilised plants as 100 to 73. + +It is also a remarkable fact that the Chelsea-crossed plants exceeded +the two other lots in hardiness, as greatly as they did in height, +luxuriance, and fertility. In the early autumn most of the pots were +bedded out in the open ground; and this always injures plants which have +been long kept in a warm greenhouse. All three lots consequently +suffered greatly, but the Chelsea-crossed plants much less than the +other two lots. On the 3rd of October the Chelsea-crossed plants began +to flower again, and continued to do so for some time; whilst not a +single flower was produced by the plants of the other two lots, the +stems of which were cut almost down to the ground and seemed half dead. +Early in December there was a sharp frost, and the stems of +Chelsea-crossed were now cut down; but on the 23rd of December they +began to shoot up again from the roots, whilst all the plants of the +other two lots were quite dead. + +Although several of the self-fertilised seeds, from which the plants in +the right hand column in Table 3/20 were raised, germinated (and were of +course rejected) before any of those of the other two lots, yet in only +one of the ten pots did a self-fertilised plant flower before the +Chelsea-crossed or the intercrossed plants growing in the same pots. The +plants of these two latter lots flowered at the same time, though the +Chelsea-crossed grew so much taller and more vigorously than the +intercrossed. + +As already stated, the flowers of the plants originally raised from the +Chelsea seeds were yellow; and it deserves notice that every one of the +twenty-eight seedlings raised from the tall white variety fertilised, +without being castrated, with pollen from the Chelsea plants, produced +yellow flowers; and this shows how prepotent this colour, which is the +natural one of the species, is over the white colour. + +THE EFFECTS ON THE OFFSPRING OF INTERCROSSING FLOWERS ON THE SAME PLANT, +INSTEAD OF CROSSING DISTINCT INDIVIDUALS. + +In all the foregoing experiments the crossed plants were the product of +a cross between distinct plants. I now selected a very vigorous plant in +Table 3/20, raised by fertilising a plant of the eighth self-fertilised +generation with pollen from the Chelsea stock. Several flowers on this +plant were crossed with pollen from other flowers on the same plant, and +several other flowers were fertilised with their own pollen. The seed +thus produced was allowed to germinate on bare sand; and the seedlings +were planted in the usual manner on the opposite sides of six pots. All +the remaining seeds, whether or not in a state of germination, were sown +thickly in Pot 7; the three tallest plants on each side of this latter +pot being alone measured. As I was in a hurry to learn the result, some +of these seeds were sown late in the autumn, but the plants grew so +irregularly during the winter, that one crossed plant was 28 1/2 inches, +and two others only 4, or less than 4 inches in height, as may be seen +in Table 3/21. Under such circumstances, as I have observed in many +other cases, the result is not in the least trustworthy; nevertheless I +feel bound to give the measurements. + +TABLE 3/21. Mimulus luteus. + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Plants raised from a Cross between different Flowers on the +same Plant. + +Column 3: Plants raised from Flowers fertilised with their own Pollen. + +Pot 1 : 17 : 17. +Pot 1 : 9 : 3 1/8. + +Pot 2 : 28 2/8 : 19 1/8. +Pot 2 : 16 4/8 : 6. +Pot 2 : 13 5/8 : 2. + +Pot 3 : 4 : 15 6/8. +Pot 3 : 2 2/8 : 10. + +Pot 4 : 23 4/8 : 6 2/8. +Pot 4 : 15 4/8 : 7 1/8. + +Pot 5 : 7 : 13 4/8. + +Pot 6 : 18 3/8 : 1 4/8. +Pot 6 : 11 : 2. + +Pot 7 : 21 : 15 1/8. +Pot 7 : 11 6/8 : 11. +Pot 7 : 12 1/8 : 11 2/8. +Crowded. + +Total : 210.88 : 140.75. + +The fifteen crossed plants here average 14.05, and the fifteen +self-fertilised plants 9.38 in height, or as 100 to 67. But if all the +plants under ten inches in height are struck out, the ratio of the +eleven crossed plants to the eight self-fertilised plants is as 100 to +82. + +On the following spring, some remaining seeds of the two lots were +treated in exactly the same manner; and the measurements of the +seedlings are given in Table 3/22. + +TABLE 3/22. Mimulus luteus. + +Heights of Plants in inches: + +Column 1: Number (Name) of Pot. + +Column 2: Plants raised from a Cross between different Flowers on the +same Plant. + +Column 3: Plants raised from Flowers fertilised with their own Pollen. + +Pot 1 : 15 1/8 : 19 1/8. +Pot 1 : 12 : 20 5/8. +Pot 1 : 10 1/8 : 12 6/8. + +Pot 2 : 16 2/8 : 11 2/8. +Pot 2 : 13 5/8 : 19 3/8. +Pot 2 : 20 1/8 : 17 4/8. + +Pot 3 : 18 7/8 : 12 6/8. +Pot 3 : 15 : 15 6/8. +Pot 3 : 13 7/8 : 17. + +Pot 4 : 19 2/8 : 16 2/8. +Pot 4 : 19 6/8 : 21 5/8. + +Pot 5 : 25 3/8 : 22 5/8. + +Pot 6 : 15 : 19 5/8. +Pot 6 : 20 2/8 : 16 2/8. +Pot 6 : 27 2/8 : 19 5/8. + +Pot 7 : 7 6/8 : 7 6/8. +Pot 7 : 14 : 8. +Pot 7 : 13 4/8 : 7. + +Pot 8 : 18 2/8 : 20 3/8. +Pot 8 : 18 6/8 : 17 6/8. +Pot 8 : 18 3/8 : 15 4/8. +Pot 8 : 18 3/8 : 15 1/8. +Crowded. + +Total : 370.88 : 353.63. + +Here the average height of the twenty-two crossed plants is 16.85, and +that of the twenty-two self-fertilised plants 16.07; or as 100 to 95. +But if four of the plants in Pot 7, which are much shorter than any of +the others, are struck out (and this would be the fairest plan), the +twenty-one crossed are to the nineteen self-fertilised plants in height +as 100 to 100.6--that is, are equal. All the plants, except the crowded +ones in Pot 8, after being measured were cut down, and the eighteen +crossed plants weighed 10 ounces, whilst the same number of +self-fertilised plants weighed 10 1/4 ounces, or as 100 to 102.5; but if +the dwarfed plants in Pot 7 had been excluded, the self-fertilised would +have exceeded the crossed in weight in a higher ratio. In all the +previous experiments in which seedlings were raised from a cross between +distinct plants, and were put into competition with self-fertilised +plants, the former generally flowered first; but in the present case, in +seven out of the eight pots a self-fertilised plant flowered before a +crossed one on the opposite side. Considering all the evidence with +respect to the plants in Table3/ 22, a cross between two flowers on the +same plant seems to give no advantage to the offspring thus produced, +the self-fertilised plants being in weight superior. But this conclusion +cannot be absolutely trusted, owing to the measurements given in Table +3/21, though these latter, from the cause already assigned, are very +much less trustworthy than the present ones.] + +SUMMARY OF OBSERVATIONS ON Mimulus luteus. + +In the three first generations of crossed and self-fertilised plants, +the tallest plants alone on each side of the several pots were measured; +and the average height of the ten crossed to that of the ten +self-fertilised plants was as 100 to 64. The crossed were also much more +fertile than the self-fertilised, and so much more vigorous that they +exceeded them in height, even when sown on the opposite side of the same +pot after an interval of four days. The same superiority was likewise +shown in a remarkable manner when both kinds of seeds were sown on the +opposite sides of a pot with very poor earth full of the roots of +another plant. In one instance crossed and self-fertilised seedlings, +grown in rich soil and not put into competition with each other, +attained to an equal height. When we come to the fourth generation the +two tallest crossed plants taken together exceeded by only a little the +two tallest self-fertilised plants, and one of the latter beat its +crossed opponent,--a circumstance which had not occurred in the previous +generations. This victorious self-fertilised plant consisted of a new +white-flowered variety, which grew taller than the old yellowish +varieties. From the first it seemed to be rather more fertile, when +self-fertilised, than the old varieties, and in the succeeding +self-fertilised generations became more and more self-fertile. In the +sixth generation the self-fertilised plants of this variety compared +with the crossed plants produced capsules in the proportion of 147 to +100, both lots being allowed to fertilise themselves spontaneously. In +the seventh generation twenty flowers on one of these plants +artificially self-fertilised yielded no less than nineteen very fine +capsules! + +This variety transmitted its characters so faithfully to all the +succeeding self-fertilised generations, up to the last or ninth, that +all the many plants which were raised presented a complete uniformity of +character; thus offering a remarkable contrast with the seedlings raised +from the purchased seeds. Yet this variety retained to the last a latent +tendency to produce yellow flowers; for when a plant of the eighth +self-fertilised generation was crossed with pollen from a +yellow-flowered plant of the Chelsea stock, every single seedling bore +yellow flowers. A similar variety, at least in the colour of its +flowers, also appeared amongst the crossed plants of the third +generation. No attention was at first paid to it, and I know not how far +it was at first used either for crossing or self-fertilisation. In the +fifth generation most of the self-fertilised plants, and in the sixth +and all the succeeding generations every single plant consisted of this +variety; and this no doubt was partly due to its great and increasing +self-fertility. On the other hand, it disappeared from amongst the +crossed plants in the later generations; and this was probably due to +the continued intercrossing of the several plants. From the tallness of +this variety, the self-fertilised plants exceeded the crossed plants in +height in all the generations from the fifth to the seventh inclusive; +and no doubt would have done so in the later generations, had they been +grown in competition with one another. In the fifth generation the +crossed plants were in height to the self-fertilised, as 100 to 126; in +the sixth, as 100 to 147; and in the seventh generation, as 100 to 137. +This excess of height may be attributed not only to this variety +naturally growing taller than the other plants, but to its possessing a +peculiar constitution, so that it did not suffer from continued +self-fertilisation. + +This variety presents a strikingly analogous case to that of the plant +called the Hero, which appeared in the sixth self-fertilised generation +of Ipomoea. If the seeds produced by Hero had been as greatly in excess +of those produced by the other plants, as was the case with Mimulus, and +if all the seeds had been mingled together, the offspring of Hero would +have increased to the entire exclusion of the ordinary plants in the +later self-fertilised generations, and from naturally growing taller +would have exceeded the crossed plants in height in each succeeding +generation. + +Some of the self-fertilised plants of the sixth generation were +intercrossed, as were some in the eighth generation; and the seedlings +from these crosses were grown in competition with self-fertilised plants +of the two corresponding generations. In the first trial the +intercrossed plants were less fertile than the self-fertilised, and less +tall in the ratio of 100 to 110. In the second trial, the intercrossed +plants were more fertile than the self-fertilised in the ratio of 100 to +73, and taller in the ratio of 100 to 92. Notwithstanding that the +self-fertilised plants in the second trial were the product of two +additional generations of self-fertilisation, I cannot understand this +discordance in the results of the two analogous experiments. + +The most important of all the experiments on Mimulus are those in which +flowers on plants of the eighth self-fertilised generation were again +self-fertilised; other flowers on distinct plants of the same lot were +intercrossed; and others were crossed with a new stock of plants from +Chelsea. The Chelsea-crossed seedlings were to the intercrossed in +height as 100 to 56, and in fertility as 100 to 4; and they were to the +self-fertilised plants, in height as 100 to 52, and in fertility as 100 +to 3. These Chelsea-crossed plants were also much more hardy than the +plants of the other two lots; so that altogether the gain from the cross +with a fresh stock was wonderfully great. + +Lastly, seedlings raised from a cross between flowers on the same plant +were not superior to those from flowers fertilised with their own +pollen; but this result cannot be absolutely trusted, owing to some +previous observations, which, however, were made under very unfavourable +circumstances. + +[Digitalis purpurea. + +The flowers of the common Foxglove are proterandrous; that is, the +pollen is mature and mostly shed before the stigma of the same flower is +ready for fertilisation. This is effected by the larger humble-bees, +which, whilst in search of nectar, carry pollen from flower to flower. +The two upper and longer stamens shed their pollen before the two lower +and shorter ones. The meaning of this fact probably is, as Dr. Ogle +remarks, that the anthers of the longer stamens stand near to the +stigma, so that they would be the most likely to fertilise it (3/3. +'Popular Science Review' January 1870 page 50.); and as it is an +advantage to avoid self-fertilisation, they shed their pollen first, +thus lessening the chance. There is, however, but little danger of +self-fertilisation until the bifid stigma opens; for Hildebrand found +that pollen placed on the stigma before it had opened produced no +effect. (3/4. 'Geschlechter-Vertheilung bei den Pflanzen' 1867 page 20.) +The anthers, which are large, stand at first transversely with respect +to the tubular corolla, and if they were to dehisce in this position +they would, as Dr. Ogle also remarks, smear with pollen the whole back +and sides of an entering humble-bee in a useless manner; but the anthers +twist round and place themselves longitudinally before they dehisce. The +lower and inner side of the mouth of the corolla is thickly clothed with +hairs, and these collect so much of the fallen pollen that I have seen +the under surface of a humble-bee thickly dusted with it; but this can +never be applied to the stigma, as the bees in retreating do not turn +their under surfaces upwards. I was therefore puzzled whether these +hairs were of any use; but Mr. Belt has, I think, explained their use: +the smaller kinds of bees are not fitted to fertilise the flowers, and +if they were allowed to enter easily they would steal much nectar, and +fewer large bees would haunt the flowers. Humble-bees can crawl into the +dependent flowers with the greatest ease, using the "hairs as footholds +while sucking the honey; but the smaller bees are impeded by them, and +when, having at length struggled through them, they reach the slippery +precipice above, they are completely baffled." Mr. Belt says that he +watched many flowers during a whole season in North Wales, and "only +once saw a small bee reach the nectary, though many were seen trying in +vain to do so." (3/5. 'The Naturalist in Nicaragua' 1874 page 132. But +it appears from H. Muller 'Die Befruchtung der Blumen' 1873 page 285, +that small insects sometimes succeed in entering the flowers.) + +I covered a plant growing in its native soil in North Wales with a net, +and fertilised six flowers each with its own pollen, and six others with +pollen from a distinct plant growing within the distance of a few feet. +The covered plant was occasionally shaken with violence, so as to +imitate the effects of a gale of wind, and thus to facilitate as far as +possible self-fertilisation. It bore ninety-two flowers (besides the +dozen artificially fertilised), and of these only twenty-four produced +capsules; whereas almost all the flowers on the surrounding uncovered +plants were fruitful. Of the twenty-four spontaneously self-fertilised +capsules, only two contained their full complement of seed; six +contained a moderate supply; and the remaining sixteen extremely few +seeds. A little pollen adhering to the anthers after they had dehisced, +and accidentally falling on the stigma when mature, must have been the +means by which the above twenty-four flowers were partially +self-fertilised; for the margins of the corolla in withering do not curl +inwards, nor do the flowers in dropping off turn round on their axes, so +as to bring the pollen-covered hairs, with which the lower surface is +clothed, into contact with the stigma--by either of which means +self-fertilisation might be effected. + +Seeds from the above crossed and self-fertilised capsules, after +germinating on bare sand, were planted in pairs on the opposite sides of +five moderately-sized pots, which were kept in the greenhouse. The +plants after a time appeared starved, and were therefore, without being +disturbed, turned out of their pots, and planted in the open ground in +two close parallel rows. They were thus subjected to tolerably severe +competition with one another; but not nearly so severe as if they had +been left in the pots. At the time when they were turned out, their +leaves were between 5 and 8 inches in length, and the longest leaf on +the finest plant on each side of each pot was measured, with the result +that the leaves of the crossed plants exceeded, on an average, those of +the self-fertilised plants by .4 of an inch. + +In the following summer the tallest flower-stem on each plant, when +fully grown, was measured. There were seventeen crossed plants; but one +did not produce a flower-stem. There were also, originally, seventeen +self-fertilised plants, but these had such poor constitutions that no +less than nine died in the course of the winter and spring, leaving only +eight to be measured, as in Table 3/23. + +TABLE 3/23. Digitalis purpurea. + +The tallest Flower-stem on each Plant measured in inches: 0 means that +the Plant died before a Flower-stem was produced. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 53 6/8 : 27 4/8. +Pot 1 : 57 4/8 : 55 6/8. +Pot 1 : 57 6/8 : 0. +Pot 1 : 65 : 0. + +Pot 2 : 34 4/8 : 39. +Pot 2 : 52 4/8 : 32. +Pot 2 : 63 6/8 : 21. + +Pot 3 : 57 4/8 : 53 4/8. +Pot 3 : 53 4/8 : 0. +Pot 3 : 50 6/8 : 0. +Pot 3 : 37 2/8 : 0. + +Pot 4 : 64 4/8 : 34 4/8. +Pot 4 : 37 4/8 : 23 6/8. +Pot 4 : -- : 0. + +Pot 5 : 53 : 0. +Pot 5 : 47 6/8 : 0. +Pot 5 : 34 6/8 : 0. + +Total : 821.25 : 287.00. + +The average height of the flower-stems of the sixteen crossed plants is +here 51.33 inches; and that of the eight self-fertilised plants, 35.87; +or as 100 to 70. But this difference in height does not give at all a +fair idea of the vast superiority of the crossed plants. These latter +produced altogether sixty-four flower-stems, each plant producing, on an +average, exactly four flower-stems, whereas the eight self-fertilised +plants produced only fifteen flower-stems, each producing an average +only of 1.87 stems, and these had a less luxuriant appearance. We may +put the result in another way: the number of flower-stems on the crossed +plants was to those on an equal number of self-fertilised plants as 100 +to 48. + +Three crossed seeds in a state of germination were also planted in three +separate pots; and three self-fertilised seeds in the same state in +three other pots. These plants were therefore at first exposed to no +competition with one another, and when turned out of their pots into the +open ground they were planted at a moderate distance apart, so that they +were exposed to much less severe competition than in the last case. The +longest leaves on the three crossed plants, when turned out, exceeded +those on the self-fertilised plants by a mere trifle, namely, on an +average by .17 of an inch. When fully grown the three crossed plants +produced twenty-six flower-stems; the two tallest of which on each plant +were on an average 54.04 inches in height. The three self-fertilised +plants produced twenty-three flower-stems, the two tallest of which on +each plant had an average height of 46.18 inches. So that the difference +between these two lots, which hardly competed together, is much less +than in the last case when there was moderately severe competition, +namely, as 100 to 85, instead of as 100 to 70. + +THE EFFECTS ON THE OFFSPRING OF INTERCROSSING DIFFERENT FLOWERS ON THE +SAME PLANT, INSTEAD OF CROSSING DISTINCT INDIVIDUALS. + +A fine plant growing in my garden (one of the foregoing seedlings) was +covered with a net, and six flowers were crossed with pollen from +another flower on the same plant, and six others were fertilised with +their own pollen. All produced good capsules. The seeds from each were +placed in separate watch-glasses, and no difference could be perceived +by the eye between the two lots of seeds; and when they were weighed +there was no difference of any significance, as the seeds from the +self-fertilised capsules weighed 7.65 grains, whilst those from the +crossed capsules weighed 7.7 grains. Therefore the sterility of the +present species, when insects are excluded, is not due to the impotence +of pollen on the stigma of the same flower. Both lots of seeds and +seedlings were treated in exactly the same manner as in Table 3/23, +excepting that after the pairs of germinating seeds had been planted on +the opposite sides of eight pots, all the remaining seeds were thickly +sown on the opposite sides of Pots 9 and 10 in Table 3/24. The young +plants during the following spring were turned out of their pots, +without being disturbed, and planted in the open ground in two rows, not +very close together, so that they were subjected to only moderately +severe competition with one another. Very differently to what occurred +in the first experiment, when the plants were subjected to somewhat +severe mutual competition, an equal number on each side either died or +did not produce flower-stems. The tallest flower-stems on the surviving +plants were measured, as shown in Table 3/24. + +TABLE 3/24. Digitalis purpurea. + +The tallest Flower-stem on each Plant measured in inches: 0 signifies +that the Plant died, or did not produce a Flower-stem. + +Column 1: Number (Name) of Pot. + +Column 2: Plants raised from a Cross between different Flowers on the +same Plant. + +Column 3: Plants raised from Flowers fertilised with their own Pollen. + +Pot 1 : 49 4/8 : 45 5/8. +Pot 1 : 46 7/8 : 52. +Pot 1 : 43 6/8 : 0. + +Pot 2 : 38 4/8 : 54 4/8. +Pot 2 : 47 4/8 : 47 4/8. +Pot 2 : 0 : 32 5/8. + +Pot 3 : 54 7/8 : 46 5/8. + +Pot 4 : 32 1/8 : 41 3/8. +Pot 4 : 0 : 29 7/8. +Pot 4 : 43 7/8 : 37 1/8. + +Pot 5 : 46 6/8 : 42 1/8. +Pot 5 : 40 4/8 : 42 1/8. +Pot 5 : 43 : 0. + +Pot 6 : 48 2/8 : 47 7/8. +Pot 6 : 46 2/8 : 48 3/8. + +Pot 7 : 48 5/8 : 25. +Pot 7 : 42 : 40 5/8. + +Pot 8 : 46 7/8 : 39 1/8. + +Pot 9 : 49 : 30 3/8. +Pot 9 : 50 3/8 : 15. +Pot 9 : 46 3/8 : 36 7/8. +Pot 9 : 47 6/8 : 44 1/8. +Pot 9 : 0 : 31 6/8. +Crowded Plants. + +Pot 10 : 46 4/8 : 47 7/8. +Pot 10 : 35 2/8 : 0. +Pot 10 : 24 5/8 : 34 7/8. +Pot 10 : 41 4/8 : 40 7/8. +Pot 10 : 17 3/8 : 41 1/8. +Crowded Plants. + +Total : 1078.00 : 995.38. + +The average height of the flower-stems on the twenty-five crossed plants +in all the pots taken together is 43.12 inches, and that of the +twenty-five self-fertilised plants 39.82, or as 100 to 92. In order to +test this result, the plants planted in pairs in Pots 1 and 8 were +considered by themselves, and the average height of the sixteen crossed +plants is here 44.9, and that of the sixteen self-fertilised plants +42.03, or as 100 to 94. Again, the plants raised from the thickly sown +seed in Pots 9 and 10, which were subjected to very severe mutual +competition, were taken by themselves, and the average height of the +nine crossed plants is 39.86, and that of the nine self-fertilised +plants 35.88, or as 100 to 90. The plants in these two latter pots (9 +and 10), after being measured, were cut down close to the ground and +weighed: the nine crossed plants weighed 57.66 ounces, and the nine +self-fertilised plants 45.25 ounces, or as 100 to 78. On the whole we +may conclude, especially from the evidence of weight, that seedlings +from a cross between flowers on the same plant have a decided, though +not great, advantage over those from flowers fertilised with their own +pollen, more especially in the case of the plants subjected to severe +mutual competition. But the advantage is much less than that exhibited +by the crossed offspring of distinct plants, for these exceeded the +self-fertilised plants in height as 100 to 70, and in the number of +flower-stems as 100 to 48. Digitalis thus differs from Ipomoea, and +almost certainly from Mimulus, as with these two species a cross between +flowers on the same plant did no good. + +CALCEOLARIA. + +A BUSHY GREENHOUSE VARIETY, WITH YELLOW FLOWERS BLOTCHED WITH PURPLE. + +The flowers in this genus are constructed so as to favour or almost +ensure cross-fertilisation (3/6. Hildebrand as quoted by H. Muller 'Die +Befruchtung der Blumen' 1873 page 277.); and Mr. Anderson remarks that +extreme care is necessary to exclude insects in order to preserve any +kind true. (3/7. 'Gardeners' Chronicle' 1853 page 534.) He adds the +interesting statement, that when the corolla is cut quite away, insects, +as far as he has seen, never discover or visit the flowers. This plant +is, however, self-fertile if insects are excluded. So few experiments +were made by me, that they are hardly worth giving. Crossed and +self-fertilised seeds were sown on opposite sides of a pot, and after a +time the crossed seedlings slightly exceeded the self-fertilised in +height. When a little further grown, the longest leaves on the former +were very nearly 3 inches in length, whilst those on the self-fertilised +plants were only 2 inches. Owing to an accident, and to the pot being +too small, only one plant on each side grew up and flowered; the crossed +plant was 19 1/2 inches in height, and the self-fertilised one 15 +inches; or as 100 to 77. + +Linaria vulgaris. + +It has been mentioned in the introductory chapter that two large beds of +this plant were raised by me many years ago from crossed and +self-fertilised seeds, and that there was a conspicuous difference in +height and general appearance between the two lots. The trial was +afterwards repeated with more care; but as this was one of the first +plants experimented on, my usual method was not followed. Seeds were +taken from wild plants growing in this neighbourhood and sown in poor +soil in my garden. Five plants were covered with a net, the others being +left exposed to the bees, which incessantly visit the flowers of this +species, and which, according to H. Muller, are the exclusive +fertilisers. This excellent observer remarks that, as the stigma lies +between the anthers and is mature at the same time with them, +self-fertilisation is possible. (3/8. 'Die Befruchtung' etc. page 279.) +But so few seeds are produced by protected plants, that the pollen and +stigma of the same flower seem to have little power of mutual +interaction. The exposed plants bore numerous capsules forming solid +spikes. Five of these capsules were examined and appeared to contain an +equal number of seeds; and these being counted in one capsule, were +found to be 166. The five protected plants produced altogether only +twenty-five capsules, of which five were much finer than all the others, +and these contained an average of 23.6 seeds, with a maximum in one +capsule of fifty-five. So that the number of seeds in the capsules on +the exposed plants to the average number in the finest capsules on the +protected plants was as 100 to 14. + +Some of the spontaneously self-fertilised seeds from under the net, and +some seeds from the uncovered plants naturally fertilised and almost +certainly intercrossed by the bees, were sown separately in two large +pots of the same size; so that the two lots of seedlings were not +subjected to any mutual competition. Three of the crossed plants when in +full flower were measured, but no care was taken to select the tallest +plants; their heights were 7 4/8, 7 2/8, and 6 4/8 inches; averaging +7.08 in height. The three tallest of all the self-fertilised plants were +then carefully selected, and their heights were 6 3/8, 5 5/8, and 5 2/8, +averaging 5.75 in height. So that the naturally crossed plants were to +the spontaneously self-fertilised plants in height, at least as much as +100 to 81. + +Verbascum thapsus. + +The flowers of this plant are frequented by various insects, chiefly by +bees, for the sake of the pollen. Hermann Muller, however, has shown +('Die Befruchtung' etc. page 277) that V. nigrum secretes minute drops +of nectar. The arrangement of the reproductive organs, though not at all +complex, favours cross-fertilisation; and even distinct species are +often crossed, for a greater number of naturally produced hybrids have +been observed in this genus than in almost any other. (3/9. I have given +a striking case of a large number of such hybrids between Verbascum +thapsus and lychnitis found growing wild: 'Journal of Linnean Society +Botany' volume 10 page 451.) Nevertheless the present species is +perfectly self-fertile, if insects are excluded; for a plant protected +by a net was as thickly loaded with fine capsules as the surrounding +uncovered plants. Verbascum lychnitis is rather less self-fertile, for +some protected plants did not yield quite so many capsules as the +adjoining uncovered plants. + +Plants of Verbascum thapsus had been raised for a distinct purpose from +self-fertilised seeds; and some flowers on these plants were again +self-fertilised, yielding seed of the second self-fertilised generation; +and other flowers were crossed with pollen from a distinct plant. The +seeds thus produced were sown on the opposite sides of four large pots. +They germinated, however, so irregularly (the crossed seedlings +generally coming up first) that I was able to save only six pairs of +equal age. These when in full flower were measured, as in Table 3/25. + +TABLE 3/25. Verbascum thapsus. + +Heights of Plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants of the Second Generation. + +Pot 1 : 76 : 53 4/8. + +Pot 2 : 54 : 66. + +Pot 3 : 62 : 75. +Pot 3 : 60 5/8 : 30 4/8. + +Pot 4 : 73 : 62. +Pot 4 : 66 4/8 : 52. + +Total : 392.13 : 339.00. + +We here see that two of the self-fertilised plants exceed in height +their crossed opponents. Nevertheless the average height of the six +crossed plants is 65.34 inches, and that of the six self-fertilised +plants 56.5 inches; or as 100 to 86. + +Vandellia nummularifolia. + +Seeds were sent to me by Mr. J. Scott from Calcutta of this small Indian +weed, which bears perfect and cleistogene flowers. (3/10. The convenient +term of CLEISTOGENE was proposed by Kuhn in an article on the present +genus in 'Bot. Zeitung' 1867 page 65.) The latter are extremely small, +imperfectly developed, and never expand, yet yield plenty of seeds. The +perfect and open flowers are also small, of a white colour with purple +marks; they generally produce seed, although the contrary has been +asserted; and they do so even if protected from insects. They have a +rather complicated structure, and appear to be adapted for +cross-fertilisation, but were not carefully examined by me. They are not +easy to fertilise artificially, and it is possible that some of the +flowers which I thought that I had succeeded in crossing were afterwards +spontaneously self-fertilised under the net. Sixteen capsules from the +crossed perfect flowers contained on an average ninety-three seeds (with +a maximum in one capsule of 137), and thirteen capsules from the +self-fertilised perfect flowers contained sixty-two seeds (with a +maximum in one capsule of 135); or as 100 to 67. But I suspect that this +considerable excess was accidental, as on one occasion nine crossed +capsules were compared with seven self-fertilised capsules (both +included in the above number), and they contained almost exactly the +same average number of seed. I may add that fifteen capsules from +self-fertilised cleistogene flowers contained on an average sixty-four +seeds, with a maximum in one of eighty-seven. + +Crossed and self-fertilised seeds from the perfect flowers, and other +seeds from the self-fertilised cleistogene flowers, were sown in five +pots, each divided superficially into three compartments. The seedlings +were thinned at an early age, so that twenty plants were left in each of +the three divisions. The crossed plants when in full flower averaged 4.3 +inches, and the self-fertilised plants from the perfect flowers 4.27 +inches in height; or as 100 to 99. The self-fertilised plants from the +cleistogene flowers averaged 4.06 inches in height; so that the crossed +were in height to these latter plants as 100 to 94. + +I determined to compare again the growth of plants raised from crossed +and self-fertilised perfect flowers, and obtained two fresh lots of +seeds. These were sown on opposite sides of five pots, but they were not +sufficiently thinned, so that they grew rather crowded. When fully +grown, all those above 2 inches in height were selected, all below this +standard being rejected; the former consisted of forty-seven crossed and +forty-one self-fertilised plants; thus a greater number of the crossed +than of the self-fertilised plants grew to a height of above 2 inches. +Of the crossed plants, the twenty-four tallest were on an average 3.6 +inches in height; whilst the twenty-four tallest self-fertilised plants +were 3.38 inches in average height; or as 100 to 94. All these plants +were then cut down close to the ground, and the forty-seven crossed +plants weighed 1090.3 grains, and the forty-one self-fertilised plants +weighed 887.4 grains. Therefore an equal number of crossed and +self-fertilised would have been to each other in weight as 100 to 97. +From these several facts we may conclude that the crossed plants had +some real, though very slight, advantage in height and weight over the +self-fertilised plants, when grown in competition with one another. + +The crossed plants were, however, inferior in fertility to the +self-fertilised. Six of the finest plants were selected out of the +forty-seven crossed plants, and six out of the forty-one self-fertilised +plants; and the former produced 598 capsules, whilst the latter or +self-fertilised plants produced 752 capsules. All these capsules were +the product of cleistogene flowers, for the plants did not bear during +the whole of this season any perfect flowers. The seeds were counted in +ten cleistogene capsules produced by crossed plants, and their average +number was 46.4 per capsule; whilst the number in ten cleistogene +capsules produced by the self-fertilised plants was 49.4; or as 100 to +106. + +3. GESNERIACEAE.--Gesneria pendulina. + +In Gesneria the several parts of the flower are arranged on nearly the +same plan as in Digitalis, and most or all of the species are +dichogamous. (3/11. Dr. Ogle 'Popular Science Review' January 1870 page +51.) Plants were raised from seed sent me by Fritz Muller from South +Brazil. Seven flowers were crossed with pollen from a distinct plant, +and produced seven capsules containing by weight 3.01 grains of seeds. +Seven flowers on the same plants were fertilised with their own pollen, +and their seven capsules contained exactly the same weight of seeds. +Germinating seeds were planted on opposite sides of four pots, and when +fully grown measured to the tips of their leaves. + +TABLE 3/26. Gesneria pendulina. + +Heights of Plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 42 2/8 : 39. +Pot 1 : 24 4/8 : 27 3/8. + +Pot 2 : 33 : 30 6/8. +Pot 2 : 27 : 19 2/8. + +Pot 3 : 33 4/8 : 31 7/8. +Pot 3 : 29 4/8 : 28 6/8. + +Pot 4 : 30 6/8 : 29 6/8. +Pot 4 : 36 : 26 3/8. + +Total : 256.50 : 233.13. + +The average height of the eight crossed plants is 32.06 inches, and that +of the eight self-fertilised plants 29.14; or as 100 to 90. + +4. LABIATAE.--Salvia coccinea. (3/12. The admirable mechanical +adaptations in this genus for favouring or ensuring cross-fertilisation, +have been fully described by Sprengel, Hildebrand, Delpino, H. Muller, +Ogle, and others, in their several works.) + +This species, unlike most of the others in the same genus, yields a good +many seeds when insects are excluded. I gathered ninety-eight capsules +produced by flowers spontaneously self-fertilised under a net, and they +contained on an average 1.45 seeds, whilst flowers artificially +fertilised with their own pollen, in which case the stigma will have +received plenty of pollen, yielded on an average 3.3 seeds, or more than +twice as many. Twenty flowers were crossed with pollen from a distinct +plant, and twenty-six were self-fertilised. There was no great +difference in the proportional number of flowers which produced capsules +by these two processes, or in the number of the contained seeds, or in +the weight of an equal number of seeds. + +Seeds of both kinds were sown rather thickly on opposite sides of three +pots. When the seedlings were about 3 inches in height, the crossed +showed a slight advantage over the self-fertilised. When two-thirds +grown, the two tallest plants on each side of each pot were measured; +the crossed averaged 16.37 inches, and the self-fertilised 11.75 in +height; or as 100 to 71. When the plants were fully grown and had done +flowering, the two tallest plants on each side were again measured, with +the results shown in Table 3/27. + +TABLE 3/27. Salvia coccinea. + +Heights of Plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 32 6/8 : 25. +Pot 1 : 20 : 18 6/8. + +Pot 2 : 32 3/8 : 20 6/8. +Pot 2 : 24 4/8 : 19 4/8. + +Pot 3 : 29 4/8 : 25. +Pot 3 : 28 : 18. + +Total : 167.13 : 127.00. + +It may be here seen that each of the six tallest crossed plants exceeds +in height its self-fertilised opponent; the former averaged 27.85 +inches, whilst the six tallest self-fertilised plants averaged 21.16 +inches; or as 100 to 76. In all three pots the first plant which +flowered was a crossed one. All the crossed plants together produced 409 +flowers, whilst all the self-fertilised together produced only 232 +flowers; or as 100 to 57. So that the crossed plants in this respect +were far more productive than the self-fertilised. + +Origanum vulgare. + +This plant exists, according to H. Muller, under two forms; one +hermaphrodite and strongly proterandrous, so that it is almost certain +to be fertilised by pollen from another flower; the other form is +exclusively female, has a smaller corolla, and must of course be +fertilised by pollen from a distinct plant in order to yield any seeds. +The plants on which I experimented were hermaphrodites; they had been +cultivated for a long period as a pot-herb in my kitchen garden, and +were, like so many long-cultivated plants, extremely sterile. As I felt +doubtful about the specific name I sent specimens to Kew, and was +assured that the species was Origanum vulgare. My plants formed one +great clump, and had evidently spread from a single root by stolons. In +a strict sense, therefore, they all belonged to the same individual. My +object in experimenting on them was, firstly, to ascertain whether +crossing flowers borne by plants having distinct roots, but all derived +asexually from the same individual, would be in any respect more +advantageous than self-fertilisation; and, secondly, to raise for future +trial seedlings which would constitute really distinct individuals. +Several plants in the above clump were covered by a net, and about two +dozen seeds (many of which, however, were small and withered) were +obtained from the flowers thus spontaneously self-fertilised. The +remainder of the plants were left uncovered and were incessantly visited +by bees, so that they were doubtless crossed by them. These exposed +plants yielded rather more and finer seed (but still very few) than did +the covered plants. The two lots of seeds thus obtained were sown on +opposite sides of two pots; the seedlings were carefully observed from +their first growth to maturity, but they did not differ at any period in +height or in vigour, the importance of which latter observation we shall +presently see. When fully grown, the tallest crossed plant in one pot +was a very little taller than the tallest self-fertilised plant on the +opposite side, and in the other pot exactly the reverse occurred. So +that the two lots were in fact equal; and a cross of this kind did no +more good than crossing two flowers on the same plant of Ipomoea or +Mimulus. + +The plants were turned out of the two pots without being disturbed and +planted in the open ground, in order that they might grow more +vigorously. In the following summer all the self-fertilised and some of +the quasi-crossed plants were covered by a net. Many flowers on the +latter were crossed by me with pollen from a distinct plant, and others +were left to be crossed by the bees. These quasi-crossed plants produced +rather more seed than did the original ones in the great clump when left +to the action of the bees. Many flowers on the self-fertilised plants +were artificially self-fertilised, and others were allowed to fertilise +themselves spontaneously under the net, but they yielded altogether very +few seeds. These two lots of seeds--the product of a cross between +distinct seedlings, instead of as in the last case between plants +multiplied by stolons, and the product of self-fertilised flowers--were +allowed to germinate on bare sand, and several equal pairs were planted +on opposite sides of two LARGE pots. At a very early age the crossed +plants showed some superiority over the self-fertilised, which was ever +afterwards retained. When the plants were fully grown, the two tallest +crossed and the two tallest self-fertilised plants in each pot were +measured, as shown in Table 3/28. I regret that from want of time I did +not measure all the pairs; but the tallest on each side seemed fairly to +represent the average difference between the two lots. + +TABLE 3/28. Origanum vulgare. + +Heights of Plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants (two tallest in each pot). + +Column 3: Self-fertilised Plants (two tallest in each pot). + +Pot 1 : 26 : 24. +Pot 1 : 21 : 21. + +Pot 2 : 17 : 12. +Pot 2 : 16 : 11 4/8. + +Total : 80.0 : 68.5. + +The average height of the crossed plants is here 20 inches, and that of +the self-fertilised 17.12; or as 100 to 86. But this excess of height by +no means gives a fair idea of the vast superiority in vigour of the +crossed over the self-fertilised plants. The crossed flowered first and +produced thirty flower-stems, whilst the self-fertilised produced only +fifteen, or half the number. The pots were then bedded out, and the +roots probably came out of the holes at the bottom and thus aided their +growth. Early in the following summer the superiority of the crossed +plants, owing to their increase by stolons, over the self-fertilised +plants was truly wonderful. In Pot 1, and it should be remembered that +very large pots had been used, the oval clump of crossed plants was 10 +by 4 1/2 inches across, with the tallest stem, as yet young, 5 1/2 +inches in height; whilst the clump of self-fertilised plants, on the +opposite side of the same pot, was only 3 1/2 by 2 1/2 inches across, +with the tallest young stem 4 inches in height. In Pot 2, the clump of +crossed plants was 18 by 9 inches across, with the tallest young stem 8 +1/2 inches in height; whilst the clump of self-fertilised plants on the +opposite side of the same pot was 12 by 4 1/2 inches across, with the +tallest young stem 6 inches in height. The crossed plants during this +season, as during the last, flowered first. Both the crossed and +self-fertilised plants being left freely exposed to the visits of bees, +manifestly produced much more seed than their grand-parents,--the plants +of the original clump still growing close by in the same garden, and +equally left to the action of the bees. + +5. ACANTHACEAE.--Thunbergia alata. + +It appears from Hildebrand's description ('Botanische Zeitung' 1867 page +285) that the conspicuous flowers of this plant are adapted for +cross-fertilisation. Seedlings were twice raised from purchased seed; +but during the early summer, when first experimented on, they were +extremely sterile, many of the anthers containing hardly any pollen. +Nevertheless, during the autumn these same plants spontaneously produced +a good many seeds. Twenty-six flowers during the two years were crossed +with pollen from a distinct plant, but they yielded only eleven +capsules; and these contained very few seeds! Twenty-eight flowers were +fertilised with pollen from the same flower, and these yielded only ten +capsules, which, however, contained rather more seed than the crossed +capsules. Eight pairs of germinating seeds were planted on opposite +sides of five pots; and exactly half the crossed and half the +self-fertilised plants exceeded their opponents in height. Two of the +self-fertilised plants died young, before they were measured, and their +crossed opponents were thrown away. The six remaining pairs of these +grew very unequally, some, both of the crossed and self-fertilised +plants, being more than twice as tall as the others. The average height +of the crossed plants was 60 inches, and that of the self-fertilised +plants 65 inches, or as 100 to 108. A cross, therefore, between distinct +individuals here appears to do no good; but this result deduced from so +few plants in a very sterile condition and growing very unequally, +obviously cannot be trusted.] + + + +CHAPTER IV. + +CRUCIFERAE, PAPAVERACEAE, RESEDACEAE, ETC. + +Brassica oleracea, crossed and self-fertilised plants. +Great effect of a cross with a fresh stock on the weight of the +offspring. +Iberis umbellata. +Papaver vagum. +Eschscholtzia californica, seedlings from a cross with a fresh stock not +more vigorous, but more fertile than the self-fertilised seedlings. +Reseda lutea and odorata, many individuals sterile with their own pollen. +Viola tricolor, wonderful effects of a cross. +Adonis aestivalis. +Delphinium consolida. +Viscaria oculata, crossed plants hardly taller, but more fertile than +the self-fertilised. +Dianthus caryophyllus, crossed and self-fertilised plants compared for +four generations. +Great effects of a cross with a fresh stock. +Uniform colour of the flowers on the self-fertilised plants. +Hibiscus africanus. + +[6. CRUCIFERAE.--Brassica oleracea. + +VAR. CATTELL'S EARLY BARNES CABBAGE. + +The flowers of the common cabbage are adapted, as shown by H. Muller, +for cross-fertilisation, and should this fail, for self-fertilisation. +(4/1. 'Die Befruchtung' etc. page 139.) It is well known that the +varieties are crossed so largely by insects, that it is impossible to +raise pure kinds in the same garden, if more than one kind is in flower +at the same time. Cabbages, in one respect, were not well fitted for my +experiments, as, after they had formed heads, they were often difficult +to measure. The flower-stems also differ much in height; and a poor +plant will sometimes throw up a higher stem than that of a fine plant. +In the later experiments, the fully-grown plants were cut down and +weighed, and then the immense advantage from a cross became manifest. + +A single plant of the above variety was covered with a net just before +flowering, and was crossed with pollen from another plant of the same +variety growing close by; and the seven capsules thus produced contained +on an average 16.3 seeds, with a maximum of twenty in one capsule. Some +flowers were artificially self-fertilised, but their capsules did not +contain so many seeds as those from flowers spontaneously +self-fertilised under the net, of which a considerable number were +produced. Fourteen of these latter capsules contained on an average 4.1 +seeds, with a maximum in one of ten seeds; so that the seeds in the +crossed capsules were in number to those in the self-fertilised capsules +as 100 to 25. The self-fertilised seeds, fifty-eight of which weighed +3.88 grains, were, however, a little finer than those from the crossed +capsules, fifty-eight of which weighed 3.76 grains. When few seeds are +produced, these seem often to be better nourished and to be heavier than +when many are produced. + +The two lots of seeds in an equal state of germination were planted, +some on opposite sides of a single pot, and some in the open ground. The +young crossed plants in the pot at first exceeded by a little in height +the self-fertilised; then equalled them; were then beaten; and lastly +were again victorious. The plants, without being disturbed, were turned +out of the pot, and planted in the open ground; and after growing for +some time, the crossed plants, which were all of nearly the same height, +exceeded the self-fertilised ones by 2 inches. When they flowered, the +flower-stems of the tallest crossed plant exceeded that of the tallest +self-fertilised plant by 6 inches. The other seedlings which were +planted in the open ground stood separate, so that they did not compete +with one another; nevertheless the crossed plants certainly grew to a +rather greater height than the self-fertilised; but no measurements were +made. The crossed plants which had been raised in the pot, and those +planted in the open ground, all flowered a little before the +self-fertilised plants. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION. + +Some flowers on the crossed plants of the last generation were again +crossed with pollen from another crossed plant, and produced fine +capsules. The flowers on the self-fertilised plants of the last +generation were allowed to fertilise themselves spontaneously under a +net, and they produced some remarkably fine capsules. The two lots of +seeds thus produced germinated on sand, and eight pairs were planted on +opposite sides of four pots. These plants were measured to the tips of +their leaves on the 20th of October of the same year, and the eight +crossed plants averaged in height 8.4 inches, whilst the self-fertilised +averaged 8.53 inches, so that the crossed were a little inferior in +height, as 100 to 101.5. By the 5th of June of the following year these +plants had grown much bulkier, and had begun to form heads. The crossed +had now acquired a marked superiority in general appearance, and +averaged 8.02 inches in height, whilst the self-fertilised averaged 7.31 +inches; or as 100 to 91. The plants were then turned out of their pots +and planted undisturbed in the open ground. By the 5th of August their +heads were fully formed, but several had grown so crooked that their +heights could hardly be measured with accuracy. The crossed plants, +however, were on the whole considerably taller than the self-fertilised. +In the following year they flowered; the crossed plants flowering before +the self-fertilised in three of the pots, and at the same time in Pot 2. +The flower-stems were now measured, as shown in Table 4/29. + +TABLE 3/29. Brassica oleracea. + +Measured in inches to tops of flower-stems: 0 signifies that a +Flower-stem was not formed. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 49 2/8 : 44. +Pot 1 : 39 4/8 : 41. + +Pot 2 : 37 4/8 : 38. +Pot 2 : 33 4/8 : 35 4/8. + +Pot 3 : 47 : 51 1/8. +Pot 3 : 40 : 41 2/8. +Pot 3 : 42 : 46 4/8. + +Pot 4 : 43 6/8 : 20 2/8. +Pot 4 : 37 2/8 : 33 3/8. +Pot 4 : 0 : 0. + +Total : 369.75 : 351.00. + +The nine flower-stems on the crossed plants here average 41.08 inches, +and the nine on the self-fertilised plants 39 inches in height, or as +100 to 95. But this small difference, which, moreover, depended almost +wholly on one of the self-fertilised plants being only 20 inches high, +does not in the least show the vast superiority of the crossed over the +self-fertilised plants. Both lots, including the two plants in Pot 4, +which did not flower, were now cut down close to the ground and weighed, +but those in Pot 2 were excluded, for they had been accidentally injured +by a fall during transplantation, and one was almost killed. The eight +crossed plants weighed 219 ounces, whilst the eight self-fertilised +plants weighed only 82 ounces, or as 100 to 37; so that the superiority +of the former over the latter in weight was great. + +THE EFFECTS OF A CROSS WITH A FRESH STOCK. + +Some flowers on a crossed plant of the last or second generation were +fertilised, without being castrated, by pollen taken from a plant of the +same variety, but not related to my plants, and brought from a nursery +garden (whence my seeds originally came) having a different soil and +aspect. The flowers on the self-fertilised plants of the last or second +generation (Table 4/29) were allowed to fertilise themselves +spontaneously under a net, and yielded plenty of seeds. These latter and +the crossed seeds, after germinating on sand, were planted in pairs on +the opposite sides of six large pots, which were kept at first in a cool +greenhouse. Early in January their heights were measured to the tips of +their leaves. The thirteen crossed plants averaged 13.16 inches in +height, and the twelve (for one had died) self-fertilised plants +averaged 13.7 inches, or as 100 to 104; so that the self-fertilised +plants exceeded by a little the crossed plants. + +TABLE 3/30. Brassica oleracea. + +Weights in ounces of plants after they had formed heads. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants from Pollen of fresh Stock. + +Column 3: Self-fertilised Plants of the Third Generation. + +Pot 1 : 130 : 18 2/4. + +Pot 2 : 74 : 34 3/4. + +Pot 3 : 121 : 17 2/4. + +Pot 4 : 127 2/4 : 14. + +Pot 5 : 90 : 11 2/4. + +Pot 6 : 106 2/4 : 46. + +Total : 649.00 : 142.25. + +Early in the spring the plants were gradually hardened, and turned out +of their pots into the open ground without being disturbed. By the end +of August the greater number had formed fine heads, but several grew +extremely crooked, from having been drawn up to the light whilst in the +greenhouse. As it was scarcely possible to measure their heights, the +finest plant on each side of each pot was cut down close to the ground +and weighed. In Table 4/30 we have the result. + +The six finest crossed plants average 108.16 ounces, whilst the six +finest self-fertilised plants average only 23.7 ounces, or as 100 to 22. +This difference shows in the clearest manner the enormous benefit which +these plants derived from a cross with another plant belonging to the +same sub-variety, but to a fresh stock, and grown during at least the +three previous generations under somewhat different conditions. + +THE OFFSPRING FROM A CUT-LEAVED, CURLED, AND VARIEGATED WHITE-GREEN +CABBAGE CROSSED WITH A CUT-LEAVED, CURLED, AND VARIEGATED CRIMSON-GREEN +CABBAGE, COMPARED WITH THE SELF-FERTILISED OFFSPRING FROM THE TWO +VARIETIES. + +These trials were made, not for the sake of comparing the growth of the +crossed and self-fertilised seedlings, but because I had seen it stated +that these varieties would not naturally intercross when growing +uncovered and near one another. This statement proved quite erroneous; +but the white-green variety was in some degree sterile in my garden, +producing little pollen and few seeds. It was therefore no wonder that +seedlings raised from the self-fertilised flowers of this variety were +greatly exceeded in height by seedlings from a cross between it and the +more vigorous crimson-green variety; and nothing more need be said about +this experiment. + +The seedlings from the reciprocal cross, that is, from the crimson-green +variety fertilised with pollen from the white-green variety, offer a +somewhat more curious case. A few of these crossed seedlings reverted to +a pure green variety with their leaves less cut and curled, so that they +were altogether in a much more natural state, and these plants grew more +vigorously and taller than any of the others. Now it is a strange fact +that a much larger number of the self-fertilised seedlings from the +crimson-green variety than of the crossed seedlings thus reverted; and +as a consequence the self-fertilised seedlings grew taller by 2 1/2 +inches on an average than the crossed seedlings, with which they were +put into competition. At first, however, the crossed seedlings exceeded +the self-fertilised by an average of a quarter of an inch. We thus see +that reversion to a more natural condition acted more powerfully in +favouring the ultimate growth of these plants than did a cross; but it +should be remembered that the cross was with a semi-sterile variety +having a feeble constitution. + +Iberis umbellata. + +VAR. KERMESIANA. + +This variety produced plenty of spontaneously self-fertilised seed under +a net. Other plants in pots in the greenhouse were left uncovered, and +as I saw small flies visiting the flowers, it seemed probable that they +would be intercrossed. Consequently seeds supposed to have been thus +crossed and spontaneously self-fertilised seeds were sown on opposite +sides of a pot. The self-fertilised seedlings grew from the first +quicker than the supposed crossed seedlings, and when both lots were in +full flower the former were from 5 to 6 inches higher than the crossed! +I record in my notes that the self-fertilised seeds from which these +self-fertilised plants were raised were not so well ripened as the +crossed; and this may possibly have caused the great difference in their +growth, in a somewhat analogous manner as occurred with the +self-fertilised plants of the eighth generation of Ipomoea raised from +unhealthy parents. It is a curious circumstance, that two other lots of +the above seeds were sown in pure sand mixed with burnt earth, and +therefore without any organic matter; and here the supposed crossed +seedlings grew to double the height of the self-fertilised, before both +lots died, as necessarily occurred at an early period. We shall +hereafter meet with another case apparently analogous to this of Iberis +in the third generation of Petunia. + +The above self-fertilised plants were allowed to fertilise themselves +again under a net, yielding self-fertilised plants of the second +generation, and the supposed crossed plants were crossed by pollen of a +distinct plant; but from want of time this was done in a careless +manner, namely, by smearing one head of expanded flowers over another. I +should have thought that this would have succeeded, and perhaps it did +so; but the fact of 108 of the self-fertilised seeds weighing 4.87 +grains, whilst the same number of the supposed crossed seeds weighed +only 3.57 grains, does not look like it. Five seedlings from each lot of +seeds were raised, and the self-fertilised plants, when fully grown, +exceeded in average height by a trifle (namely .4 of an inch) the five +probably crossed plants. I have thought it right to give this case and +the last, because had the supposed crossed plants proved superior to the +self-fertilised in height, I should have assumed without doubt that the +former had really been crossed. As it is, I do not know what to +conclude. + +Being much surprised at the two foregoing trials, I determined to make +another, in which there should be no doubt about the crossing. I +therefore fertilised with great care (but as usual without castration) +twenty-four flowers on the supposed crossed plants of the last +generation with pollen from distinct plants, and thus obtained +twenty-one capsules. The self-fertilised plants of the last generation +were allowed to fertilise themselves again under a net, and the +seedlings reared from these seeds formed the third self-fertilised +generation. Both lots of seeds, after germinating on bare sand, were +planted in pairs on the opposite sides of two pots. All the remaining +seeds were sown crowded on opposite sides of a third pot; but as all the +self-fertilised seedlings in this latter pot died before they grew to +any considerable height, they were not measured. The plants in Pots 1 +and 2 were measured when between 7 and 8 inches in height, and the +crossed exceeded the self-fertilised in average height by 1.57 inches. +When fully grown they were again measured to the summits of their +flower-heads, with the following result:-- + +TABLE 4/31. Iberis umbellata. + +Heights of plants to the summits of their flower-heads, in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants of the Third Generation. + +Pot 1 : 18 : 19. +Pot 1 : 21 : 21. +Pot 1 : 18 2/8 : 19 4/8. + +Pot 2 : 19 : 16 6/8. +Pot 2 : 18 4/8 : 7 4/8. +Pot 2 : 17 6/8 : 14 4/8. +Pot 2 : 21 3/8 : 16 4/8. + +Total : 133.88 : 114.75. + +The average height of the seven crossed plants is here 19.12 inches, and +that of the seven self-fertilised plants 16.39, or as 100 to 86. But as +the plants on the self-fertilised side grew very unequally, this ratio +cannot be fully trusted, and is probably too high. In both pots a +crossed plant flowered before any one of the self-fertilised. These +plants were left uncovered in the greenhouse; but from being too much +crowded they were not very productive. The seeds from all seven plants +of both lots were counted; the crossed produced 206, and the +self-fertilised 154; or as 100 to 75. + +CROSS BY A FRESH STOCK. + +From the doubts caused by the two first trials, in which it was not +known with certainty that the plants had been crossed; and from the +crossed plants in the last experiment having been put into competition +with plants self-fertilised for three generations, which moreover grew +very unequally, I resolved to repeat the trial on a larger scale, and in +a rather different manner. I obtained seeds of the same crimson variety +of Iberis umbellata from another nursery garden, and raised plants from +them. Some of these plants were allowed to fertilise themselves +spontaneously under a net; others were crossed by pollen taken from +plants raised from seed sent me by Dr. Durando from Algiers, where the +parent-plants had been cultivated for some generations. These latter +plants differed in having pale pink instead of crimson flowers, but in +no other respect. That the cross had been effective (though the flowers +on the crimson mother-plant had NOT been castrated) was well shown when +the thirty crossed seedlings flowered, for twenty-four of them produced +pale pink flowers, exactly like those of their father; the six others +having crimson flowers exactly like those of their mother and like those +of all the self-fertilised seedlings. This case offers a good instance +of a result which not rarely follows from crossing varieties having +differently coloured flowers; namely, that the colours do not blend, but +resemble perfectly those either of the father or mother plant. The seeds +of both lots, after germinating on sand, were planted on opposite sides +of eight pots. When fully grown, the plants were measured to the summits +of the flower-heads, as shown in Table 4/32. + +TABLE 4/32. Iberis umbellata. + +Height of Plants to the summits of the flower-heads, measured in inches: +0 signifies that the Plant died. + +Column 1: Number (Name) of Pot. + +Column 2: Plants from a Cross with a fresh Stock. + +Column 3: Plants from Spontaneously Self-fertilised Seeds. + +Pot 1 : 18 6/8 : 17 3/8. +Pot 1 : 17 5/8 : 16 7/8. +Pot 1 : 17 6/8 : 13 1/8. +Pot 1 : 20 1/8 : 15 3/8. + +Pot 2 : 20 2/8 : 0. +Pot 2 : 15 7/8 : 16 6/8. +Pot 2 : 17 : 15 2/8. + +Pot 3 : 19 2/8 : 13 6/8. +Pot 3 : 18 1/8 : 14 2/8. +Pot 3 : 15 2/8 : 13 4/8. + +Pot 4 : 17 1/8 : 16 4/8. +Pot 4 : 18 7/8 : 14 4/8. +Pot 4 : 17 5/8 : 16. +Pot 4 : 15 6/8 : 15 3/8. +Pot 4 : 14 4/8 : 14 7/8. + +Pot 5 : 18 1/8 : 16 4/8. +Pot 5 : 14 7/8 : 16 2/8. +Pot 5 : 16 2/8 : 14 2/8. +Pot 5 : 15 5/8 : 14 2/8. +Pot 5 : 12 4/8 : 16 1/8. + +Pot 6 : 18 6/8 : 16 1/8. +Pot 6 : 18 6/8 : 15. +Pot 6 : 17 3/8 : 15 2/8. + +Pot 7 : 18 : 16 3/8. +Pot 7 : 16 4/8 : 14 4/8. +Pot 7 : 18 2/8 : 13 5/8. + +Pot 8 : 20 6/8 : 15 6/8. +Pot 8 : 17 7/8 : 16 3/8. +Pot 8 : 13 5/8 : 20 2/8. +Pot 8 : 19 2/8 : 15 6/8. + +Total : 520.38 : 449.88. + +The average height of the thirty crossed plants is here 17.34, and that +of the twenty-nine self-fertilised plants (one having died) 15.51, or as +100 to 89. I am surprised that the difference did not prove somewhat +greater, considering that in the last experiment it was as 100 to 86; +but this latter ratio, as before explained, was probably too great. It +should, however, be observed that in the last experiment (Table 4/31), +the crossed plants competed with plants of the third self-fertilised +generation; whilst in the present case, plants derived from a cross with +a fresh stock competed with self-fertilised plants of the first +generation. + +The crossed plants in the present case, as in the last, were more +fertile than the self-fertilised, both lots being left uncovered in the +greenhouse. The thirty crossed plants produced 103 seed-bearing +flowers-heads, as well as some heads which yielded no seeds; whereas the +twenty-nine self-fertilised plants produced only 81 seed-bearing heads; +therefore thirty such plants would have produced 83.7 heads. We thus get +the ratio of 100 to 81, for the number of seed-bearing flower-heads +produced by the crossed and self-fertilised plants. Moreover, a number +of seed-bearing heads from the crossed plants, compared with the same +number from the self-fertilised, yielded seeds by weight, in the ratio +of 100 to 92. Combining these two elements, namely, the number of +seed-bearing heads and the weight of seeds in each head, the +productiveness of the crossed to the self-fertilised plants was as 100 +to 75. + +The crossed and self-fertilised seeds, which remained after the above +pairs had been planted, (some in a state of germination and some not +so), were sown early in the year out of doors in two rows. Many of the +self-fertilised seedlings suffered greatly, and a much larger number of +them perished than of the crossed. In the autumn the surviving +self-fertilised plants were plainly less well-grown than the crossed +plants. + +7. PAPAVERACEAE.--Papaver vagum. + +A SUB-SPECIES OF Papaver dubium, FROM THE SOUTH OF FRANCE. + +The poppy does not secrete nectar, but the flowers are highly +conspicuous and are visited by many pollen-collecting bees, flies and +beetles. The anthers shed their pollen very early, and in the case of +Papaver rhoeas, it falls on the circumference of the radiating stigmas, +so that this species must often be self-fertilised; but with Papaver +dubium the same result does not follow (according to H. Muller 'Die +Befruchtung' page 128), owing to the shortness of the stamens, unless +the flower happens to stand inclined. The present species, therefore, +does not seem so well fitted for self-fertilisation as most of the +others. Nevertheless Papaver vagum produced plenty of capsules in my +garden when insects were excluded, but only late in the season. I may +here add that Papaver somniferum produces an abundance of spontaneously +self-fertilised capsules, as Professor H. Hoffmann likewise found to be +the case. (4/2. 'Zur Speciesfrage' 1875 page 53.) Some species of +Papaver cross freely when growing in the same garden, as I have known to +be the case with Papaver bracteatum and orientale. + +Plants of Papaver vagum were raised from seeds sent me from Antibes +through the kindness of Dr. Bornet. Some little time after the flowers +had expanded, several were fertilised with their own pollen, and others +(not castrated) with pollen from a distinct individual; but I have +reason to believe, from observations subsequently made, that these +flowers had been already fertilised by their own pollen, as this process +seems to take place soon after their expansion. (4/3. Mr. J. Scott found +'Report on the Experimental Culture of the Opium Poppy' Calcutta 1874 +page 47, in the case of Papaver somniferum, that if he cut away the +stigmatic surface before the flower had expanded, no seeds were +produced; but if this was done "on the second day, or even a few hours +after the expansion of the flower on the first day, a partial +fertilisation had already been effected, and a few good seeds were +almost invariably produced." This proves at how early a period +fertilisation takes place.) I raised, however, a few seedlings of both +lots, and the self-fertilised rather exceeded the crossed plants in +height. + +Early in the following year I acted differently, and fertilised seven +flowers, very soon after their expansion, with pollen from another +plant, and obtained six capsules. From counting the seeds in a +medium-sized one, I estimated that the average number in each was at +least 120. Four out of twelve capsules, spontaneously self-fertilised at +the same time, were found to contain no good seeds; and the remaining +eight contained on an average 6.6 seeds per capsule. But it should be +observed that later in the season the same plants produced under a net +plenty of very fine spontaneously self-fertilised capsules. + +The above two lots of seeds, after germinating on sand, were planted in +pairs on opposite sides of five pots. The two lots of seedlings, when +half an inch in height, and again when 6 inches high, were measured to +the tips of their leaves, but presented no difference. When fully grown, +the flower-stalks were measured to the summits of the seed capsules, +with the following result:-- + +TABLE 4/33. Papaver vagum. + +Heights of flower-stalks to the summits of the seed capsules measured in +inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 24 2/8 : 21. +Pot 1 : 30 : 26 5/8. +Pot 1 : 18 4/8 : 16. + +Pot 2 : 14 4/8 : 15 3/8. +Pot 2 : 22 : 20 1/8. +Pot 2 : 19 5/8 : 14 1/8. +Pot 2 : 21 5/8 : 16 4/8. + +Pot 3 : 20 6/8 : 19 2/8. +Pot 3 : 20 2/8 : 13 2/8. +Pot 3 : 20 6/8 : 18. + +Pot 4 : 25 3/8 : 23 2/8. +Pot 4 : 24 2/8 : 23. + +Pot 5 : 20 : 18 3/8. +Pot 5 : 27 7/8 : 27. +Pot 5 : 19 : 21 2/8. + +Total : 328.75 : 293.13. + +The fifteen crossed plants here average 21.91 inches, and the fifteen +self-fertilised plants 19.54 inches in height, or as 100 to 89. These +plants did not differ in fertility, as far as could be judged by the +number of capsules produced, for there were seventy-five on the crossed +side and seventy-four on the self-fertilised side. + +Eschscholtzia californica. + +This plant is remarkable from the crossed seedlings not exceeding in +height or vigour the self-fertilised. On the other hand, a cross greatly +increases the productiveness of the flowers on the parent-plant, and is +indeed sometimes necessary in order that they should produce any seed; +moreover, plants thus derived are themselves much more fertile than +those raised from self-fertilised flowers; so that the whole advantage +of a cross is confined to the reproductive system. It will be necessary +for me to give this singular case in considerable detail. + +Twelve flowers on some plants in my flower-garden were fertilised with +pollen from distinct plants, and produced twelve capsules; but one of +these contained no good seed. The seeds of the eleven good capsules +weighed 17.4 grains. Eighteen flowers on the same plants were fertilised +with their own pollen and produced twelve good capsules, which contained +13.61 grains weight of seed. Therefore an equal number of crossed and +self-fertilised capsules would have yielded seed by weight as 100 to 71. +(4/4. Professor Hildebrand experimented on plants in Germany on a larger +scale than I did, and found them much more self-fertile. Eighteen +capsules, produced by cross-fertilisation, contained on an average +eighty-five seeds, whilst fourteen capsules from self-fertilised flowers +contained on an average only nine seeds; that is, as 100 to 11: 'Jahrb. +fur Wissen Botanik.' B. 7 page 467.) If we take into account of the fact +that a much greater proportion of flowers produced capsules when crossed +than when self-fertilised, the relative fertility of the crossed to the +self-fertilised flowers was as 100 to 52. Nevertheless these plants, +whilst still protected by the net, spontaneously produced a considerable +number of self-fertilised capsules. + +The seeds of the two lots after germinating on sand were planted in +pairs on the opposite sides of four large pots. At first there was no +difference in their growth, but ultimately the crossed seedlings +exceeded the self-fertilised considerably in height, as shown in Table +4/34. But I believe from the cases which follow that this result was +accidental, owing to only a few plants having been measured, and to one +of the self-fertilised plants having grown only to a height of 15 +inches. The plants had been kept in the greenhouse, and from being drawn +up to the light had to be tied to sticks in this and the following +trials. They were measured to the summits of their flower-stems. + +TABLE 4/34. Eschscholtzia californica. + +Heights of Plants to the summits of their flower-stems measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 33 4/8 : 25. + +Pot 2 : 34 2/8 : 35. + +Pot 3 : 29 : 27 2/8. + +Pot 4 : 22 : 15. + +Total : 118.75 : 102.25. + +The four crossed plants here average 29.68 inches, and the four +self-fertilised 25.56 in height; or as 100 to 86. The remaining seeds +were sown in a large pot in which a Cineraria had long been growing; and +in this case again the two crossed plants on the one side greatly +exceeded in height the two self-fertilised plants on the opposite side. +The plants in the above four pots from having been kept in the +greenhouse did not produce on this or any other similar occasion many +capsules; but the flowers on the crossed plants when again crossed were +much more productive than the flowers on the self-fertilised plants when +again self-fertilised. These plants after seeding were cut down and kept +in the greenhouse; and in the following year, when grown again, their +relative heights were reversed, as the self-fertilised plants in three +out of the four pots were now taller than and flowered before the +crossed plants. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION. + +The fact just given with respect to the growth of the cut-down plants +made me doubtful about my first trial, so I determined to make another +on a larger scale with crossed and self-fertilised seedlings raised from +the crossed and self-fertilised plants of the last generation. Eleven +pairs were raised and grown in competition in the usual manner; and now +the result was different, for the two lots were nearly equal during +their whole growth. It would therefore be superfluous to give a table of +their heights. When fully grown and measured, the crossed averaged +32.47, and the self-fertilised 32.81 inches in height; or as 100 to 101. +There was no great difference in the number of flowers and capsules +produced by the two lots when both were left freely exposed to the +visits of insects. + +PLANTS RAISED FROM BRAZILIAN SEED. + +Fritz Muller sent me from South Brazil seeds of plants which were there +absolutely sterile when fertilised with pollen from the same plant, but +were perfectly fertile when fertilised with pollen from any other plant. +The plants raised by me in England from these seeds were examined by +Professor Asa Gray, and pronounced to belong to E. Californica, with +which they were identical in general appearance. Two of these plants +were covered by a net, and were found not to be so completely +self-sterile as in Brazil. But I shall recur to this subject in another +part of this work. Here it will suffice to state that eight flowers on +these two plants, fertilised with pollen from another plant under the +net, produced eight fine capsules, each containing on an average about +eighty seeds. Eight flowers on these same plants, fertilised with their +own pollen, produced seven capsules, which contained on an average only +twelve seeds, with a maximum in one of sixteen seeds. Therefore the +cross-fertilised capsules, compared with the self-fertilised, yielded +seeds in the ratio of about 100 to 15. These plants of Brazilian +parentage differed also in a marked manner from the English plants in +producing extremely few spontaneously self-fertilised capsules under a +net. + +Crossed and self-fertilised seeds from the above plants, after +germinating on bare sand, were planted in pairs on the opposite sides of +five large pots. The seedlings thus raised were the grandchildren of the +plants which grew in Brazil; the parents having been grown in England. +As the grandparents in Brazil absolutely require cross-fertilisation in +order to yield any seeds, I expected that self-fertilisation would have +proved very injurious to these seedlings, and that the crossed ones +would have been greatly superior in height and vigour to those raised +from self-fertilised flowers. But the result showed that my anticipation +was erroneous; for as in the last experiment with plants of the English +stock, so in the present one, the self-fertilised plants exceeded the +crossed by a little in height. It will be sufficient to state that the +fourteen crossed plants averaged 44.64, and the fourteen self-fertilised +45.12 inches in height; or as 100 to 101. + +THE EFFECTS OF A CROSS WITH A FRESH STOCK. + +I now tried a different experiment. Eight flowers on the self-fertilised +plants of the last experiment (i.e., grandchildren of the plants which +grew in Brazil) were again fertilised with pollen from the same plant, +and produced five capsules, containing on an average 27.4 seeds, with a +maximum in one of forty-two seeds. The seedlings raised from these seeds +formed the second SELF-FERTILISED generation of the Brazilian stock. + +Eight flowers on one of the crossed plants of the last experiment were +crossed with pollen from another grandchild, and produced five capsules. +These contained on an average 31.6 seeds, with a maximum in one of +forty-nine seeds. The seedlings raised from these seeds may be called +the INTERCROSSED. + +Lastly, eight other flowers on the crossed plants of the last experiment +were fertilised with pollen from a plant of the English stock, growing +in my garden, and which must have been exposed during many previous +generations to very different conditions from those to which the +Brazilian progenitors of the mother-plant had been subjected. These +eight flowers produced only four capsules, containing on an average 63.2 +seeds, with a maximum in one of ninety. The plants raised from these +seeds may be called the ENGLISH-CROSSED. As far as the above averages +can be trusted from so few capsules, the English-crossed capsules +contained twice as many seeds as the intercrossed, and rather more than +twice as many as the self-fertilised capsules. The plants which yielded +these capsules were grown in pots in the greenhouse, so that their +absolute productiveness must not be compared with that of plants growing +out of doors. + +The above three lots of seeds, namely, the self-fertilised, +intercrossed, and English-crossed, were planted in an equal state of +germination (having been as usual sown on bare sand) in nine large pots, +each divided into three parts by superficial partitions. Many of the +self-fertilised seeds germinated before those of the two crossed lots, +and these were of course rejected. The seedlings thus raised are the +great-grandchildren of the plants which grew in Brazil. When they were +from 2 to 4 inches in height, the three lots were equal. They were +measured when four-fifths grown, and again when fully grown, and as +their relative heights were almost exactly the same at these two ages, I +will give only the last measurements. The average height of the nineteen +English-crossed plants was 45.92 inches; that of the eighteen +intercrossed plants (for one died), 43.38; and that of the nineteen +self-fertilised plants, 50.3 inches. So that we have the following +ratios in height:-- + +The English-crossed to the self-fertilised plants, as 100 to 109. + +The English-crossed to the intercrossed plants, as 100 to 94. + +The intercrossed to the self-fertilised plants, as 100 to 116. + +After the seed-capsules had been gathered, all these plants were cut +down close to the ground and weighed. The nineteen English crossed +plants weighed 18.25 ounces; the intercrossed plants (with their weight +calculated as if there had been nineteen) weighed 18.2 ounces; and the +nineteen self-fertilised plants, 21.5 ounces. We have therefore for the +weights of the three lots of plants the following ratios:-- + +The English-crossed to the self-fertilised plants, as 100 to 118. + +The English-crossed to the intercrossed plants, as 100 to 100. + +The intercrossed to the self-fertilised plants, as 100 to 118. + +We thus see that in weight, as in height, the self-fertilised plants had +a decided advantage over the English-crossed and intercrossed plants. + +The remaining seeds of the three kinds, whether or not in a state of +germination, were sown in three long parallel rows in the open ground; +and here again the self-fertilised seedlings exceeded in height by +between 2 and 3 inches the seedlings in the two other rows, which were +of nearly equal heights. The three rows were left unprotected throughout +the winter, and all the plants were killed, with the exception of two of +the self-fertilised; so that as far as this little bit of evidence goes, +some of the self-fertilised plants were more hardy than any of the +crossed plants of either lot. + +We thus see that the self-fertilised plants which were grown in the nine +pots were superior in height (as 116 to 100), and in weight (as 118 to +100), and apparently in hardiness, to the intercrossed plants derived +from a cross between the grandchildren of the Brazilian stock. The +superiority is here much more strongly marked than in the second trial +with the plants of the English stock, in which the self-fertilised were +to the crossed in height as 101 to 100. It is a far more remarkable +fact--if we bear in mind the effects of crossing plants with pollen from +a fresh stock in the cases of Ipomoea, Mimulus, Brassica, and +Iberis--that the self-fertilised plants exceeded in height (as 109 to +100), and in weight (as 118 to 100), the offspring of the Brazilian +stock crossed by the English stock; the two stocks having been long +subjected to widely different conditions. + +If we now turn to the fertility of the three lots of plants we find a +very different result. I may premise that in five out of the nine pots +the first plant which flowered was one of the English-crossed; in four +of the pots it was a self-fertilised plant; and in not one did an +intercrossed plant flower first; so that these latter plants were beaten +in this respect, as in so many other ways. The three closely adjoining +rows of plants growing in the open ground flowered profusely, and the +flowers were incessantly visited by bees, and certainly thus +intercrossed. The manner in which several plants in the previous +experiments continued to be almost sterile as long as they were covered +by a net, but set a multitude of capsules immediately that they were +uncovered, proves how effectually the bees carry pollen from plant to +plant. My gardener gathered, at three successive times, an equal number +of ripe capsules from the plants of the three lots, until he had +collected forty-five from each lot. It is not possible to judge from +external appearance whether or not a capsule contains any good seeds; so +that I opened all the capsules. Of the forty-five from the +English-crossed plants, four were empty; of those from the intercrossed, +five were empty; and of those from the self-fertilised, nine were empty. +The seeds were counted in twenty-one capsules taken by chance out of +each lot, and the average number of seeds in the capsules from the +English-crossed plants was 67; from the intercrossed, 56; and from the +self-fertilised, 48.52. It therefore follows that:-- + +The forty-five capsules (the four empty ones included) from the +English-crossed plants contained 2747 seeds. + +The forty-five capsules (the five empty ones included) from the +intercrossed plants contained 2240 seeds. + +The forty-five capsules (the nine empty ones included) from the +self-fertilised plants contained 1746.7 seeds. + +The reader should remember that these capsules are the product of +cross-fertilisation, effected by the bees; and that the difference in +the number of the contained seeds must depend on the constitution of the +plants;--that is, on whether they were derived from a cross with a +distinct stock, or from a cross between plants of the same stock, or +from self-fertilisation. From the above facts we obtain the following +ratios:-- + +Number of seeds contained in an equal number of naturally fertilised +capsules produced:-- + +By the English-crossed and self-fertilised plants, as 100 to 63. + +By the English-crossed and intercrossed plants, as 100 to 81. + +By the intercrossed and self-fertilised plants, as 100 to 78. + +But to have ascertained the productiveness of the three lots of plants, +it would have been necessary to know how many capsules were produced by +the same number of plants. The three long rows, however, were not of +quite equal lengths, and the plants were much crowded, so that it would +have been extremely difficult to have ascertained how many capsules were +produced by them, even if I had been willing to undertake so laborious a +task as to collect and count all the capsules. But this was feasible +with the plants grown in pots in the greenhouse; and although these were +much less fertile than those growing out of doors, their relative +fertility appeared, after carefully observing them, to be the same. The +nineteen plants of the English-crossed stock in the pots produced +altogether 240 capsules; the intercrossed plants (calculated as +nineteen) produced 137.22 capsules; and the nineteen self-fertilised +plants, 152 capsules. Now, knowing the number of seeds contained in +forty-five capsules of each lot, it is easy to calculate the relative +numbers of seeds produced by an equal number of the plants of the three +lots. + +Number of seeds produced by an equal number of naturally-fertilised +plants:-- + +Plants of English-crossed and self-fertilised parentage, as 100 to 40 +seeds. + +Plants of English-crossed and intercrossed parentage, as 100 to 45 +seeds. + +Plants of intercrossed and self-fertilised parentage, as 100 to 89 +seeds. + +The superiority in productiveness of the intercrossed plants (that is, +the product of a cross between the grandchildren of the plants which +grew in Brazil) over the self-fertilised, small as it is, is wholly due +to the larger average number of seeds contained in the capsules; for the +intercrossed plants produced fewer capsules in the greenhouse than did +the self-fertilised plants. The great superiority in productiveness of +the English-crossed over the self-fertilised plants is shown by the +larger number of capsules produced, the larger average number of +contained seeds, and the smaller number of empty capsules. As the +English-crossed and intercrossed plants were the offspring of crosses in +every previous generation (as must have been the case from the flowers +being sterile with their own pollen), we may conclude that the great +superiority in productiveness of the English-crossed over the +intercrossed plants is due to the two parents of the former having been +long subjected to different conditions. + +The English-crossed plants, though so superior in productiveness, were, +as we have seen, decidedly inferior in height and weight to the +self-fertilised, and only equal to, or hardly superior to, the +intercrossed plants. Therefore, the whole advantage of a cross with a +distinct stock is here confined to productiveness, and I have met with +no similar case. + +8. RESEDACEAE.--Reseda lutea. + +Seeds collected from wild plants growing in this neighbourhood were sown +in the kitchen-garden; and several of the seedlings thus raised were +covered with a net. Of these, some were found (as will hereafter be more +fully described) to be absolutely sterile when left to fertilise +themselves spontaneously, although plenty of pollen fell on their +stigmas; and they were equally sterile when artificially and repeatedly +fertilised with their own pollen; whilst other plants produced a few +spontaneously self-fertilised capsules. The remaining plants were left +uncovered, and as pollen was carried from plant to plant by the hive and +humble-bees which incessantly visit the flowers, they produced an +abundance of capsules. Of the necessity of pollen being carried from one +plant to another, I had ample evidence in the case of this species and +of R. odorata; for those plants, which set no seeds or very few as long +as they were protected from insects, became loaded with capsules +immediately that they were uncovered. + +Seeds from the flowers spontaneously self-fertilised under the net, and +from flowers naturally crossed by the bees, were sown on opposite sides +of five large pots. The seedlings were thinned as soon as they appeared +above ground, so that an equal number were left on the two sides. After +a time the pots were plunged into the open ground. The same number of +plants of crossed and self-fertilised parentage were measured up to the +summits of their flower-stems, with the result given in Table 4/35. +Those which did not produce flower-stems were not measured. + +TABLE 4/35. Reseda lutea, in pots. + +Heights of plants to the summits of the flower-stems measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 21 : 12 7/8. +Pot 1 : 14 2/8 : 16. +Pot 1 : 19 1/8 : 11 7/8. +Pot 1 : 7 : 15 2/8. +Pot 1 : 15 1/8 : 19 1/8. + +Pot 2 : 20 4/8 : 12 4/8. +Pot 2 : 17 3/8 : 16 2/8. +Pot 2 : 23 7/8 : 16 2/8. +Pot 2 : 17 1/8 : 13 3/8. +Pot 2 : 20 6/8 : 13 5/8. + +Pot 3 : 16 1/8 : 14 4/8. +Pot 3 : 17 6/8 : 19 4/8. +Pot 3 : 16 2/8 : 20 7/8. +Pot 3 : 10 : 7 7/8. +Pot 3 : 10 : 17 6/8. + +Pot 4 : 22 1/8 : 9. +Pot 4 : 19 : 11 4/8. +Pot 4 : 18 7/8 : 11. +Pot 4 : 16 4/8 : 16. +Pot 4 : 19 2/8 : 16 3/8. + +Pot 5 : 25 2/8 : 14 6/8. +Pot 5 : 22 : 16. +Pot 5 : 8 6/8 : 14 3/8. +Pot 5 : 14 2/8 : 14 2/8. + +Total : 412.25 : 350.86. + +The average height of the twenty-four crossed plants is here 17.17 +inches, and that of the same number of self-fertilised plants 14.61; or +as 100 to 85. Of the crossed plants all but five flowered, whilst +several of the self-fertilised did not do so. The above pairs, whilst +still in flower, but with some capsules already formed, were afterwards +cut down and weighed. The crossed weighed 90.5 ounces; and an equal +number of the self-fertilised only 19 ounces, or as 100 to 21; and this +is an astonishing difference. + +Seeds of the same two lots were also sown in two adjoining rows in the +open ground. There were twenty crossed plants in the one row and +thirty-two self-fertilised plants in the other row, so that the +experiment was not quite fair; but not so unfair as it at first appears, +for the plants in the same row were not crowded so much as seriously to +interfere with each other's growth, and the ground was bare on the +outside of both rows. These plants were better nourished than those in +the pots and grew to a greater height. The eight tallest plants in each +row were measured in the same manner as before, with the following +result:-- + +TABLE 4/36. Reseda lutea, growing in the open ground. + +Heights of plants to the summits of the flower-stems measured in inches. + +Column 1: Crossed Plants. + +Column 2: Self-fertilised Plants. + + 28 : 33 2/8. + 27 3/8 : 23. + 27 5/8 : 21 5/8. + 28 6/8 : 20 4/8. + 29 7/8 : 21 5/8. + 26 6/8 : 22. + 26 2/8 : 21 2/8. + 30 1/8 : 21 7/8. + +Total : 224.75 : 185.13 + +The average height of the crossed plants, whilst in full flower, was +here 28.09, and that of the self-fertilised 23.14 inches; or as 100 to +82. It is a singular fact that the tallest plant in the two rows, was +one of the self-fertilised. The self-fertilised plants had smaller and +paler green leaves than the crossed. All the plants in the two rows were +afterwards cut down and weighed. The twenty crossed plants weighed 65 +ounces, and twenty self-fertilised (by calculation from the actual +weight of the thirty-two self-fertilised plants) weighed 26.25 ounces; +or as 100 to 40. Therefore the crossed plants did not exceed in weight +the self-fertilised plants in nearly so great a degree as those growing +in the pots, owing probably to the latter having been subjected to more +severe mutual competition. On the other hand, they exceeded the +self-fertilised in height in a slightly greater degree. + +Reseda odorata. + +Plants of the common mignonette were raised from purchased seed, and +several of them were placed under separate nets. Of these some became +loaded with spontaneously self-fertilised capsules; others produced a +few, and others not a single one. It must not be supposed that these +latter plants produced no seed because their stigmas did not receive any +pollen, for they were repeatedly fertilised with pollen from the same +plant with no effect; but they were perfectly fertile with pollen from +any other plant. Spontaneously self-fertilised seeds were saved from one +of the highly self-fertile plants, and other seeds were collected from +the plants growing outside the nets, which had been crossed by the bees. +These seeds after germinating on sand were planted in pairs on the +opposite sides of five pots. The plants were trained up sticks, and +measured to the summits of their leafy stems--the flower-stems not being +included. We here have the result:-- + +TABLE 4/37. Reseda odorata (seedlings from a highly self-fertile plant). + +Heights of plants to the summits of the leafy stems, flower-stems not +included, measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 20 7/8 : 22 4/8. +Pot 1 : 34 7/8 : 28 5/8. +Pot 1 : 26 6/8 : 23 2/8. +Pot 1 : 32 6/8 : 30 4/8. + +Pot 2 : 34 3/8 : 28 5/8. +Pot 2 : 34 5/8 : 30 5/8. +Pot 2 : 11 6/8 : 23. +Pot 2 : 33 3/8 : 30 1/8. + +Pot 3 : 17 7/8 : 4 4/8. +Pot 3 : 27 : 25. +Pot 3 : 30 1/8 : 26 3/8. +Pot 3 : 30 2/8 : 25 1/8. + +Pot 4 : 21 5/8 : 22 6/8. +Pot 4 : 28 : 25 4/8. +Pot 4 : 32 5/8 : 15 1/8. +Pot 4 : 32 3/8 : 24 6/8. + +Pot 5 : 21 : 11 6/8. +Pot 5 : 25 2/8 : 19 7/8. +Pot 5 : 26 6/8 : 10 4/8. + +Total : 522.25 : 428.50. + +The average height of the nineteen crossed plants is here 27.48, and +that of the nineteen self-fertilised 22.55 inches; or as 100 to 82. All +these plants were cut down in the early autumn and weighed: the crossed +weighed 11.5 ounces, and the self-fertilised 7.75 ounces, or as 100 to +67. These two lots having been left freely exposed to the visits of +insects, did not present any difference to the eye in the number of +seed-capsules which they produced. + +The remainder of the same two lots of seeds were sown in two adjoining +rows in the open ground; so that the plants were exposed to only +moderate competition. The eight tallest on each side were measured, as +shown in Table 4/38. + +TABLE 4/38. Reseda odorata, growing in the open ground. + +Heights of plants measured in inches. + +Column 1: Crossed Plants. + +Column 2: Self-fertilised Plants. + + 24 4/8 : 26 5/8. + 27 2/8 : 25 7/8. + 24 : 25. + 26 6/8 : 28 3/8. + 25 : 29 7/8. + 26 2/8 : 25 7/8. + 27 2/8 : 26 7/8. + 25 1/8 : 28 2/8. + +Total : 206.13 : 216.75 + +The average height of the eight crossed plants is 25.76, and that of the +eight self-fertilised 27.09; or as 100 to 105. + +We here have the anomalous result of the self-fertilised plants being a +little taller than the crossed; of which fact I can offer no +explanation. It is of course possible, but not probable, that the labels +may have been interchanged by accident. + +Another experiment was now tried: all the self-fertilised capsules, +though very few in number, were gathered from one of the +semi-self-sterile plants under a net; and as several flowers on this +same plant had been fertilised with pollen from a distinct individual, +crossed seeds were thus obtained. I expected that the seedlings from +this semi-self-sterile plant would have profited in a higher degree from +a cross, than did the seedlings from the fully self-fertile plants. But +my anticipation was quite wrong, for they profited in a less degree. An +analogous result followed in the case of Eschscholtzia, in which the +offspring of the plants of Brazilian parentage (which were partially +self-sterile) did not profit more from a cross, than did the plants of +the far more self-fertile English stock. The above two lots of crossed +and self-fertilised seeds from the same plant of Reseda odorata, after +germinating on sand, were planted on opposite sides of five pots, and +measured as in the last case, with the result in Table 4/39. + +TABLE 4/39. Reseda odorata (seedlings from a semi-self-sterile plant). + +Heights of plants to the summits of the leafy stems, flower-stems not +included, measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 33 4/8 : 31. +Pot 1 : 30 6/8 : 28. +Pot 1 : 29 6/8 : 13 2/8. +Pot 1 : 20 : 32. + +Pot 2 : 22 : 21 6/8. +Pot 2 : 33 4/8 : 26 6/8. +Pot 2 : 31 2/8 : 25 2/8. +Pot 2 : 32 4/8 : 30 4/8. + +Pot 3 : 30 1/8 : 17 2/8. +Pot 3 : 32 1/8 : 29 6/8. +Pot 3 : 31 4/8 : 24 6/8. +Pot 3 : 32 2/8 : 34 2/8. + +Pot 4 : 19 1/8 : 20 6/8. +Pot 4 : 30 1/8 : 32 6/8. +Pot 4 : 24 3/8 : 31 4/8. +Pot 4 : 30 6/8 : 36 6/8. + +Pot 5 : 34 6/8 : 24 5/8. +Pot 5 : 37 1/8 : 34. +Pot 5 : 31 2/8 : 22 2/8. +Pot 5 : 33 : 37 1/8. + +Total : 599.75 : 554.25. + +The average height of the twenty crossed plants is here 29.98, and that +of the twenty self-fertilised 27.71 inches; or as 100 to 92. These +plants were then cut down and weighed; and the crossed in this case +exceeded the self-fertilised in weight by a mere trifle, namely, in the +ratio of 100 to 99. The two lots, left freely exposed to insects, seemed +to be equally fertile. + +The remainder of the seed was sown in two adjoining rows in the open +ground; and the eight tallest plants in each row were measured, with the +result in Table 4/40. + +TABLE 4/40. Reseda odorata, (seedlings from a semi-self-sterile plant, +planted in the open ground). + +Heights of plants measured in inches. + +Column 1: Crossed Plants. + +Column 2: Self-fertilised Plants. + + 28 2/8 : 22 3/8. + 22 4/8 : 24 3/8. + 25 7/8 : 23 4/8. + 25 3/8 : 21 4/8. + 29 4/8 : 22 5/8. + 27 1/8 : 27 3/8. + 22 4/8 : 27 3/8. + 26 2/8 : 19 2/8. + +Total : 207.38 : 188.38. + +The average height of the eight crossed plants is here 25.92, and that +of the eight self-fertilised plants 23.54 inches; or as 100 to 90. + +9. VIOLACEAE.--Viola tricolor. + +Whilst the flowers of the common cultivated heartsease are young, the +anthers shed their pollen into a little semi-cylindrical passage, formed +by the basal portion of the lower petal, and surrounded by papillae. The +pollen thus collected lies close beneath the stigma, but can seldom gain +access into its cavity, except by the aid of insects, which pass their +proboscides down this passage into the nectary. (4/5. The flowers of +this plant have been fully described by Sprengel, Hildebrand, Delpino, +and H. Muller. The latter author sums up all the previous observations +in his 'Befruchtung der Blumen' and in 'Nature' November 20, 1873 page +44. See also Mr. A.W. Bennett in 'Nature' May 15, 1873 page 50 and some +remarks by Mr. Kitchener ibid page 143. The facts which follow on the +effects of covering up a plant of V. tricolor have been quoted by Sir J. +Lubbock in his 'British Wild Flowers' etc. page 62.) Consequently when I +covered up a large plant of a cultivated variety, it set only eighteen +capsules, and most of these contained very few good seeds--several from +only one to three; whereas an equally fine uncovered plant of the same +variety, growing close by, produced 105 fine capsules. The few flowers +which produce capsules when insects are excluded, are perhaps fertilised +by the curling inwards of the petals as their wither, for by this means +pollen-grains adhering to the papillae might be inserted into the cavity +of the stigma. But it is more probable that their fertilisation is +effected, as Mr. Bennett suggests, by Thrips and certain minute beetles +which haunt the flowers, and which cannot be excluded by any net. +Humble-bees are the usual fertilisers; but I have more than once seen +flies (Rhingia rostrata) at work, with the under sides of their bodies, +heads and legs dusted with pollen; and having marked the flowers which +they visited, I found them after a few days fertilised. (4/6. I should +add that this fly apparently did not suck the nectar, but was attracted +by the papillae which surround the stigma. Hermann Muller also saw a +small bee, an Andrena, which could not reach the nectar, repeatedly +inserting its proboscis beneath the stigma, where the papillae are +situated; so that these papillae must be in some way attractive to +insects. A writer asserts 'Zoologist' volume 3-4 page 1225, that a moth +(Plusia) frequently visits the flowers of the pansy. Hive-bees do not +ordinarily visit them, but a case has been recorded 'Gardeners' +Chronicle' 1844 page 374, of these bees doing so. Hermann Muller has +also seen the hive-bee at work, but only on the wild small-flowered +form. He gives a list 'Nature' 1873 page 45, of all the insects which he +has seen visiting both the large and small-flowered forms. From his +account, I suspect that the flowers of plants in a state of nature are +visited more frequently by insects than those of the cultivated +varieties. He has seen several butterflies sucking the flowers of wild +plants, and this I have never observed in gardens, though I have watched +the flowers during many years.) It is curious for how long a time the +flowers of the heartsease and of some other plants may be watched +without an insect being seen to visit them. During the summer of 1841, I +observed many times daily for more than a fortnight some large clumps of +heartsease growing in my garden, before I saw a single humble-bee at +work. During another summer I did the same, but at last saw some +dark-coloured humble-bees visiting on three successive days almost every +flower in several clumps; and almost all these flowers quickly withered +and produced fine capsules. I presume that a certain state of the +atmosphere is necessary for the secretion of nectar, and that as soon as +this occurs the insects discover the fact by the odour emitted, and +immediately frequent the flowers. + +As the flowers require the aid of insects for their complete +fertilisation, and as they are not visited by insects nearly so often as +most other nectar-secreting flowers, we can understand the remarkable +fact discovered by H. Muller and described by him in 'Nature,' namely, +that this species exists under two forms. One of these bears conspicuous +flowers, which, as we have seen, require the aid of insects, and are +adapted to be cross-fertilised by them; whilst the other form has much +smaller and less conspicuously coloured flowers, which are constructed +on a slightly different plan, favouring self-fertilisation, and are thus +adapted to ensure the propagation of the species. The self-fertile form, +however, is occasionally visited, and may be crossed by insects, though +this is rather doubtful. + +In my first experiments on Viola tricolor I was unsuccessful in raising +seedlings, and obtained only one full-grown crossed and self-fertilised +plant. The former was 12 1/2 inches and the latter 8 inches in height. +On the following year several flowers on a fresh plant were crossed with +pollen from another plant, which was known to be a distinct seedling; +and to this point it is important to attend. Several other flowers on +the same plant were fertilised with their own pollen. The average number +of seeds in the ten crossed capsules was 18.7, and in the twelve +self-fertilised capsules 12.83; or as 100 to 69. These seeds, after +germinating on bare sand, were planted in pairs on the opposite sides of +five pots. They were first measured when about a third of their full +size, and the crossed plants then averaged 3.87 inches, and the +self-fertilised only 2.00 inches in height; or as 100 to 52. They were +kept in the greenhouse, and did not grow vigorously. Whilst in flower +they were again measured to the summits of their stems (see Table 4/41), +with the following result:-- + +TABLE 4/41. Viola tricolor. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 8 2/8 : 0 2/8. +Pot 1 : 7 4/8 : 2 4/8. +Pot 1 : 5 : 1 2/8. + +Pot 2 : 5 : 6. +Pot 2 : 4 : 4. +Pot 2 : 4 4/8 : 3 1/8. + +Pot 3 : 9 4/8 : 3 1/8. +Pot 3 : 3 3/8 : 1 7/8. +Pot 3 : 8 4/8 : 0 5/8. + +Pot 4 : 4 7/8 : 2 1/8. +Pot 4 : 4 2/8 : 1 6/8. +Pot 4 : 4 : 2 1/8. + +Pot 5 : 6 : 3. +Pot 5 : 3 3/8 : 1 4/8. + +Total : 78.13 : 33.25. + +The average height of the fourteen crossed plants is here 5.58 inches, +and that of the fourteen self-fertilised 2.37; or as 100 to 42. In four +out of the five pots, a crossed plant flowered before any one of the +self-fertilised; as likewise occurred with the pair raised during the +previous year. These plants without being disturbed were now turned out +of their pots and planted in the open ground, so as to form five +separate clumps. Early in the following summer (1869) they flowered +profusely, and being visited by humble-bees set many capsules, which +were carefully collected from all the plants on both sides. The crossed +plants produced 167 capsules, and the self-fertilised only 17; or as 100 +to 10. So that the crossed plants were more than twice the height of the +self-fertilised, generally flowered first, and produced ten times as +many naturally fertilised capsules. + +By the early part of the summer of 1870 the crossed plants in all the +five clumps had grown and spread so much more than the self-fertilised, +that any comparison between them was superfluous. The crossed plants +were covered with a sheet of bloom, whilst only a single self-fertilised +plant, which was much finer than any of its brethren, flowered. The +crossed and self-fertilised plants had now grown all matted together on +the respective sides of the superficial partitions still separating +them; and in the clump which included the finest self-fertilised plant, +I estimated that the surface covered by the crossed plants was about +nine times as large as that covered by the self-fertilised plants. The +extraordinary superiority of the crossed over the self-fertilised plants +in all five clumps, was no doubt due to the crossed plants at first +having had a decided advantage over the self-fertilised, and then +robbing them more and more of their food during the succeeding seasons. +But we should remember that the same result would follow in a state of +nature even to a greater degree; for my plants grew in ground kept clear +of weeds, so that the self-fertilised had to compete only with the +crossed plants; whereas the whole surface of the ground is naturally +covered with various kinds of plants, all of which have to struggle +together for existence. + +The ensuing winter was very severe, and in the following spring (1871) +the plants were again examined. All the self-fertilised were now dead, +with the exception of a single branch on one plant, which bore on its +summit a minute rosette of leaves about as large as a pea. On the other +hand, all the crossed plants without exception were growing vigorously. +So that the self-fertilised plants, besides their inferiority in other +respects, were more tender. + +Another experiment was now tried for the sake of ascertaining how far +the superiority of the crossed plants, or to speak more correctly, the +inferiority of the self-fertilised plants, would be transmitted to their +offspring. The one crossed and one self-fertilised plant, which were +first raised, had been turned out of their pot and planted in the open +ground. Both produced an abundance of very fine capsules, from which +fact we may safely conclude that they had been cross-fertilised by +insects. Seeds from both, after germinating on sand, were planted in +pairs on the opposite sides of three pots. The naturally crossed +seedlings derived from the crossed plants flowered in all three pots +before the naturally crossed seedlings derived from the self-fertilised +plants. When both lots were in full flower, the two tallest plants on +each side of each pot were measured, and the result is shown in Table +4/42. + +TABLE 4/42. Viola tricolor: seedlings from crossed and self-fertilised +plants, the parents of both sets having been left to be naturally +fertilised. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Naturally Crossed Plants from artificially crossed Plants. + +Column 3: Naturally Crossed Plants from Self-fertilised Plants. + +Pot 1 : 12 1/8 : 9 6/8. +Pot 1 : 11 6/8 : 8 3/8. + +Pot 2 : 13 2/8 : 9 6/8. +Pot 2 : 10 : 11 4/8. + +Pot 3 : 14 4/8 : 11 1/8. +Pot 3 : 13 6/8 : 11 3/8. + +Total : 75.38 : 61.88. + +The average height of the six tallest plants derived from the crossed +plants is 12.56 inches; and that of the six tallest plants derived from +the self-fertilised plants is 10.31 inches; or as 100 to 82. We here see +a considerable difference in height between the two sets, though very +far from equalling that in the previous trials between the offspring +from crossed and self-fertilised flowers. This difference must be +attributed to the latter set of plants having inherited a weak +constitution from their parents, the offspring of self-fertilised +flowers; notwithstanding that the parents themselves had been freely +intercrossed with other plants by the aid of insects. + +10. RANUNCULACEAE.--Adonis aestivalis. + +The results of my experiments on this plant are hardly worth giving, as +I remark in my notes made at the time, "seedlings, from some unknown +cause, all miserably unhealthy." Nor did they ever become healthy; yet I +feel bound to give the present case, as it is opposed to the general +results at which I have arrived. Fifteen flowers were crossed and all +produced fruit, containing on an average 32.5 seeds; nineteen flowers +were fertilised with their own pollen, and they likewise all yielded +fruit, containing a rather larger average of 34.5 seeds; or as 100 to +106. Seedlings were raised from these seeds. In one of the pots all the +self-fertilised plants died whilst quite young; in the two others, the +measurements were as follows: + +TABLE 4/43. Adonis aestivalis. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 14 : 13 4/8. +Pot 1 : 13 4/8 : 13 4/8. + +Pot 2 : 16 2/8 : 15 2/8. +Pot 2 : 13 2/8 : 15. + +Total : 57.00 : 57.25. + +The average height of the four crossed plants is 14.25, and that of the +four self-fertilised plants 14.31; or as 100 to 100.4; so that they were +in fact of equal height. According to Professor H. Hoffman, this plant +is proterandrous (4/7. 'Zur Speciesfrage' 1875 page 11.); nevertheless +it yields plenty of seeds when protected from insects. + +Delphinium consolida. + +It has been said in the case of this plant, as of so many others, that +the flowers are fertilised in the bud, and that distinct plants or +varieties can never naturally intercross. (4/8. Decaisne +'Comptes-Rendus' July 1863 page 5.) But this is an error, as we may +infer, firstly from the flowers being proterandrous,--the mature stamens +bending up, one after the other, into the passage which leads to the +nectary, and afterwards the mature pistils bending in the same +direction; secondly, from the number of humble-bees which visit the +flowers (4/9. Their structure is described by H. Muller 'Befruchtung' +etc., page 122.); and thirdly, from the greater fertility of the flowers +when crossed with pollen from a distinct plant than when spontaneously +self-fertilised. In the year 1863 I enclosed a large branch in a net, +and crossed five flowers with pollen from a distinct plant; these +yielded capsules containing on an average 35.2 very fine seeds, with a +maximum of forty-two in one capsule. Thirty-two other flowers on the +same branch produced twenty-eight spontaneously self-fertilised +capsules, containing on an average 17.2 seeds, with a maximum in one of +thirty-six seeds. But six of these capsules were very poor, yielding +only from one to five seeds; if these are excluded, the remaining +twenty-two capsules give an average of 20.9 seeds, though many of these +seeds were small. The fairest ratio, therefore, for the number of seeds +produced by a cross and by spontaneous self-fertilisation is as 100 to +59. These seeds were not sown, as I had too many other experiments in +progress. + +In the summer of 1867, which was a very unfavourable one, I again +crossed several flowers under a net with pollen from a distinct plant, +and fertilised other flowers on the same plant with their own pollen. +The former yielded a much larger proportion of capsules than the latter; +and many of the seeds in the self-fertilised capsules, though numerous, +were so poor that an equal number of seeds from the crossed and +self-fertilised capsules were in weight as 100 to 45. The two lots were +allowed to germinate on sand, and pairs were planted on the opposite +sides of four pots. When nearly two-thirds grown they were measured, as +shown in Table 4/44. + +TABLE 4/44. Delphinium consolida. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 11 : 11. + +Pot 2 : 19 : 16 2/8. +Pot 2 : 16 2/8 : 11 4/8. + +Pot 3 : 26 : 22. + +Pot 4 : 9 4/8 : 8 2/8. +Pot 4 : 8 : 6 4/8. + +Total : 89.75 : 75.50. + +The six crossed plants here average 14.95, and the six self-fertilised +12.50 inches in height; or as 100 to 84. When fully grown they were +again measured, but from want of time only a single plant on each side +was measured; so that I have thought it best to give the earlier +measurements. At the later period the three tallest crossed plants still +exceeded considerably in height the three tallest self-fertilised, but +not in quite so great a degree as before. The pots were left uncovered +in the greenhouse, but whether the flowers were intercrossed by bees or +self-fertilised I do not know. The six crossed plants produced 282 +mature and immature capsules, whilst the six self-fertilised plants +produced only 159; or as 100 to 56. So that the crossed plants were very +much more productive than the self-fertilised. + +11. CARYOPHYLLACEAE.--Viscaria oculata. + +Twelve flowers were crossed with pollen from another plant, and yielded +ten capsules, containing by weight 5.77 grains of seeds. Eighteen +flowers were fertilised with their own pollen and yielded twelve +capsules, containing by weight 2.63 grains. Therefore the seeds from an +equal number of crossed and self-fertilised flowers would have been in +weight as 100 to 38. I had previously selected a medium-sized capsule +from each lot, and counted the seeds in both; the crossed one contained +284, and the self-fertilised one 126 seeds; or as 100 to 44. These seeds +were sown on opposite sides of three pots, and several seedlings raised; +but only the tallest flower-stem of one plant on each side was measured. +The three on the crossed side averaged 32.5 inches, and the three on the +self-fertilised side 34 inches in height; or as 100 to 104. But this +trial was on much too small a scale to be trusted; the plants also grew +so unequally that one of the three flower-stems on the crossed plants +was very nearly twice as tall as that on one of the others; and one of +the three flower-stems on the self-fertilised plants exceeded in an +equal degree one of the others. + +In the following year the experiment was repeated on a larger scale: ten +flowers were crossed on a new set of plants and yielded ten capsules +containing by weight 6.54 grains of seed. Eighteen spontaneously +self-fertilised capsules were gathered, of which two contained no seed; +the other sixteen contained by weight 6.07 grains of seed. Therefore the +weight of seed from an equal number of crossed and spontaneously +self-fertilised flowers (instead of artificially fertilised as in the +previous case) was as 100 to 58. + +The seeds after germinating on sand were planted in pairs on the +opposite sides of four pots, with all the remaining seeds sown crowded +in the opposite sides of a fifth pot; in this latter pot only the +tallest plant on each side was measured. Until the seedlings had grown +about 5 inches in height no difference could be perceived in the two +lots. Both lots flowered at nearly the same time. When they had almost +done flowering, the tallest flower-stem on each plant was measured, as +shown in Table 4/45. + +TABLE 4/45. Viscaria oculata. + +Tallest flower-stem on each plant measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 19 : 32 3/8. +Pot 1 : 33 : 38. +Pot 1 : 41 : 38. +Pot 1 : 41 : 28 7/8. + +Pot 2 : 37 4/8 : 36. +Pot 2 : 36 4/8 : 32 3/8. +Pot 2 : 38 : 35 6/8. + +Pot 3 : 44 4/8 : 36. +Pot 3 : 39 4/8 : 20 7/8. +Pot 3 : 39 : 30 5/8. + +Pot 4 : 30 2/8 : 36. +Pot 4 : 31 : 39. +Pot 4 : 33 1/8 : 29. +Pot 4 : 24 : 38 4/8. + +Pot 5 : 30 2/8 : 32. +Crowded. + +Total : 517.63 : 503.36. + +The fifteen crossed plants here average 34.5, and the fifteen +self-fertilised 33.55 inches in height; or as 100 to 97. So that the +excess of height of the crossed plants is quite insignificant. In +productiveness, however, the difference was much more plainly marked. +All the capsules were gathered from both lots of plants (except from the +crowded and unproductive ones in Pot 5), and at the close of the season +the few remaining flowers were added in. The fourteen crossed plants +produced 381, whilst the fourteen self-fertilised plants produced only +293 capsules and flowers; or as 100 to 77. + +Dianthus caryophyllus. + +The common carnation is strongly proterandrous, and therefore depends to +a large extent upon insects for fertilisation. I have seen only +humble-bees visiting the flowers, but I dare say other insects likewise +do so. It is notorious that if pure seed is desired, the greatest care +is necessary to prevent the varieties which grow in the same garden from +intercrossing. (4/10. 'Gardeners' Chronicle' 1847 page 268.) The pollen +is generally shed and lost before the two stigmas in the same flower +diverge and are ready to be fertilised. I was therefore often forced to +use for self-fertilisation pollen from the same plant instead of from +the same flower. But on two occasions, when I attended to this point, I +was not able to detect any marked difference in the number of seeds +produced by these two forms of self-fertilisation. + +Several single-flowered carnations were planted in good soil, and were +all covered with a net. Eight flowers were crossed with pollen from a +distinct plant and yielded six capsules, containing on an average 88.6 +seeds, with a maximum in one of 112 seeds. Eight other flowers were +self-fertilised in the manner above described, and yielded seven +capsules containing on an average 82 seeds, with a maximum in one of 112 +seeds. So that there was very little difference in the number of seeds +produced by cross-fertilisation and self-fertilisation, namely, as 100 +to 92. As these plants were covered by a net, they produced +spontaneously only a few capsules containing any seeds, and these few +may perhaps be attributed to the action of Thrips and other minute +insects which haunt the flowers. A large majority of the spontaneously +self-fertilised capsules produced by several plants contained no seeds, +or only a single one. Excluding these latter capsules, I counted the +seeds in eighteen of the finest ones, and these contained on an average +18 seeds. One of the plants was spontaneously self-fertile in a higher +degree than any of the others. On another occasion a single covered-up +plant produced spontaneously eighteen capsules, but only two of these +contained any seed, namely 10 and 15. + +CROSSED AND SELF-FERTILISED PLANTS OF THE FIRST GENERATION. + +The many seeds obtained from the above crossed and artificially +self-fertilised flowers were sown out of doors, and two large beds of +seedlings, closely adjoining one another, thus raised. This was the +first plant on which I experimented, and I had not then formed any +regular scheme of operation. When the two lots were in full flower, I +measured roughly a large number of plants but record only that the +crossed were on an average fully 4 inches taller than the +self-fertilised. Judging from subsequent measurements, we may assume +that the crossed plants were about 28 inches, and the self-fertilised +about 24 inches in height; and this will give us a ratio of 100 to 86. +Out of a large number of plants, four of the crossed ones flowered +before any one of the self-fertilised plants. + +Thirty flowers on these crossed plants of the first generation were +again crossed with pollen from a distinct plant of the same lot, and +yielded twenty-nine capsules, containing on an average 55.62 seeds, with +a maximum in one of 110 seeds. + +Thirty flowers on the self-fertilised plants were again self-fertilised; +eight of them with pollen from the same flower, and the remainder with +pollen from another flower on the same plant; and these produced +twenty-two capsules, containing on an average 35.95 seeds, with a +maximum in one of sixty-one seeds. We thus see, judging by the number of +seeds per capsule, that the crossed plants again crossed were more +productive than the self-fertilised again self-fertilised, in the ratio +of 100 to 65. Both the crossed and self-fertilised plants, from having +grown much crowded in the two beds, produced less fine capsules and +fewer seeds than did their parents. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION. + +The crossed and self-fertilised seeds from the crossed and +self-fertilised plants of the last generation were sown on opposite +sides of two pots; but the seedlings were not thinned enough, so that +both lots grew very irregularly, and most of the self-fertilised plants +after a time died from being smothered. My measurements were, therefore, +very incomplete. From the first the crossed seedlings appeared the +finest, and when they were on an average, by estimation, 5 inches high, +the self-fertilised plants were only 4 inches. In both pots the crossed +plants flowered first. The two tallest flower-stems on the crossed +plants in the two pots were 17 and 16 1/2 inches in height; and the two +tallest flower-stems on the self-fertilised plants 10 1/2 and 9 inches; +so that their heights were as 100 to 58. But this ratio, deduced from +only two pairs, obviously is not in the least trustworthy, and would not +have been given had it not been otherwise supported. I state in my notes +that the crossed plants were very much more luxuriant than their +opponents, and seemed to be twice as bulky. This latter estimate may be +believed from the ascertained weights of the two lots in the next +generation. Some flowers on these crossed plants were again crossed with +pollen from another plant of the same lot, and some flowers on the +self-fertilised plants again self-fertilised; and from the seeds thus +obtained the plants of the next generation were raised. + +CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION. + +The seeds just alluded to were allowed to germinate on bare sand, and +were planted in pairs on the opposite sides of four pots. When the +seedlings were in full flower, the tallest stem on each plant was +measured to the base of the calyx. The measurements are given in Table +4/46. In Pot 1 the crossed and self-fertilised plants flowered at the +same time; but in the other three pots the crossed flowered first. These +latter plants also continued flowering much later in the autumn than the +self-fertilised. + +TABLE 4/46. Dianthus caryophyllus (third generation). + +Tallest flower-stem on each plant measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 28 6/8 : 30. +Pot 1 : 27 3/8 : 26. + +Pot 2 : 29 : 30 7/8. +Pot 2 : 29 4/8 : 27 4/8. + +Pot 3 : 28 4/8 : 31 6/8. +Pot 3 : 23 4/8 : 24 5/8. + +Pot 4 : 27 : 30. +Pot 4 : 33 4/8 : 25. + +Total : 227.13 : 225.75. + +The average height of the eight crossed plants is here 28.39 inches, and +of the eight self-fertilised 28.21; or as 100 to 99. So that there was +no difference in height worth speaking of; but in general vigour and +luxuriance there was an astonishing difference, as shown by their +weights. After the seed-capsules had been gathered, the eight crossed +and the eight self-fertilised plants were cut down and weighed; the +former weighed 43 ounces, and the latter only 21 ounces; or as 100 to +49. + +These plants were all kept under a net, so that the capsules which they +produced must have been all spontaneously self-fertilised. The eight +crossed plants produced twenty-one such capsules, of which only twelve +contained any seed, averaging 8.5 per capsule. On the other hand, the +eight self-fertilised plants produced no less than thirty-six capsules, +of which I examined twenty-five, and, with the exception of three, all +contained seeds, averaging 10.63 seeds per capsule. Thus the +proportional number of seeds per capsule produced by the plants of +crossed origin to those produced by the plants of self-fertilised origin +(both lots being spontaneously self-fertilised) was as 100 to 125. This +anomalous result is probably due to some of the self-fertilised plants +having varied so as to mature their pollen and stigmas more nearly at +the same time than is proper to the species; and we have already seen +that some plants in the first experiment differed from the others in +being slightly more self-fertile. + +THE EFFECTS OF A CROSS WITH A FRESH STOCK. + +Twenty flowers on the self-fertilised plants of the last or third +generation, in Table 4/46, were fertilised with their own pollen, but +taken from other flowers on the same plants. These produced fifteen +capsules, which contained (omitting two with only three and six seeds) +on an average 47.23 seeds, with a maximum of seventy in one. The +self-fertilised capsules from the self-fertilised plants of the first +generation yielded the much lower average of 35.95 seeds; but as these +latter plants grew extremely crowded, nothing can be inferred with +respect to this difference in their self-fertility. The seedlings raised +from the above seeds constitute the plants of the fourth self-fertilised +generation in Table 4/47. + +Twelve flowers on the same plants of the third self-fertilised +generation, in Table 4/46, were crossed with pollen from the crossed +plants in the same table. These crossed plants had been intercrossed for +the three previous generations; and many of them, no doubt, were more or +less closely inter-related, but not so closely as in some of the +experiments with other species; for several carnation plants had been +raised and crossed in the earlier generations. They were not related, or +only in a distant degree, to the self-fertilised plants. The parents of +both the self-fertilised and crossed plants had been subjected to as +nearly as possible the same conditions during the three previous +generations. The above twelve flowers produced ten capsules, containing +on an average 48.66 seeds, with a maximum in one of seventy-two seeds. +The plants raised from these seeds may be called the INTERCROSSED. + +Lastly, twelve flowers on the same self-fertilised plants of the third +generation were crossed with pollen from plants which had been raised +from seeds purchased in London. It is almost certain that the plants +which produced these seeds had grown under very different conditions to +those to which my self-fertilised and crossed plants had been subjected; +and they were in no degree related. The above twelve flowers thus +crossed all produced capsules, but these contained the low average of +37.41 seeds per capsule, with a maximum in one of sixty-four seeds. It +is surprising that this cross with a fresh stock did not give a much +higher average number of seeds; for, as we shall immediately see, the +plants raised from these seeds, which may be called the LONDON-CROSSED, +benefited greatly by the cross, both in growth and fertility. + +The above three lots of seeds were allowed to germinate on bare sand. +Many of the London-crossed germinated before the others, and were +rejected; and many of the intercrossed later than those of the other two +lots. The seeds after thus germinating were planted in ten pots, made +tripartite by superficial divisions; but when only two kinds of seeds +germinated at the same time, they were planted on the opposite sides of +other pots; and this is indicated by blank spaces in one of the three +columns in Table 4/47. A 0 in the table signifies that the seedling died +before it was measured; and a + signifies that the plant did not produce +a flower-stem, and therefore was not measured. It deserves notice that +no less than eight out of the eighteen self-fertilised plants either +died or did not flower; whereas only three out of the eighteen +intercrossed, and four out of the twenty London-crossed plants, were in +this predicament. The self-fertilised plants had a decidedly less +vigorous appearance than the plants of the other two lots, their leaves +being smaller and narrower. In only one pot did a self-fertilised plant +flower before one of the two kinds of crossed plants, between which +there was no marked difference in the period of flowering. The plants +were measured to the base of the calyx, after they had completed their +growth, late in the autumn. + +TABLE 4/47. Dianthus caryophyllus. + +Heights of plants to the base of the calyx, measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: London-Crossed Plants. + +Column 3: Intercrossed Plants. + +Column 4: Self-fertilised Plants. + +Pot 1 : 39 5/8 : 25 1/8 : 29 2/8. +Pot 1 : 30 7/8 : 21 6/8 : +. + +Pot 2 : 36 2/8 : : 22 3/8. +Pot 2 : 0 : : +. + +Pot 3 : 28 5/8 : 30 2/8 : . +Pot 3 : + : 23 1/8 : . + +Pot 4 : 33 4/8 : 35 5/8 : 30. +Pot 4 : 28 7/8 : 32 : 24 4/8. + +Pot 5 : 28 : 34 4/8 : +. +Pot 5 : 0 : 24 2/8 : +. + +Pot 6 : 32 5/8 : 24 7/8 : 30 3/8. +Pot 6 : 31 : 26 : 24 4/8. + +Pot 7 : 41 7/8 : 29 7/8 : 27 7/8. +Pot 7 : 34 7/8 : 26 4/8 : 27. + +Pot 8 : 34 5/8 : 29 : 26 6/8. +Pot 8 : 28 5/8 : 0 : +. + +Pot 9 : 25 5/8 : 28 5/8 : +. +Pot 9 : 0 : + : 0. + +Pot 10 : 38 : 28 4/8 : 22 7/8. +Pot 10 : 32 1/8 : + : 0. + +Total : 525.13 : 420.00 : 265.50. + +The average height of the sixteen London-crossed plants in Table 4/47 is +32.82 inches; that of the fifteen intercrossed plants, 28 inches; and +that of the ten self-fertilised plants, 26.55. + +So that in height we have the following ratios:-- + +The London-crossed to the self-fertilised as 100 to 81. + +The London-crossed to the intercrossed as 100 to 85. + +The intercrossed to the self-fertilised as 100 to 95. + +These three lots of plants, which it should be remembered were all +derived on the mother-side from plants of the third self-fertilised +generation, fertilised in three different ways, were left exposed to the +visits of insects, and their flowers were freely crossed by them. As the +capsules of each lot became ripe they were gathered and kept separate, +the empty or bad ones being thrown away. But towards the middle of +October, when the capsules could no longer ripen, all were gathered and +were counted, whether good or bad. The capsules were then crushed, and +the seed cleaned by sieves and weighed. For the sake of uniformity the +results are given from calculation, as if there had been twenty plants +in each lot. + +The sixteen London-crossed plants actually produced 286 capsules; +therefore twenty such plants would have produced 357.5 capsules; and +from the actual weight of the seeds, the twenty plants would have +yielded 462 grains weight of seeds. + +The fifteen intercrossed plants actually produced 157 capsules; +therefore twenty of them would have produced 209.3 capsules and the +seeds would have weighed 208.48 grains. + +The ten self-fertilised plants actually produced 70 capsules, therefore +twenty of them would have produced 140 capsules; and the seeds would +have weighed 153.2 grains. + +From these data we get the following ratios:-- + +NUMBER OF CAPSULES PRODUCED BY AN EQUAL NUMBER OF PLANTS OF THE THREE +LOTS. + +NUMBER OF CAPSULES: + +The London-crossed to the self-fertilised as 100 to 39. + +The London-crossed to the intercrossed as 100 to 45. + +The intercrossed to the self-fertilised as 100 to 67. + +WEIGHT OF SEEDS PRODUCED BY AN EQUAL NUMBER OF PLANTS OF THE THREE LOTS. + +WEIGHT OF SEED: + +The London-crossed to the self-fertilised as 100 to 33. + +The London-crossed to the intercrossed as 100 to 45. + +The intercrossed to the self-fertilised as 100 to 73. + +We thus see how greatly the offspring from the self-fertilised plants of +the third generation crossed by a fresh stock, had their fertility +increased, whether tested by the number of capsules produced or by the +weight of the contained seeds; this latter being the more trustworthy +method. Even the offspring from the self-fertilised plants crossed by +one of the crossed plants of the same stock, notwithstanding that both +lots had been long subjected to the same conditions, had their fertility +considerably increased, as tested by the same two methods. + +In conclusion it may be well to repeat in reference to the fertility of +these three lots of plants, that their flowers were left freely exposed +to the visits of insects and were undoubtedly crossed by them, as may be +inferred from the large number of good capsules produced. These plants +were all the offspring of the same mother-plants, and the strongly +marked difference in their fertility must be attributed to the nature of +the pollen employed in fertilising their parents; and the difference in +the nature of the pollen must be attributed to the different treatment +to which the pollen-bearing parents had been subjected during several +previous generations. + +COLOUR OF THE FLOWERS. + +The flowers produced by the self-fertilised plants of the last or fourth +generation were as uniform in tint as those of a wild species, being of +a pale pink or rose colour. Analogous cases with Mimulus and Ipomoea, +after several generations of self-fertilisation, have been already +given. The flowers of the intercrossed plants of the fourth generation +were likewise nearly uniform in colour. On the other hand, the flowers +of the London-crossed plants, or those raised from a cross with the +fresh stock which bore dark crimson flowers, varied extremely in colour, +as might have been expected, and as is the general rule with seedling +carnations. It deserves notice that only two or three of the +London-crossed plants produced dark crimson flowers like those of their +fathers, and only a very few of a pale pink like those of their mothers. +The great majority had their petals longitudinally and variously striped +with the two colours,--the groundwork tint being, however, in some cases +darker than that of the mother-plants. + +12. MALVACEAE.--Hibiscus africanus. + +Many flowers on this Hibiscus were crossed with pollen from a distinct +plant, and many others were self-fertilised. A rather larger +proportional number of the crossed than of the self-fertilised flowers +yielded capsules, and the crossed capsules contained rather more seeds. +The self-fertilised seeds were a little heavier than an equal number of +the crossed seeds, but they germinated badly, and I raised only four +plants of each lot. In three out of the four pots, the crossed plants +flowered first. + +TABLE 4/48. Hibiscus africanus. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 13 4/8 : 16 2/8. + +Pot 2 : 14 : 14. + +Pot 3 : 8 : 7. + +Pot 4 : 17 4/8 : 20 4/8. + +Total : 53.00 : 57.75. + +The four crossed plants average 13.25, and the four self-fertilised +14.43 inches in height; or as 100 to 109. Here we have the unusual case +of self-fertilised plants exceeding the crossed in height; but only four +pairs were measured, and these did not grow well or equally. I did not +compare the fertility of the two lots. + + + +CHAPTER V. + +GERANIACEAE, LEGUMINOSAE, ONAGRACEAE, ETC. + +Pelargonium zonale, a cross between plants propagated by cuttings does +no good. +Tropaeolum minus. +Limnanthes douglasii. +Lupinus luteus and pilosus. +Phaseolus multiflorus and vulgaris. +Lathyrus odoratus, varieties of, never naturally intercross in England. +Pisum sativum, varieties of, rarely intercross, but a cross between them +highly beneficial. +Sarothamnus scoparius, wonderful effects of a cross. +Ononis minutissima, cleistogene flowers of. +Summary on the Leguminosae. +Clarkia elegans. +Bartonia aurea. +Passiflora gracilis. +Apium petroselinum. +Scabiosa atropurpurea. +Lactuca sativa. +Specularia speculum. +Lobelia ramosa, advantages of a cross during two generations. +Lobelia fulgens. +Nemophila insignis, great advantages of a cross. +Borago officinalis. +Nolana prostrata. + +13. GERANIACEAE.--Pelargonium zonale. + +This plant, as a general rule, is strongly proterandrous, and is +therefore adapted for cross-fertilisation by the aid of insects. (5/1. +Mr. J. Denny, a great raiser of new varieties of pelargoniums, after +stating that this species is proterandrous, adds 'The Florist and +Pomologist' January 1872 page 11, "there are some varieties, especially +those with petals of a pink colour, or which possess a weakly +constitution, where the pistil expands as soon as or even before the +pollen-bag bursts, and in which also the pistil is frequently short, so +when it expands it is smothered as it were by the bursting anthers; +these varieties are great seeders, each pip being fertilised by its own +pollen. I would instance Christine as an example of this fact." We have +here an interesting case of variability in an important functional +point.) Some flowers on a common scarlet variety were self-fertilised, +and other flowers were crossed with pollen from another plant; but no +sooner had I done so, than I remembered that these plants had been +propagated by cuttings from the same stock, and were therefore parts in +a strict sense of the same individual. Nevertheless, having made the +cross I resolved to save the seeds, which, after germinating on sand, +were planted on the opposite sides of three pots. In one pot the +quasi-crossed plant was very soon and ever afterwards taller and finer +than the self-fertilised. In the two other pots the seedlings on both +sides were for a time exactly equal; but when the self-fertilised plants +were about 10 inches in height, they surpassed their antagonists by a +little, and ever afterwards showed a more decided and increasing +advantage; so that the self-fertilised plants, taken altogether, were +somewhat superior to the quasi-crossed plants. In this case, as in that +of the Origanum, if individuals which have been asexually propagated +from the same stock, and which have been long subjected to the same +conditions, are crossed, no advantage whatever is gained. + +Several flowers on another plant of the same variety were fertilised +with pollen from the younger flowers on the same plant, so as to avoid +using the old and long-shed pollen from the same flower, as I thought +that this latter might be less efficient than fresh pollen. Other +flowers on the same plant were crossed with fresh pollen from a plant +which, although closely similar, was known to have arisen as a distinct +seedling. The self-fertilised seeds germinated rather before the others; +but as soon as I got equal pairs they were planted on the opposite sides +of four pots. + +TABLE 5/49. Pelargonium zonale. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 22 3/8 : 25 5/8. +Pot 1 : 19 6/8 : 12 4/8. + +Pot 2 : 15 : 19 6/8. +Pot 2 : 12 2/8 : 22 3/8. + +Pot 3 : 30 5/8 : 19 4/8. +Pot 3 : 18 4/8 : 7 4/8. + +Pot 4 : 38 : 9 1/8. + +Total : 156.50 : 116.38. + +When the two lots of seedlings were between 4 and 5 inches in height +they were equal, excepting in Pot 4, in which the crossed plant was much +the tallest. When between 11 and 14 inches in height, they were measured +to the tips of their uppermost leaves; the crossed averaged 13.46, and +the self-fertilised 11.07 inches in height, or as 100 to 82. Five months +later they were again measured in the same manner, and the results are +given in Table 5/49. + +The seven crossed plants now averaged 22.35, and the seven +self-fertilised 16.62 inches in height, or as 100 to 74. But from the +great inequality of the several plants, the result is less trustworthy +than in most other cases. In Pot 2 the two self-fertilised plants always +had an advantage, except whilst quite young over the two crossed plants. + +As I wished to ascertain how these plants would behave during a second +growth, they were cut down close to the ground whilst growing freely. +The crossed plants now showed their superiority in another way, for only +one out of the seven was killed by the operation, whilst three of the +self-fertilised plants never recovered. There was, therefore, no use in +keeping any of the plants excepting those in Pots 1 and 3; and in the +following year the crossed plants in these two pots showed during their +second growth nearly the same relative superiority over the +self-fertilised plants as before. + +Tropaeolum minus. + +The flowers are proterandrous, and are manifestly adapted for +cross-fertilisation by insects, as shown by Sprengel and Delpino. Twelve +flowers on some plants growing out of doors were crossed with pollen +from a distinct plant and produced eleven capsules, containing +altogether twenty-four good seeds. Eighteen flowers were fertilised with +their own pollen and produced only eleven capsules, containing +twenty-two good seeds; so that a much larger proportion of the crossed +than of the self-fertilised flowers produced capsules, and the crossed +capsules contained rather more seed than the self-fertilised in the +ratio of 100 to 92. The seeds from the self-fertilised capsules were +however the heavier of the two, in the ratio of 100 to 87. + +Seeds in an equal state of germination were planted on the opposite +sides of four pots, but only the two tallest plants on each side of each +pot were measured to the tops of their stems. The pots were placed in +the greenhouse, and the plants trained up sticks, so that they ascended +to an unusual height. In three of the pots the crossed plants flowered +first, but in the fourth at the same time with the self-fertilised. When +the seedlings were between 6 and 7 inches in height, the crossed began +to show a slight advantage over their opponents. When grown to a +considerable height the eight tallest crossed plants averaged 44.43, and +the eight tallest self-fertilised plants 37.34 inches, or as 100 to 84. +When their growth was completed they were again measured, as shown in +Table 5/50. + +TABLE 5/50. Tropaeolum minus. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 65 : 31. +Pot 1 : 50 : 45. + +Pot 2 : 69 : 42. +Pot 2 : 35 : 45. + +Pot 3 : 70 : 50 4/8. +Pot 3 : 59 4/8 : 55 4/8. + +Pot 4 : 61 4/8 : 37 4/8. +Pot 4 : 57 4/8 : 61 4/8. + +Total : 467.5 : 368.0. + +The eight tallest crossed plants now averaged 58.43, and the eight +tallest self-fertilised plants 46 inches in height, or as 100 to 79. + +There was also a great difference in the fertility of the two lots which +were left uncovered in the greenhouse. On the 17th of September the +capsules from all the plants were gathered, and the seeds counted. The +crossed plants yielded 243, whilst the same number of self-fertilised +plants yielded only 155 seeds, or as 100 to 64. + +Limnanthes douglasii. + +Several flowers were crossed and self-fertilised in the usual manner, +but there was no marked difference in the number of seeds which they +yielded. A vast number of spontaneously self-fertilised capsules were +also produced under the net. Seedlings were raised in five pots from the +above seeds, and when the crossed were about 3 inches in height they +showed a slight advantage over the self-fertilised. When double this +height, the sixteen crossed and sixteen self-fertilised plants were +measured to the tips of their leaves; the former averaged 7.3 inches, +and the self-fertilised 6.07 inches in height, or as 100 to 83. In all +the pots, excepting 4, a crossed plant flowered before any one of the +self-fertilised plants. The plants, when fully grown, were again +measured to the summits of their ripe capsules, with the result in Table +5/51. + +TABLE 5/51. Limnanthes douglasii. + +Heights of plants to the summits of their ripe capsules, measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 17 7/8 : 15 1/8. +Pot 1 : 17 6/8 : 16 4/8. +Pot 1 : 13 : 11. + +Pot 2 : 20 : 14 4/8. +Pot 2 : 22 : 15 6/8. +Pot 2 : 21 : 16 1/8. +Pot 2 : 18 4/8 : 17. + +Pot 3 : 15 6/8 : 11 4/8. +Pot 3 : 17 2/8 : 10 4/8. +Pot 3 : 14 : 0. + +Pot 4 : 20 4/8 : 13 4/8. +Pot 4 : 14 : 13. +Pot 4 : 18 : 12 2/8. + +Pot 5 : 17 : 14 2/8. +Pot 5 : 18 5/8 : 14 1/8. +Pot 5 : 14 2/8 : 12 5/8. + +Total : 279.50 : 207.75. + +The sixteen crossed plants now averaged 17.46, and the fifteen (for one +had died) self-fertilised plants 13.85 inches in height, or as 100 to +79. Mr. Galton considers that a higher ratio would be fairer, namely, +100 to 76. He made a graphical representation of the above measurements, +and adds the words "very good" to the curvature thus formed. Both lots +of plants produced an abundance of seed-capsules, and, as far as could +be judged by the eye, there was no difference in their fertility.] + +14. LEGUMINOSAE. + +In this family I experimented on the following six genera, Lupinus, +Phaseolus, Lathyrus, Pisum, Sarothamnus, and Ononis. + +[Lupinus luteus. (5/2. The structure of the flowers of this plant, and +their manner of fertilisation, have been described by H. Muller +'Befruchtung' etc. page 243. The flowers do not secrete free nectar, and +bees generally visit them for their pollen. Mr. Farrer, however, remarks +'Nature' 1872 page 499, that "there is a cavity at the back and base of +the vexillum, in which I have not been able to find nectar. But the +bees, which constantly visit these flowers, certainly go to this cavity +for what they want, and not to the staminal tube.") + +A few flowers were crossed with pollen from a distinct plant, but owing +to the unfavourable season only two crossed seeds were produced. Nine +seeds were saved from flowers spontaneously self-fertilised under a net, +on the same plant which yielded the two crossed seeds. One of these +crossed seeds was sown in a pot with two self-fertilised seeds on the +opposite side; the latter came up between two and three days before the +crossed seed. The second crossed seed was sown in like manner with two +self-fertilised seeds on the opposite side; these latter also came up +about a day before the crossed one. In both pots, therefore, the crossed +seedlings from germinating later, were at first completely beaten by the +self-fertilised; nevertheless, this state of things was afterwards +completely reversed. The seeds were sown late in the autumn, and the +pots, which were much too small, were kept in the greenhouse. The plants +in consequence grew badly, and the self-fertilised suffered most in both +pots. The two crossed plants when in flower during the following spring +were 9 inches in height; one of the self-fertilised plants was 8, and +the three others only 3 inches in height, being thus mere dwarfs. The +two crossed plants produced thirteen pods, whilst the four +self-fertilised plants produced only a single one. Some other +self-fertilised plants which had been raised separately in larger pots +produced several spontaneously self-fertilised pods under a net, and +seeds from these were used in the following experiment. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION. + +The spontaneously self-fertilised seeds just mentioned, and crossed +seeds obtained by intercrossing the two crossed plants of the last +generation, after germinating on sand, were planted in pairs on the +opposite sides of three large pots. When the seedlings were only 4 +inches in height, the crossed had a slight advantage over their +opponents. When grown to their full height, every one of the crossed +plants exceeded its opponent in height. Nevertheless the self-fertilised +plants in all three pots flowered before the crossed! The measurements +are given in Table 5/52. + +TABLE 5/52. Lupinus luteus. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 33 2/8 : 24 4/8. +Pot 1 : 30 4/8 : 18 4/8. +Pot 1 : 30 : 28. + +Pot 2 : 29 4/8 : 26. +Pot 2 : 30 : 25. + +Pot 3 : 30 4/8 : 28. +Pot 3 : 31 : 27 2/8. +Pot 3 : 31 4/8 : 24 4/8. + +Total : 246.25 : 201.75. + +The eight crossed plants here average 30.78, and the eight +self-fertilised 25.21 inches in height; or as 100 to 82. These plants +were left uncovered in the greenhouse to set their pods, but they +produced very few good ones, perhaps in part owing to few bees visiting +them. The crossed plants produced nine pods, containing on an average +3.4 seeds, and the self-fertilised plants seven pods, containing on an +average 3 seeds, so that the seeds from an equal number of plants were +as 100 to 88. + +Two other crossed seedlings, each with two self-fertilised seedlings on +the opposite sides of the same large pot, were turned out of their pots +early in the season, without being disturbed, into open ground of good +quality. They were thus subjected to but little competition with one +another, in comparison with the plants in the above three pots. In the +autumn the two crossed plants were about 3 inches taller than the four +self-fertilised plants; they looked also more vigorous and produced many +more pods. + +Two other crossed and self-fertilised seeds of the same lot, after +germinating on sand, were planted on the opposite sides of a large pot, +in which a Calceolaria had long been growing, and were therefore exposed +to unfavourable conditions: the two crossed plants ultimately attained a +height of 20 1/2 and 20 inches, whilst the two self-fertilised were only +18 and 9 1/2 inches high. + +Lupinus pilosus. + +From a series of accidents I was again unfortunate in obtaining a +sufficient number of crossed seedlings; and the following results would +not be worth giving, did they not strictly accord with those just given +with respect to Lupinus luteus. I raised at first only a single crossed +seedling, which was placed in competition with two self-fertilised ones +on the opposite side of the same pot. These plants, without being +disturbed, were soon afterwards turned into the open ground. By the +autumn the crossed plant had grown to so large a size that it almost +smothered the two self-fertilised plants, which were mere dwarfs; and +the latter died without maturing a single pod. Several self-fertilised +seeds had been planted at the same time separately in the open ground; +and the two tallest of these were 33 and 32 inches, whereas the one +crossed plant was 38 inches in height. This latter plant also produced +many more pods than did any one of the self-fertilised plants, although +growing separately. A few flowers on the one crossed plant were crossed +with pollen from one of the self-fertilised plants, for I had no other +crossed plant from which to obtain pollen. One of the self-fertilised +plants having been covered by a net produced plenty of spontaneously +self-fertilised pods. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION. + +From crossed and self-fertilised seeds obtained in the manner just +described, I succeeded in raising to maturity only a pair of plants, +which were kept in a pot in the greenhouse. The crossed plant grew to a +height of 33 inches, and the self-fertilised to that of 26 1/2 inches. +The former produced, whilst still kept in the greenhouse, eight pods, +containing on an average 2.77 seeds; and the latter only two pods, +containing on an average 2.5 seeds. The average height of the two +crossed plants of the two generations taken together was 35.5, and that +of the three self-fertilised plants of the same two generations 30.5; or +as 100 to 86. (5/3. We here see that both Lupinus luteus and pilosus +seed freely when insects are excluded; but Mr. Swale, of Christchurch, +in New Zealand, informs me 'Gardeners' Chronicle' 1858 page 828, that +the garden varieties of the lupine are not there visited by any bees, +and that they seed less freely than any other introduced leguminous +plant, with the exception of red clover. He adds "I have, for amusement, +during the summer, released the stamens with a pin, and a pod of seed +has always rewarded me for my trouble, the adjoining flowers not so +served having all proved blind." I do not know to what species this +statement refers.) + +Phaseolus multiflorus. + +This plant, the scarlet-runner of English gardeners and the Phaseolus +coccineus of Lamarck, originally came from Mexico, as I am informed by +Mr. Bentham. The flowers are so constructed that hive and humble-bees, +which visit them incessantly, almost always alight on the left +wing-petal, as they can best suck the nectar from this side. Their +weight and movements depress the petal, and this causes the stigma to +protrude from the spirally-wound keel, and a brush of hairs round the +stigma pushes out the pollen before it. The pollen adheres to the head +or proboscis of the bee which is at work, and is thus placed either on +the stigma of the same flower, or is carried to another flower. (5/4. +The flowers have been described by Delpino, and in an admirable manner +by Mr. Farrer in the 'Annals and Magazine of Natural History' volume 2 +4th series October 1868 page 256. My son Francis has explained 'Nature' +January 8, 1874 page 189, the use of one peculiarity in their structure, +namely, a little vertical projection on the single free stamen near its +base, which seems placed as if to guard the entrance into the two +nectar-holes in the staminal sheath. He shows that this projection +prevents the bees reaching the nectar, unless they go to the left side +of the flower, and it is absolutely necessary for cross-fertilisation +that they should alight on the left wing-petal.) Several years ago I +covered some plants under a large net, and these produced on one +occasion about one-third, and on another occasion about one-eighth, of +the number of pods which the same number of uncovered plants growing +close alongside produced. (5/5. 'Gardeners' Chronicle' 1857 page 725 and +more especially ibid 1858 page 828. Also 'Annals and Magazine of Natural +History' 3rd series volume 2 1858 page 462.) This lessened fertility was +not caused by any injury from the net, as I moved the wing-petals of +several protected flowers, in the same manner as bees do, and these +produced remarkably fine pods. When the net was taken off, the flowers +were immediately visited by bees, and it was interesting to observe how +quickly the plants became covered with young pods. As the flowers are +much frequented by Thrips, the self-fertilisation of most of the flowers +under the net may have been due to the action of these minute insects. +Dr. Ogle likewise covered up a large portion of a plant, and "out of a +vast number of blossoms thus protected not a single one produced a pod, +while the unprotected blossoms were for the most part fruitful." Mr. +Belt gives a more curious case; this plant grows well and flowers in +Nicaragua; but as none of the native bees visit the flowers, not a +single pod is ever produced. (5/6. Dr. Ogle 'Popular Science Review' +1870 page 168. Mr. Belt 'The Naturalist in Nicaragua' 1874 page 70. The +latter author gives a case 'Nature' 1875 page 26, of a late crop of +Phaseolus multiflorus near London which "was rendered barren" by the +humble-bees cutting, as they frequently do, holes at the bases of the +flowers instead of entering them in the proper manner.) + +From the facts now given we may feel nearly sure that individuals of the +same variety or of different varieties, if growing near each other and +in flower at the same time, would intercross; but I cannot myself +advance any direct evidence of such an occurrence, as only a single +variety is commonly cultivated in England. I have, however, received an +account from the Reverend W.A. Leighton, that plants raised by him from +ordinary seed produced seeds differing in an extraordinary manner in +colour and shape, leading to the belief that their parents must have +been crossed. In France M. Fermond more than once planted close together +varieties which ordinarily come true and which bear differently coloured +flowers and seeds; and the offspring thus raised varied so greatly that +there could hardly be a doubt that they had intercrossed. (5/7. +'Fécondation chez les Végétaux' 1859 pages 34-40. He adds that M. +Villiers has described a spontaneous hybrid, which he calls Phaseolus +coccineus hybridus, in the 'Annales de la Soc. R. de Horticulture' June +1844.) On the other hand, Professor H. Hoffman does not believe in the +natural crossing of the varieties; for although seedlings raised from +two varieties growing close together produced plants which yielded seeds +of a mixed character, he found that this likewise occurred with plants +separated by a space of from 40 to 150 paces from any other variety; he +therefore attributes the mixed character of the seed to spontaneous +variability. (5/8. 'Bestimmung des Werthes von Species und Varietat' +1869 pages 47-72.) But the above distance would be very far from +sufficient to prevent intercrossing: cabbages have been known to cross +at several times this distance; and the careful Gartner gives many +instances of plants growing at from 600 to 800 yards apart fertilising +one another. (5/9. 'Kenntnis der Befruchtung' 1844 pages 573, 577.) +Professor Hoffman even maintains that the flowers of the kidney-bean are +specially adapted for self-fertilisation. He enclosed several flowers in +bags; and as the buds often dropped off, he attributes the partial +sterility of these flowers to the injurious effects of the bags, and not +to the exclusion of insects. But the only safe method of experimenting +is to cover up a whole plant, which then never suffers. + +Self-fertilised seeds were obtained by moving up and down in the same +manner as bees do the wing-petals of flowers protected by a net; and +crossed seeds were obtained by crossing two of the plants under the same +net. The seeds after germinating on sand were planted on the opposite +sides of two large pots, and equal-sized sticks were given them to twine +up. When 8 inches in height, the plants on the two sides were equal. The +crossed plants flowered before the self-fertilised in both pots. As soon +as one of each pair had grown to the summit of its stick both were +measured. + +TABLE 5/53. Phaseolus multiflorus. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 87 : 84 6/8. +Pot 1 : 88 : 87. +Pot 1 : 82 4/8 : 76. + +Pot 2 : 90 : 76 4/8. +Pot 2 : 82 4/8 : 87 4/8. + +Total : 430.00 : 411.75. + +The average height of the five crossed plants is 86 inches, and that of +the five self-fertilised plants 82.35; or as 100 to 96. The pots were +kept in the greenhouse, and there was little or no difference in the +fertility of the two lots. Therefore as far as these few observations +serve, the advantage gained by a cross is very small. + +Phaseolus vulgaris. + +With respect to this species, I merely ascertained that the flowers were +highly fertile when insects were excluded, as indeed must be the case, +for the plants are often forced during the winter when no insects are +present. Some plants of two varieties (namely Canterbury and Fulmer's +Forcing Bean) were covered with a net, and they seemed to produce as +many pods, containing as many beans, as some uncovered plants growing +alongside; but neither the pods nor the beans were actually counted. +This difference in self-fertility between Phaseolus vulgaris and +multifloris is remarkable, as these two species are so closely related +that Linnaeus thought that they formed one. When the varieties of +Phaseolus vulgaris grow near one another in the open ground, they +sometimes cross largely, notwithstanding their capacity for +self-fertilisation. Mr. Coe has given me a remarkable instance of this +fact with respect to the negro and a white-seeded and a brown-seeded +variety, which were all grown together. The diversity of character in +the seedlings of the second generation raised by me from his plants was +wonderful. I could add other analogous cases, and the fact is well-known +to gardeners. (5/10. I have given Mr. Coe's case in the 'Gardeners' +Chronicle' 1858 page 829. See also for another case ibid page 845.) + +Lathyrus odoratus. + +Almost everyone who has studied the structure of papilionaceous flowers +has been convinced that they are specially adapted for +cross-fertilisation, although many of the species are likewise capable +of self-fertilisation. The case therefore of Lathyrus odoratus or the +sweet-pea is curious, for in this country it seems invariably to +fertilise itself. I conclude that this is so, as five varieties, +differing greatly in the colour of their flowers but in no other +respect, are commonly sold and come true; yet on inquiry from two great +raisers of seed for sale, I find that they take no precautions to insure +purity--the five varieties being habitually grown close together. (5/11. +See Mr. W. Earley in 'Nature' 1872 page 242, to the same effect. He +once, however, saw bees visiting the flowers, and supposed that on this +occasion they would have been intercrossed.) I have myself purposely +made similar trials with the same result. Although the varieties always +come true, yet, as we shall presently see, one of the five well-known +varieties occasionally gives birth to another, which exhibits all its +usual characters. Owing to this curious fact, and to the darker-coloured +varieties being the most productive, these increase, to the exclusion of +the others, as I was informed by the late Mr. Masters, if there be no +selection. + +In order to ascertain what would be the effect of crossing two +varieties, some flowers on the Purple sweet-pea, which has a dark +reddish-purple standard-petal with violet-coloured wing-petals and keel, +were castrated whilst very young, and were fertilised with pollen of the +Painted Lady. This latter variety has a pale cherry-coloured standard, +with almost white wings and keel. On two occasions I raised from a +flower thus crossed plants perfectly resembling both parent-forms; but +the greater number resembled the paternal variety. So perfect was the +resemblance, that I should have suspected some mistake in the label, had +not the plants, which were at first identical in appearance with the +father or Painted Lady, later in the season produced flowers blotched +and streaked with dark purple. This is an interesting example of partial +reversion in the same individual plant as it grows older. The +purple-flowered plants were thrown away, as they might possibly have +been the product of the accidental self-fertilisation of the +mother-plant, owing to the castration not having been effectual. But the +plants which resembled in the colour of their flowers the paternal +variety or Painted Lady were preserved, and their seeds saved. Next +summer many plants were raised from these seeds, and they generally +resembled their grandfather the Painted Lady, but most of them had their +wing-petals streaked and stained with dark pink; and a few had pale +purple wings with the standard of a darker crimson than is natural to +the Painted Lady, so that they formed a new sub-variety. Amongst these +plants a single one appeared having purple flowers like those of the +grandmother, but with the petals slightly streaked with a paler tint: +this was thrown away. Seeds were again saved from the foregoing plants, +and the seedlings thus raised still resembled the Painted Lady, or +great-grandfather; but they now varied much, the standard petal varying +from pale to dark red, in a few instances with blotches of white; and +the wing-petals varied from nearly white to purple, the keel being in +all nearly white. + +As no variability of this kind can be detected in plants raised from +seeds, the parents of which have grown during many successive +generations in close proximity, we may infer that they cannot have +intercrossed. What does occasionally occur is that in a row of plants +raised from seeds of one variety, another variety true of its kind +appears; for instance, in a long row of Scarlets (the seeds of which had +been carefully gathered from Scarlets for the sake of this experiment) +two Purples and one Painted Lady appeared. Seeds from these three +aberrant plants were saved and sown in separate beds. The seedlings from +both the Purples were chiefly Purples, but with some Painted Ladies and +some Scarlets. The seedlings from the aberrant Painted Lady were chiefly +Painted Ladies with some Scarlets. Each variety, whatever its parentage +may have been, retained all its characters perfect, and there was no +streaking or blotching of the colours, as in the foregoing plants of +crossed origin. Another variety, however, is often sold, which is +striped and blotched with dark purple; and this is probably of crossed +origin, for I found, as well as Mr. Masters, that it did not transmit +its characters at all truly. + +From the evidence now given, we may conclude that the varieties of the +sweet-pea rarely or never intercross in this country; and this is a +highly remarkable fact, considering, firstly, the general structure of +the flowers; secondly, the large quantity of pollen produced, far more +than is requisite for self-fertilisation; and thirdly, the occasional +visit of insects. That insects should sometimes fail to cross-fertilise +the flowers is intelligible, for I have thrice seen humble-bees of two +kinds, as well as hive-bees, sucking the nectar, and they did not +depress the keel-petals so as to expose the anthers and stigma; they +were therefore quite inefficient for fertilising the flowers. One of +these bees, namely, Bombus lapidarius, stood on one side at the base of +the standard and inserted its proboscis beneath the single separate +stamen, as I afterwards ascertained by opening the flower and finding +this stamen prised up. Bees are forced to act in this manner from the +slit in the staminal tube being closely covered by the broad membranous +margin of the single stamen, and from the tube not being perforated by +nectar-passages. On the other hand, in the three British species of +Lathyrus which I have examined, and in the allied genus Vicia, two +nectar-passages are present. Therefore British bees might well be +puzzled how to act in the case of the sweet-pea. I may add that the +staminal tube of another exotic species, Lathyrus grandiflorus, is not +perforated by nectar-passages, and this species has rarely set any pods +in my garden, unless the wing-petals were moved up and down, in the same +manner as bees ought to do; and then pods were generally formed, but +from some cause often dropped off afterwards. One of my sons caught an +elephant sphinx-moth whilst visiting the flowers of the sweet-pea, but +this insect would not depress the wing-petals and keel. On the other +hand, I have seen on one occasion hive-bees, and two or three occasions +the Megachile willughbiella in the act of depressing the keel; and these +bees had the under sides of their bodies thickly covered with pollen, +and could not thus fail to carry pollen from one flower to the stigma of +another. Why then do not the varieties occasionally intercross, though +this would not often happen, as insects so rarely act in an efficient +manner? The fact cannot, as it appears, be explained by the flowers +being self-fertilised at a very early age; for although nectar is +sometimes secreted and pollen adheres to the viscid stigma before the +flowers are fully expanded, yet in five young flowers which were +examined by me the pollen-tubes were not exserted. Whatever the cause +may be, we may conclude, that in England the varieties never or very +rarely intercross. But it does not follow from this, that they would not +be cross by the aid of other and larger insects in their native country, +which in botanical works is said to be the south of Europe and the East +Indies. Accordingly I wrote to Professor Delpino, in Florence, and he +informs me "that it is the fixed opinion of gardeners there that the +varieties do intercross, and that they cannot be preserved pure unless +they are sown separately." + +It follows also from the foregoing facts that the several varieties of +the sweet-pea must have propagated themselves in England by +self-fertilisation for very many generations, since the time when each +new variety first appeared. From the analogy of the plants of Mimulus +and Ipomoea, which had been self-fertilised for several generations, and +from trials previously made with the common pea, which is in nearly the +same state as the sweet-pea, it appeared to me very improbable that a +cross between the individuals of the same variety would benefit the +offspring. A cross of this kind was therefore not tried, which I now +regret. But some flowers of the Painted Lady, castrated at an early age, +were fertilised with pollen from the Purple sweet-pea; and it should be +remembered that these varieties differ in nothing except in the colour +of their flowers. The cross was manifestly effectual (though only two +seeds were obtained), as was shown by the two seedlings, when they +flowered, closely resembling their father, the Purple pea, excepting +that they were a little lighter coloured, with their keels slightly +streaked with pale purple. Seeds from flowers spontaneously +self-fertilised under a net were at the same time saved from the same +mother-plant, the Painted Lady. These seeds unfortunately did not +germinate on sand at the same time with the crossed seeds, so that they +could not be planted simultaneously. One of the two crossed seeds in a +state of germination was planted in a pot (Number 1) in which a +self-fertilised seed in the same state had been planted four days +before, so that this latter seedling had a great advantage over the +crossed one. In Pot 2 the other crossed seed was planted two days before +a self-fertilised one; so that here the crossed seedling had a +considerable advantage over the self-fertilised one. But this crossed +seedling had its summit gnawed off by a slug, and was in consequence for +a time quite beaten by the self-fertilised plant. Nevertheless I allowed +it to remain, and so great was its constitutional vigour that it +ultimately beat its uninjured self-fertilised rival. When all four +plants were almost fully grown they were measured, as here shown:-- + +TABLE 5/54. Lathyrus odoratus. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 80 : 64 4/8. + +Pot 2 : 78 4/8 : 63. + +Total : 158.5 : 127.5. + +The two crossed plants here average 79.25, and the two self-fertilised +63.75 inches in height, or as 100 to 80. Six flowers on these two +crossed plants were reciprocally crossed with pollen from the other +plant, and the six pods thus produced contained on an average six peas, +with a maximum in one of seven. Eighteen spontaneously self-fertilised +pods from the Painted Lady, which, as already stated, had no doubt been +self-fertilised for many previous generations, contained on an average +only 3.93 peas, with a maximum in one of five peas; so that the number +of peas in the crossed and self-fertilised pods was as 100 to 65. The +self-fertilised peas were, however, quite as heavy as those from the +crossed pods. From these two lots of seeds, the plants of the next +generation were raised. + +PLANTS OF THE SECOND GENERATION. + +Many of the self-fertilised peas just referred to germinated on sand +before any of the crossed ones, and were rejected. As soon as I got +equal pairs, they were planted on the opposite sides of two large pots, +which were kept in the greenhouse. The seedlings thus raised were the +grandchildren of the Painted Lady, which was first crossed by the Purple +variety. When the two lots were from 4 to 6 inches in height there was +no difference between them. Nor was there any marked difference in the +period of their flowering. When fully grown they were measured, as +follows:-- + +TABLE 5/55. Lathyrus odoratus (Second Generation). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Seedlings from Plants Crossed during the two previous +Generations. + +Column 3: Seedlings from Plants Self-fertilised during many previous +Generations. + +Pot 1 : 72 4/8 : 57 4/8. +Pot 1 : 71 : 67. +Pot 1 : 52 2/8 : 56 2/8. + +Pot 2 : 81 4/8 : 66 2/8. +Pot 2 : 45 2/8 : 38 7/8. +Pot 2 : 55 : 46. + +Total : 377.50 : 331.86. + +The average height of the six crossed plants is here 62.91, and that of +the six self-fertilised 55.31 inches; or as 100 to 88. There was not +much difference in the fertility of the two lots; the crossed plants +having produced in the greenhouse thirty-five pods, and the +self-fertilised thirty-two pods. + +Seeds were saved from the self-fertilised flowers on these two lots of +plants, for the sake of ascertaining whether the seedlings thus raised +would inherit any difference in growth or vigour. It must therefore be +understood that both lots in the following trial are plants of +self-fertilised parentage; but that in the one lot the plants were the +children of plants which had been crossed during two previous +generations, having been before that self-fertilised for many +generations; and that in the other lot they were the children of plants +which had not been crossed for very many previous generations. The seeds +germinated on sand and were planted in pairs on the opposite sides of +four pots. They were measured, when fully grown, with the following +result:-- + +TABLE 5/56. Lathyrus odoratus. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Self-fertilised Plants from Crossed Plants. + +Column 3: Self-fertilised Plants from Self-fertilised Plants. + +Pot 1 : 72 : 65. +Pot 1 : 72 : 61 4/8. + +Pot 2 : 58 : 64. +Pot 2 : 68 : 68 2/8. +Pot 2 : 72 4/8 : 56 4/8. + +Pot 3 : 81 : 60 2/8. + +Pot 4 : 77 4/8 : 76 4/8. + +Total : 501 : 452. + +The average height of the seven self-fertilised plants, the offspring of +crossed plants, is 71.57, and that of the seven self-fertilised plants, +the offspring of self-fertilised plants, is 64.57; or as 100 to 90. The +self-fertilised plants from the self-fertilised produced rather more +pods--namely, thirty-six--than the self-fertilised plants from the +crossed, for these produced only thirty-one pods. + +A few seeds of the same two lots were sown in the opposite corners of a +large box in which a Brugmansia had long been growing, and in which the +soil was so exhausted that seeds of Ipomoea purpurea would hardly +vegetate; yet the two plants of the sweet-pea which were raised +flourished well. For a long time the self-fertilised plant from the +self-fertilised beat the self-fertilised plant from the crossed plant; +the former flowered first, and was at one time 77 1/2 inches, whilst the +latter was only 68 1/2 in height; but ultimately the plant from the +previous cross showed its superiority and attained a height of 108 1/2 +inches, whilst the other was only 95 inches. I also sowed some of the +same two lots of seeds in poor soil in a shady place in a shrubbery. +Here again the self-fertilised plants from the self-fertilised for a +long time exceeded considerably in height those from the previously +crossed plants; and this may probably be attributed, in the present as +in the last case, to these seeds having germinated rather sooner than +those from the crossed plants; but at the close of the season the +tallest of the self-fertilised plants from the crossed plants was 30 +inches, whilst the tallest of the self-fertilised from the +self-fertilised was 29 3/8 inches in height. + +From the various facts now given we see that plants derived from a cross +between two varieties of the sweet-pea, which differ in no respect +except in the colour of their flowers, exceed considerably in height the +offspring from self-fertilised plants, both in the first and second +generations. The crossed plants also transmit their superiority in +height and vigour to their self-fertilised offspring. + +Pisum sativum. + +The common pea is perfectly fertile when its flowers are protected from +the visits of insects; I ascertained this with two or three different +varieties, as did Dr. Ogle with another. But the flowers are likewise +adapted for cross-fertilisation; Mr. Farrer specifies the following +points, namely: "The open blossom displaying itself in the most +attractive and convenient position for insects; the conspicuous +vexillum; the wings forming an alighting place; the attachment of the +wings to the keel, by which any body pressing on the former must press +down the latter; the staminal tube enclosing nectar, and affording by +means of its partially free stamen with apertures on each side of its +base an open passage to an insect seeking the nectar; the moist and +sticky pollen placed just where it will be swept out of the apex of the +keel against the entering insect; the stiff elastic style so placed that +on a pressure being applied to the keel it will be pushed upwards out of +the keel; the hairs on the style placed on that side of the style only +on which there is space for the pollen, and in such a direction as to +sweep it out; and the stigma so placed as to meet an entering +insect,--all these become correlated parts of one elaborate mechanism, +if we suppose that the fertilisation of these flowers is effected by the +carriage of pollen from one to the other." (5/12. 'Nature' October 10, +1872 page 479. Hermann Muller gives an elaborate description of the +flowers 'Befruchtung' etc. page 247.) Notwithstanding these manifest +provisions for cross-fertilisation, varieties which have been cultivated +for very many successive generations in close proximity, although +flowering at the same time, remain pure. I have elsewhere given evidence +on this head, and if required could give more. (5/13. 'Variation of +Animals and Plants under Domestication' chapter 9 2nd edition volume 1 +page 348.) There can hardly be a doubt that some of Knight's varieties, +which were originally produced by an artificial cross and were very +vigorous, lasted for at least sixty years, and during all these years +were self-fertilised; for had it been otherwise, they would not have +kept true, as the several varieties are generally grown near together. +Most of the varieties, however, endure for a shorter period; and this +may be in part due to their weakness of constitution from long-continued +self-fertilisation. + +It is remarkable, considering that the flowers secrete much nectar and +afford much pollen, how seldom they are visited by insects either in +England, or, as H. Muller remarks, in North Germany. I have observed the +flowers for the last thirty years, and in all this time have only thrice +seen bees of the proper kind at work (one of them being Bombus +muscorum), such as were sufficiently powerful to depress the keel, so as +to get the undersides of their bodies dusted with pollen. These bees +visited several flowers, and could hardly have failed to cross-fertilise +them. Hive-bees and other small kinds sometimes collect pollen from old +and already fertilised flowers, but this is of no account. The rarity of +the visits of efficient bees to this exotic plant is, I believe, the +chief cause of the varieties so seldom intercrossing. That a cross does +occasionally take place, as might be expected from what has just been +stated, is certain, from the recorded cases of the direct action of the +pollen of one variety on the seed-coats of another. (5/14. 'Variation of +Animals and Plants under Domestication' chapter 11 2nd edition volume 1 +page 428.) The late Mr. Masters, who particularly attended to the +raising of new varieties of peas, was convinced that some of them had +originated from accidental crosses. But as such crosses are rare, the +old varieties would not often be thus deteriorated, more especially as +plants departing from the proper type are generally rejected by those +who collect seed for sale. There is another cause which probably tends +to render cross-fertilisation rare, namely, the early age at which the +pollen-tubes are exserted; eight flowers not fully expanded were +examined, and in seven of these the pollen-tubes were in this state; but +they had not as yet penetrated the stigma. Although so few insects visit +the flowers of the pea in this country or in North Germany, and although +the anthers seem here to open abnormally soon, it does not follow that +the species in its native country would be thus circumstanced. + +Owing to the varieties having been self-fertilised for many generations, +and to their having been subjected in each generation to nearly the same +conditions (as will be explained in a future chapter) I did not expect +that a cross between two such plants would benefit the offspring; and so +it proved on trial. In 1867 I covered up several plants of the Early +Emperor pea, which was not then a very new variety, so that it must +already have been propagated by self-fertilisation for at least a dozen +generations. Some flowers were crossed with pollen from a distinct plant +growing in the same row, and others were allowed to fertilise themselves +under a net. The two lots of seeds thus obtained were sown on opposite +sides of two large pots, but only four pairs came up at the same time. +The pots were kept in the greenhouse. The seedlings of both lots when +between 6 and 7 inches in height were equal. When nearly full-grown they +were measured, as in Table 5/57. + +TABLE 5/57. Pisum sativum. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 35 : 29 6/8. + +Pot 2 : 31 4/8 : 51. +Pot 2 : 35 : 45. +Pot 2 : 37 : 33. + +Total : 138.50 : 158.75. + +The average height of the four crossed plants is here 34.62, and that of +the four self-fertilised plants 39.68, or as 100 to 115. So that the +crossed plants, far from beating the self-fertilised, were completely +beaten by them. + +There can be no doubt that the result would have been widely different, +if any two varieties out of the numberless ones which exist had been +crossed. Notwithstanding that both had been self-fertilised for many +previous generations, each would almost certainly have possessed its own +peculiar constitution; and this degree of differentiation would have +been sufficient to make a cross highly beneficial. I have spoken thus +confidently of the benefit which would have been derived from crossing +any two varieties of the pea from the following facts: Andrew Knight in +speaking of the results of crossing reciprocally very tall and short +varieties, says, "I had in this experiment a striking instance of the +stimulative effects of crossing the breeds; for the smallest variety, +whose height rarely exceeded 2 feet, was increased to 6 feet; whilst the +height of the large and luxuriant kind was very little diminished." +(5/15. 'Philosophical Transactions' 1799 page 200.) Recently Mr. Laxton +has made numerous crosses, and everyone had been astonished at the +vigour and luxuriance of the new varieties which he has thus raised and +afterwards fixed by selection. He gave me seed-peas produced from +crosses between four distinct kinds; and the plants thus raised were +extraordinarily vigorous, being in each case from 1 to 2 or even 3 feet +taller than the parent-forms, which were raised at the same time close +alongside. But as I did not measure their actual height I cannot give +the exact ratio, but it must have been at least as 100 to 75. A similar +trial was subsequently made with two other peas from a different cross, +and the result was nearly the same. For instance, a crossed seedling +between the Maple and Purple-podded pea was planted in poor soil and +grew to the extraordinary height of 116 inches; whereas the tallest +plant of either parent variety, namely, a Purple-podded pea, was only 70 +inches in height; or as 100 to 60. + +Sarothamnus scoparius. + +Bees incessantly visit the flowers of the common Broom, and these are +adapted by a curious mechanism for cross-fertilisation. When a bee +alights on the wing-petals of a young flower, the keel is slightly +opened and the short stamens spring out, which rub their pollen against +the abdomen of the bee. If a rather older flower is visited for the +first time (or if the bee exerts great force on a younger flower), the +keel opens along its whole length, and the longer as well as the shorter +stamens, together with the much elongated curved pistil, spring forth +with violence. The flattened, spoon-like extremity of the pistil rests +for a time on the back of the bee, and leaves on it the load of pollen +with which it is charged. As soon as the bee flies away, the pistil +instantly curls round, so that the stigmatic surface is now upturned and +occupies a position, in which it would be rubbed against the abdomen of +another bee visiting the same flower. Thus, when the pistil first +escapes from the keel, the stigma is rubbed against the back of the bee, +dusted with pollen from the longer stamens, either of the same or +another flower; and afterwards against the lower surface of the bee +dusted with pollen from the shorter stamens, which is often shed a day +or two before that from the longer stamens. (5/16. These observations +have been quoted in an abbreviated form by the Reverend G. Henslow, in +the 'Journal of Linnean Society Botany' volume 9 1866 page 358. Hermann +Muller has since published a full and excellent account of the flower in +his 'Befruchtung' etc. page 240.) By this mechanism cross-fertilisation +is rendered almost inevitable, and we shall immediately see that pollen +from a distinct plant is more effective than that from the same flower. +I need only add that, according to H. Muller, the flowers do not secrete +nectar, and he thinks that bees insert their proboscides only in the +hope of finding nectar; but they act in this manner so frequently and +for so long a time that I cannot avoid the belief that they obtain +something palatable within the flowers. + +If the visits of bees are prevented, and if the flowers are not dashed +by the wind against any object, the keel never opens, so that the +stamens and pistil remain enclosed. Plants thus protected yield very few +pods in comparison with those produced by neighbouring uncovered bushes, +and sometimes none at all. I fertilised a few flowers on a plant growing +almost in a state of nature with pollen from another plant close +alongside, and the four crossed capsules contained on an average 9.2 +seeds. This large number no doubt was due to the bush being covered up, +and thus not exhausted by producing many pods; for fifty pods gathered +from an adjoining plant, the flowers of which had been fertilised by the +bees, contained an average of only 7.14 seeds. Ninety-three pods +spontaneously self-fertilised on a large bush which had been covered up, +but had been much agitated by the wind, contained an average of 2.93 +seeds. Ten of the finest of these ninety-three capsules yielded an +average of 4.30 seeds, that is less than half the average number in the +four artificially crossed capsules. The ratio of 7.14 to 2.93, or as 100 +to 41, is probably the fairest for the number of seeds per pod, yielded +by naturally-crossed and spontaneously self-fertilised flowers. The +crossed seeds compared with an equal number of the spontaneously +self-fertilised seeds were heavier, in the ratio of 100 to 88. We thus +see that besides the mechanical adaptations for cross-fertilisation, the +flowers are much more productive with pollen from a distinct plant than +with their own pollen. + +Eight pairs of the above crossed and self-fertilised seeds, after they +had germinated on sand, were planted (1867) on the opposite sides of two +large pots. When several of the seedlings were an inch and a half in +height, there was no marked difference between the two lots. But even at +this early age the leaves of the self-fertilised seedlings were smaller +and of not so bright a green as those of the crossed seedlings. The pots +were kept in the greenhouse, and as the plants on the following spring +(1868) looked unhealthy and had grown but little, they were plunged, +still in their pots, into the open ground. The plants all suffered much +from the sudden change, especially the self-fertilised, and two of the +latter died. The remainder were measured, and I give the measurements in +Table 5/58, because I have not seen in any other species so great a +difference between the crossed and self-fertilised seedlings at so early +an age. + +TABLE 5/58. Sarothamnus scoparius (very young plants). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 4 4/8 : 2 4/8. +Pot 1 : 6 : 1 4/8. +Pot 1 : 2 : 1. + +Pot 2 : 2 : 1 4/8. +Pot 2 : 2 4/8 : 1. +Pot 2 : 0 4/8 : 0 4/8. + +Total : 17.5 : 8.0. + +The six crossed plants here average 2.91, and the six self-fertilised +1.33 inches in height; so that the former were more than twice as high +as the latter, or as 100 to 46. + +In the spring of the succeeding year (1869) the three crossed plants in +Pot 1 had all grown to nearly a foot in height, and they had smothered +the three little self-fertilised plants so completely that two were +dead; and the third, only an inch and a half in height, was dying. It +should be remembered that these plants had been bedded out in their +pots, so that they were subjected to very severe competition. This pot +was now thrown away. + +The six plants in Pot 2 were all alive. One of the self-fertilised was +an inch and a quarter taller than any one of the crossed plants; but the +other two self-fertilised plants were in a very poor condition. I +therefore resolved to leave these plants to struggle together for some +years. By the autumn of the same year (1869) the self-fertilised plant +which had been victorious was now beaten. The measurements are shown in +Table 5/59. + +TABLE 5/59. Pot 2.--Sarothamnus scoparius. + +Heights of plants measured in inches. + +Column 1: Crossed Plants. + +Column 2: Self-fertilised Plants. + + : 15 6/8 : 13 1/8. + : 9 6/8 : 3. + : 8 2/8 : 2 4/8. + +The same plants were again measured in the autumn of the following year, +1870. + +TABLE 5/60. Pot 2.--Sarothamnus scoparius. + +Heights of plants measured in inches. + +Column 1: Crossed Plants. + +Column 2: Self-fertilised Plants. + + : 26 2/8 : 14 2/8. + : 16 4/8 : 11 4/8. + : 14 : 9 6/8. + +Total : 56.75 : 35.50. + +The three crossed plants now averaged 18.91, and the three +self-fertilised 11.83 inches in height; or as 100 to 63. The three +crossed plants in Pot 1, as already shown, had beaten the three +self-fertilised plants so completely, that any comparison between them +was superfluous. + +The winter of 1870-1871 was severe. In the spring the three crossed +plants in Pot 2 had not even the tips of their shoots in the least +injured, whereas all three self-fertilised plants were killed half-way +down to the ground; and this shows how much more tender they were. In +consequence not one of these latter plants bore a single flower during +the ensuing summer of 1871, whilst all three crossed plants flowered. + +Ononis minutissima. + +This plant, of which seeds were sent me from North Italy, produces, +besides the ordinary papilionaceous flowers, minute, imperfect, closed +or cleistogene flowers, which can never be cross-fertilised, but are +highly self-fertile. Some of the perfect flowers were crossed with +pollen from a distinct plant, and six capsules thus produced yielded on +an average 3.66 seeds, with a maximum of five in one. Twelve perfect +flowers were marked and allowed to fertilise themselves spontaneously +under a net, and they yielded eight capsules, containing on an average +2.38 seeds, with a maximum of three seeds in one. So that the crossed +and self-fertilised capsules from the perfect flowers yielded seeds in +the proportion of 100 to 65. Fifty-three capsules produced by the +cleistogene flowers contained on an average 4.1 seeds, so that these +were the most productive of all; and the seeds themselves looked finer +even than those from the crossed perfect flowers. + +The seeds from the crossed perfect flowers and from the self-fertilised +cleistogene flowers were allowed to germinate on sand; but unfortunately +only two pairs germinated at the same time. These were planted on the +opposite sides of the same pot, which was kept in the greenhouse. In the +summer of the same year, when the seedlings were about 4 1/2 inches in +height, the two lots were equal. In the autumn of the following year +(1868) the two crossed plants were of exactly the same height, namely, +11 4/8 inches, and the two self-fertilised plants 12 6/8 and 7 2/8 +inches; so that one of the self-fertilised exceeded considerably in +height all the others. By the autumn of 1869 the two crossed plants had +acquired the supremacy; their height being 16 4/8 and 15 1/8, whilst +that of the two self-fertilised plants was 14 5/8 and 11 4/8 inches. + +By the autumn of 1870, the heights were as follows:-- + +TABLE 5/61. Ononis minutissima. + +Heights of plants measured in inches. + +Column 1: Crossed Plants. + +Column 2: Self-fertilised Plants. + + : 20 3/8 : 17 4/8. + : 19 2/8 : 17 2/8. + +Total : 39.63 : 34.75. + +So that the mean height of the two crossed plants was 19.81, and that of +the two self-fertilised 17.37 inches; or as 100 to 88. It should be +remembered that the two lots were at first equal in height; that one of +the self-fertilised plants then had the advantage, the two crossed +plants being at last victorious.] + +SUMMARY ON THE LEGUMINOSAE. + +Six genera in this family were experimented on, and the results are in +some respects remarkable. The crossed plants of the two species of +Lupinus were conspicuously superior to the self-fertilised plants in +height and fertility; and when grown under very unfavourable conditions, +in vigour. The scarlet-runner (Phaseolus multiflorus) is partially +sterile if the visits of bees are prevented, and there is reason to +believe that varieties growing near one another intercross. The five +crossed plants, however, exceeded in height the five self-fertilised +only by a little. Phaseolus vulgaris is perfectly self-sterile; +nevertheless, varieties growing in the same garden sometimes intercross +largely. The varieties of Lathyrus odoratus, on the other hand, appear +never to intercross in this country; and though the flowers are not +often visited by efficient insects, I cannot account for this fact, more +especially as the varieties are believed to intercross in North Italy. +Plants raised from a cross between two varieties, differing only in the +colour of their flowers, grew much taller and were under unfavourable +conditions more vigorous than the self-fertilised plants; they also +transmitted, when self-fertilised, their superiority to their offspring. +The many varieties of the common Pea (Pisum sativum), though growing in +close proximity, very seldom intercross; and this seems due to the +rarity in this country of the visits of bees sufficiently powerful to +effect cross-fertilisation. A cross between the self-fertilised +individuals of the same variety does no good whatever to the offspring; +whilst a cross between distinct varieties, though closely allied, does +great good, of which we have excellent evidence. The flowers of the +Broom (Sarothamnus) are almost sterile if they are not disturbed and if +insects are excluded. The pollen from a distinct plant is more effective +than that from the same flower in producing seeds. The crossed seedlings +have an enormous advantage over the self-fertilised when grown together +in close competition. Lastly, only four plants of the Ononis minutissima +were raised; but as these were observed during their whole growth, the +advantage of the crossed over the self-fertilised plants may, I think, +be fully trusted. + +[15. ONAGRACEAE.--Clarkia elegans. + +Owing to the season being very unfavourable (1867), few of the flowers +which I fertilised formed capsules; twelve crossed flowers produced only +four, and eighteen self-fertilised flowers yielded only one capsule. The +seeds after germinating on sand were planted in three pots, but all the +self-fertilised plants died in one of them. When the two lots were +between 4 and 5 inches in height, the crossed began to show a slight +superiority over the self-fertilised. When in full flower they were +measured, with the following result:-- + +TABLE 5/62. Clarkia elegans. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 40 4/8 : 33. +Pot 1 : 35 : 24. +Pot 1 : 25 : 23. + +Pot 2 : 33 4/8 : 30 4/8. + +Total : 134.0 : 110.5. + +The average height of the four crossed plants is 33.5, and that of the +four self-fertilised plants 27.62 inches, or as 100 to 82. The crossed +plants altogether produced 105 and the self-fertilised plants 63 +capsules; or as 100 to 60. In both pots a self-fertilised plant flowered +before any one of the crossed plants. + +16. LOASACEAE.--Bartonia aurea. + +Some flowers were crossed and self-fertilised in the usual manner during +two seasons; but as I reared on the first occasion only two pairs, the +results are given together. On both occasions the crossed capsules +contained slightly more seeds than the self-fertilised. During the first +year, when the plants were about 7 inches in height, the self-fertilised +were the tallest, and in the second year the crossed were the tallest. +When the two lots were in full flower they were measured, as in Table +5/63. + +TABLE 5/63. Bartonia aurea. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 31 : 37. + +Pot 2 : 18 4/8 : 20 4/8. + +Pot 3 : 19 4/8 : 40 4/8. + +Pot 4 : 25 : 35. +Pot 4 : 36 : 15 4/8. + +Pot 5 : 31 : 18. +Pot 5 : 16 : 11 4/8. + +Pot 6 : 20 : 32 4/8. + +Total : 197.0 : 210.5. + +The average height of the eight crossed plants is 24.62, and that of the +eight self-fertilised 26.31 inches; or as 100 to 107. So that the +self-fertilised had a decided advantage over the crossed. But the plants +from some cause never grew well, and finally became so unhealthy that +only three crossed and three self-fertilised plants survived to set any +capsules, and these were few in number. The two lots seemed to be about +equally unproductive. + +17. PASSIFLORACEAE.--Passiflora gracilis. + +This annual species produces spontaneously numerous fruits when insects +are excluded, and behaves in this respect very differently from most of +the other species in the genus, which are extremely sterile unless +fertilised with pollen from a distinct plant. (5/17. 'Variation of +Animals and Plants under Domestication' chapter 17 2nd edition volume 2 +page 118.) Fourteen fruits from crossed flowers contained on an average +24.14 seeds. Fourteen fruits (two poor ones being rejected), +spontaneously self-fertilised under a net, contained on an average 20.58 +seeds per fruit; or as 100 to 85. These seeds were sown on the opposite +sides of three pots, but only two pairs came up at the same time; and +therefore a fair judgment cannot be formed. + +TABLE 5/64. Passiflora gracilis. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 56 : 38. + +Pot 2 : 42 : 64. + +Total : 98 : 102. + +The mean of the two crossed is 49 inches, and that of the two +self-fertilised 51 inches; or as 100 to 104. + +18. UMBELLIFERAE.--Apium petroselinum. + +The Umbelliferae are proterandrous, and can hardly fail to be +cross-fertilised by the many flies and small Hymenoptera which visit the +flowers. (5/18. Hermann Muller 'Befruchtung' etc. page 96. According to +M. Mustel as stated by Godron 'De l'espèce' tome 2 page 58 1859, +varieties of the carrot growing near each other readily intercross.) A +plant of the common parsley was covered by a net, and it apparently +produced as many and as fine spontaneously self-fertilised fruits or +seeds as the adjoining uncovered plants. The flowers on the latter were +visited by so many insects that they must have received pollen from one +another. Some of these two lots of seeds were left on sand, but nearly +all the self-fertilised seeds germinated before the others, so that I +was forced to throw all away. The remaining seeds were then sown on the +opposite sides of four pots. At first the self-fertilised seedlings were +a little taller in most of the pots than the naturally crossed +seedlings, and this no doubt was due to the self-fertilised seeds having +germinated first. But in the autumn all the plants were so equal that it +did not seem worth while to measure them. In two of the pots they were +absolutely equal; in a third, if there was any difference, it was in +favour of the crossed plants, and in a somewhat plainer manner in the +fourth pot. But neither side had any substantial advantage over the +other; so that in height they may be said to be as 100 to 100. + +19. DIPSACEAE.--Scabiosa atro-purpurea. + +The flowers, which are proterandrous, were fertilised during the +unfavourable season of 1867, so that I got few seeds, especially from +the self-fertilised heads, which were extremely sterile. The crossed and +self-fertilised plants raised from these seeds were measured before they +were in full flower, as in Table 5/65. + +TABLE 5/65. Scabiosa atro-purpurea. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 14 : 20. + +Pot 2 : 15 : 14 4/8. + +Pot 3 : 21 : 14. +Pot 3 : 18 4/8 : 13. + +Total : 68.5 : 61.5. + +The four crossed plants averaged 17.12, and the four self-fertilised +15.37 inches in height; or as 100 to 90. One of the self-fertilised +plants in Pot 3 was killed by an accident, and its fellow pulled up; so +that when they were again measured to the summits of their flowers, +there were only three on each side; the crossed now averaged in height +32.83, and the self-fertilised 30.16 inches; or as 100 to 92. + +20. COMPOSITAE.--Lactuca sativa. (5/19. The Compositae are well-adapted +for cross-fertilisation, but a nurseryman on whom I can rely, told me +that he had been in the habit of sowing several kinds of lettuce near +together for the sake of seed, and had never observed that they became +crossed. It is very improbable that all the varieties which were thus +cultivated near together flowered at different times; but two which I +selected by hazard and sowed near each other did not flower at the same +time; and my trial failed.) + +Three plants of Lettuce (Great London Cos var.) grew close together in +my garden; one was covered by a net, and produced self-fertilised seeds, +the other two were allowed to be naturally crossed by insects; but the +season (1867) was unfavourable, and I did not obtain many seeds. Only +one crossed and one self-fertilised plant were raised in Pot 1, and +their measurements are given in Table 5/66. The flowers on this one +self-fertilised plant were again self-fertilised under a net, not with +pollen from the same floret, but from other florets on the same head. +The flowers on the two crossed plants were left to be crossed by +insects, but the process was aided by some pollen being occasionally +transported by me from plant to plant. These two lots of seeds, after +germinating on sand, were planted in pairs on the opposite sides of Pots +2 and 3, which were at first kept in the greenhouse and then turned out +of doors. The plants were measured when in full flower. Table 5/66, +therefore, includes plants belonging to two generations. When the +seedlings of the two lots were only 5 or 6 inches in height they were +equal. In Pot 3 one of the self-fertilised plants died before flowering, +as has occurred in so many other cases. + +TABLE 5/66. Lactuca sativa. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 27 : 21 4/8. +Pot 1 : 25 : 20. +First generation, planted in open ground. + +Pot 2 : 29 4/8 : 24. +Pot 2 : 17 4/8 : 10. +Pot 2 : 12 4/8 : 11. +Second generation, planted in open ground. + +Pot 3 : 14 : 9 4/8. +Pot 3 : 10 4/8 : 0. +Second generation, kept in the pot. + +Total : 136 : 96. + +The average height of the seven crossed plants is 19.43, and that of the +six self-fertilised plants 16 inches; or as 100 to 82. + +21. CAMPANULACEAE.--Specularia speculum. + +In the closely allied genus, Campanula, in which Specularia was formerly +included, the anthers shed at an early period their pollen, and this +adheres to the collecting hairs which surround the pistil beneath the +stigma; so that without some mechanical aid the flowers cannot be +fertilised. For instance, I covered up a plant of Campanula carpathica, +and it did not produce a single capsule, whilst the surrounding +uncovered plants seeded profusely. On the other hand, the present +species of Specularia appears to set almost as many capsules when +covered up, as when left to the visits of the Diptera, which, as far as +I have seen, are the only insects that frequent the flowers. (5/20. It +has long been known that another species of the genus, Specularia +perfoliata, produces cleistogene as well as perfect flowers, and the +former are of course self-fertile.) I did not ascertain whether the +naturally crossed and spontaneously self-fertilised capsules contained +an equal number of seeds, but a comparison of artificially crossed and +self-fertilised flowers, showed that the former were probably the most +productive. It appears that this plant is capable of producing a large +number of self-fertilised capsules owing to the petals closing at night, +as well as during cold weather. In the act of closing, the margins of +the petals become reflexed, and their inwardly projecting midribs then +pass between the clefts of the stigma, and in doing so push the pollen +from the outside of the pistil on to the stigmatic surfaces. (5/21. Mr. +Meehan has lately shown 'Proceedings of the Academy of Natural Science +Philadelphia' May 16, 1876 page 84, that the closing of the flowers of +Claytonia virginica and Ranunculus bulbosus during the night causes +their self-fertilisation.) + +Twenty flowers were fertilised by me with their own pollen, but owing to +the bad season, only six capsules were produced; they contained on an +average 21.7 seeds, with a maximum of forty-eight in one. Fourteen +flowers were crossed with pollen from another plant, and these produced +twelve capsules, containing on an average 30 seeds, with a maximum in +one of fifty-seven seeds; so that the crossed seeds were to the +self-fertilised from an equal number of capsules as 100 to 72. The +former were also heavier than an equal number of self-fertilised seeds, +in the ratio of 100 to 86. Thus, whether we judge by the number of +capsules produced from an equal number of flowers, or by the average +number of the contained seeds, or the maximum number in any one capsule, +or by their weight, crossing does great good in comparison with +self-fertilisation. The two lots of seeds were sown on the opposite +sides of four pots; but the seedlings were not sufficiently thinned. +Only the tallest plant on each side was measured, when fully grown. The +measurements are given in Table 5/67. In all four pots the crossed +plants flowered first. When the seedlings were only about an inch and a +half in height both lots were equal. + +TABLE 5/67. Specularia speculum. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Tallest Crossed Plant in each Pot. + +Column 3: Tallest Self-fertilised Plant in each Pot. + +Pot 1 : 18 : 15 6/8. + +Pot 2 : 17 : 19. + +Pot 3 : 22 1/8 : 18. + +Pot 4 : 20 : 23. + +Total : 77.13 : 75.75. + +The four tallest crossed plants averaged 19.28, and the four tallest +self-fertilised 18.93 inches in height; or as 100 to 98. So that there +was no difference worth speaking of between the two lots in height; +though other great advantages are derived, as we have seen, from +cross-fertilisation. From being grown in pots and kept in the +greenhouse, none of the plants produced any capsules. + +Lobelia ramosa. (5/22. I have adopted the name given to this plant in +the 'Gardeners' Chronicle' 1866. Professor T. Dyer, however, informs me +that it probably is a white variety of L. tenuior of R. Brown, from W. +Australia.) + +VAR. SNOW-FLAKE. + +The well-adapted means by which cross-fertilisation is ensured in this +genus have been described by several authors. (5/23. See the works of +Hildebrand and Delpino. Mr. Farrer also has given a remarkably clear +description of the mechanism by which cross-fertilisation is effected in +this genus, in the 'Annals and Magazine of Natural History' volume 2 4th +series 1868 page 260. In the allied genus Isotoma, the curious spike +which projects rectangularly from the anthers, and which when shaken +causes the pollen to fall on the back of an entering insect, seems to +have been developed from a bristle, like one of those which spring from +the anthers in some of or all the species of Lobelia, as described by +Mr. Farrer.) The pistil as it slowly increases in length pushes the +pollen out of the conjoined anthers, by the aid of a ring of bristles; +the two lobes of the stigma being at this time closed and incapable of +fertilisation. The extrusion of the pollen is also aided by insects, +which rub against the little bristles that project from the anthers. The +pollen thus pushed out is carried by insects to the older flowers, in +which the stigma of the now freely projecting pistil is open and ready +to be fertilised. I proved the importance of the gaily-coloured corolla, +by cutting off the large flowers of Lobelia erinus; and these flowers +were neglected by the hive-bees which were incessantly visiting the +other flowers. + +A capsule was obtained by crossing a flower of L. ramosa with pollen +from another plant, and two other capsules from artificially +self-fertilised flowers. The contained seeds were sown on the opposite +sides of four pots. Some of the crossed seedlings which came up before +the others had to be pulled up and thrown away. Whilst the plants were +very small there was not much difference in height between the two lots; +but in Pot 3 the self-fertilised were for a time the tallest. When in +full flower the tallest plant on each side of each pot was measured, and +the result is shown in Table 5/68. In all four pots a crossed plant +flowered before any one of its opponents. + +TABLE 5/68. Lobelia ramosa (First Generation). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Tallest Crossed Plant in each Pot. + +Column 3: Tallest Self-fertilised Plant in each Pot. + +Pot 1 : 22 4/8 : 17 4/8. + +Pot 2 : 27 4/8 : 24. + +Pot 3 : 16 4/8 : 15. + +Pot 4 : 22 4/8 : 17. + +Total : 89.0 : 73.5. + +The four tallest crossed plants averaged 22.25, and the four tallest +self-fertilised 18.37 inches in height; or as 100 to 82. I was surprised +to find that the anthers of a good many of these self-fertilised plants +did not cohere and did not contain any pollen; and the anthers even of a +very few of the crossed plants were in the same condition. Some flowers +on the crossed plants were again crossed, four capsules being thus +obtained; and some flowers on the self-fertilised plants were again +self-fertilised, seven capsules being thus obtained. The seeds from both +lots were weighed, and it was calculated that an equal number of +capsules would have yielded seed in the proportion by weight of 100 for +the crossed to 60 for the self-fertilised capsules. So that the flowers +on the crossed plants again crossed were much more fertile than those on +the self-fertilised plants again self-fertilised. + +PLANTS OF THE SECOND GENERATION. + +The above two lots of seeds were placed on damp sand, and many of the +crossed seeds germinated, as on the last occasion, before the +self-fertilised, and were rejected. Three or four pairs in the same +state of germination were planted on the opposite sides of two pots; a +single pair in a third pot; and all the remaining seeds were sown +crowded in a fourth pot. When the seedlings were about one and a half +inches in height, they were equal on both sides of the three first pots; +but in Pot 4, in which they grew crowded and were thus exposed to severe +competition, the crossed were about a third taller than the +self-fertilised. In this latter pot, when the crossed averaged 5 inches +in height, the self-fertilised were about 4 inches; nor did they look +nearly such fine plants. In all four pots the crossed plants flowered +some days before the self-fertilised. When in full flower the tallest +plant on each side was measured; but before this time the single crossed +plant in Pot 3, which was taller than its antagonist, had died and was +not measured. So that only the tallest plant on each side of three pots +was measured, as in Table 5/69. + +TABLE 5/69. Lobelia ramosa (Second Generation). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Tallest Crossed Plant in each Pot. + +Column 3: Tallest Self-fertilised Plant in each Pot. + +Pot 1 : 27 4/8 : 18 4/8. + +Pot 2 : 21 : 19 4/8. + +Pot 3 : 21 4/8 : 19. +Crowded. + +Total : 70 : 57. + +The average height of the three tallest crossed plants is here 23.33, +and that of the tallest self-fertilised 19 inches; or as 100 to 81. +Besides this difference in height, the crossed plants were much more +vigorous and more branched than the self-fertilised plants, and it is +unfortunate that they were not weighed. + +Lobelia fulgens. + +This species offers a somewhat perplexing case. In the first generation +the self-fertilised plants, though few in number, greatly exceeded the +crossed in height; whilst in the second generation, when the trial was +made on a much larger scale, the crossed beat the self-fertilised +plants. As this species is generally propagated by off-sets, some +seedlings were first raised, in order to have distinct plants. On one of +these plants several flowers were fertilised with their own pollen; and +as the pollen is mature and shed long before the stigma of the same +flower is ready for fertilisation, it was necessary to number each +flower and keep its pollen in paper with a corresponding number. By this +means well-matured pollen was used for self-fertilisation. Several +flowers on the same plant were crossed with pollen from a distinct +individual, and to obtain this the conjoined anthers of young flowers +were roughly squeezed, and as it is naturally protruded very slowly by +the growth of the pistil, it is probable that the pollen used by me was +hardly mature, certainly less mature than that employed for +self-fertilisation. I did not at the time think of this source of error, +but I now suspect that the growth of the crossed plants was thus +injured. Anyhow the trial was not perfectly fair. Opposed to the belief +that the pollen used in crossing was not in so good a state as that used +for self-fertilisation, is the fact that a greater proportional number +of the crossed than of the self-fertilised flowers produced capsules; +but there was no marked difference in the amount of seed contained in +the capsules of the two lots. (5/24. Gartner has shown that certain +plants of Lobelia fulgens are quite sterile with pollen from the same +plant, though this pollen is efficient on any other individual; but none +of the plants on which I experimented, which were kept in the +greenhouse, were in this peculiar condition.) + +As the seeds obtained by the above two methods would not germinate when +left on bare sand, they were sown on the opposite sides of four pots; +but I succeeded in raising only a single pair of seedlings of the same +age in each pot. The self-fertilised seedlings, when only a few inches +in height, were in most of the pots taller than their opponents; and +they flowered so much earlier in all the pots, that the height of the +flower-stems could be fairly compared only in Pots 1 and 2. + +TABLE 5/70. Lobelia fulgens (First Generation). + +Heights of flower-stems measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Height of Flower-stems on the Crossed Plants. + +Column 3: Height of Flower-stems on the Self-fertilised Plants. + +Pot 1 : 33 : 50. + +Pot 2 : 36 4/8 : 38 4/8. + +Pot 3 : 21* : 43. + +Pot 4 : 12* : 35 6/8. + +*Not in full flower. + +The mean height of the flower-stems of the two crossed plants in Pots 1 +and 2 is here 34.75 inches, and that of the two self-fertilised plants +in the same pots 44.25 inches; or as 100 to 127. The self-fertilised +plants in Pots 3 and 4 were in every respect very much finer than the +crossed plants. + +I was so much surprised at this great superiority of the self-fertilised +over the crossed plants, that I determined to try how they would behave +in one of the pots during a second growth. The two plants, therefore, in +Pot 1 were cut down, and repotted without being disturbed in a much +larger pot. In the following year the self-fertilised plant showed even +a greater superiority than before; for the two tallest flower-stems +produced by the one crossed plant were only 29 4/8 and 30 1/8 inches in +height, whereas the two tallest stems on the one self-fertilised plant +were 49 4/8 and 49 6/8 inches; and this gives a ratio of 100 to 167. +Considering all the evidence, there can be no doubt that these +self-fertilised plants had a great superiority over the crossed plants. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION. + +TABLE 5/71. Lobelia fulgens (Second Generation). + +Heights of flower-stems measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 27 3/8 : 32 3/8. +Pot 1 : 26 : 26 3/8. +Pot 1 : 24 3/8 : 25 1/8. +Pot 1 : 24 4/8 : 26 2/8. + +Pot 2 : 34 : 36 2/8. +Pot 2 : 26 6/8 : 28 6/8. +Pot 2 : 25 1/8 : 30 1/8. +Pot 2 : 26 : 32 2/8. + +Pot 3 : 40 4/8 : 30 4/8. +Pot 3 : 37 5/8 : 28 2/8. +Pot 3 : 32 1/8 : 23. + +Pot 4 : 34 5/8 : 29 4/8. +Pot 4 : 32 2/8 : 28 3/8. +Pot 4 : 29 3/8 : 26. +Pot 4 : 27 1/8 : 25 2/8. + +Pot 5 : 28 1/8 : 29. +Pot 5 : 27 : 24 6/8. +Pot 5 : 25 3/8 : 23 2/8. +Pot 5 : 24 3/8 : 24. + +Pot 6 : 33 5/8 : 44 2/8. +Pot 6 : 32 : 37 6/8. +Pot 6 : 26 1/8 : 37. +Pot 6 : 25 : 35. + +Pot 7 : 30 6/8 : 27 2/8. +Pot 7 : 30 3/8 : 19 2/8. +Pot 7 : 29 2/8 : 21. + +Pot 8 : 39 3/8 : 23 1/8. +Pot 8 : 37 2/8 : 23 4/8. +Pot 8 : 36 : 25 4/8. +Pot 8 : 36 : 25 1/8. + +Pot 9 : 33 3/8 : 19 3/8. +Pot 9 : 25 : 16 3/8. +Pot 9 : 25 3/8 : 19. +Pot 9 : 21 7/8 : 18 6/8. + +Total : 1014.00 : 921.63. + +I determined on this occasion to avoid the error of using pollen of not +quite equal maturity for crossing and self-fertilisation; so that I +squeezed pollen out of the conjoined anthers of young flowers for both +operations. Several flowers on the crossed plant in Pot 1 in Table 5/70 +were again crossed with pollen from a distinct plant. Several other +flowers on the self-fertilised plant in the same pot were again +self-fertilised with pollen from the anthers of other flowers on the +SAME PLANT. Therefore the degree of self-fertilisation was not quite so +close as in the last generation, in which pollen from the SAME FLOWER, +kept in paper, was used. These two lots of seeds were thinly sown on +opposite sides of nine pots; and the young seedlings were thinned, an +equal number of nearly as possible the same age being left on the two +sides. In the spring of the following year (1870), when the seedlings +had grown to a considerable size, they were measured to the tips of +their leaves; and the twenty-three crossed plants averaged 14.04 inches +in height, whilst the twenty-three self-fertilised seedlings were 13.54 +inches; or as 100 to 96. + +In the summer of the same year several of these plants flowered, the +crossed and self-fertilised plants flowering almost simultaneously, and +all the flower-stems were measured. Those produced by eleven of the +crossed plants averaged 30.71 inches, and those by nine of the +self-fertilised plants 29.43 inches in height; or as 100 to 96. + +The plants in these nine pots, after they had flowered, were repotted +without being disturbed in much larger pots; and in the following year, +1871, all flowered freely; but they had grown into such an entangled +mass, that the separate plants on each side could no longer be +distinguished. Accordingly three or four of the tallest flower-stems on +each side of each pot were measured; and the measurements in Table 5/71 +are, I think, more trustworthy than the previous ones, from being more +numerous, and from the plants being well established and growing +vigorously. + +The average height of the thirty-four tallest flower-stems on the +twenty-three crossed plants is 29.82 inches, and that of the same number +of flower-stems on the same number of self-fertilised plants is 27.10 +inches, or as 100 to 91. So that the crossed plants now showed a decided +advantage over their self-fertilised opponents. + +22. POLEMONIACEAE.--Nemophila insignis. + +Twelve flowers were crossed with pollen from a distinct plant, but +produced only six capsules, containing on an average 18.3 seeds. +Eighteen flowers were fertilised with their own pollen and produced ten +capsules, containing on an average 12.7 seeds, so that the seeds per +capsule were as 100 to 69. (5/25. Several species of Polemoniaceae are +known to be proterandrous, but I did not attend to this point in +Nemophila. Verlot says 'Des Variétés' 1865 page 66, that varieties +growing near one another spontaneously intercross.) The crossed seeds +weighed a little less than an equal number of self-fertilised seeds, in +the proportion of 100 to 105; but this was clearly due to some of the +self-fertilised capsules containing very few seeds, and these were much +bulkier than the others, from having been better nourished. A subsequent +comparison of the number of seeds in a few capsules did not show so +great a superiority on the side of the crossed capsules as in the +present case. + +The seeds were placed on sand, and after germinating were planted in +pairs on the opposite sides of five pots, which were kept in the +greenhouse. When the seedlings were from 2 to 3 inches in height, most +of the crossed had a slight advantage over the self-fertilised. The +plants were trained up sticks, and thus grew to a considerable height. +In four out of the five pots a crossed plant flowered before any one of +the self-fertilised. The plants were first measured to the tips of their +leaves, before they had flowered and when the crossed were under a foot +in height. The twelve crossed plants averaged 11.1 inches in height, +whilst the twelve self-fertilised were less than half of this height, +namely, 5.45; or as 100 to 49. Before the plants had grown to their full +height, two of the self-fertilised died, and as I feared that this might +happen with others, they were again measured to the tops of their stems, +as shown in Table 5/72. + +TABLE 5/72. Nemophila insignis; 0 means that the plant died. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 32 4/8 : 21 2/8. + +Pot 2 : 34 4/8 : 23 5/8. + +Pot 3 : 33 1/8 : 19. +Pot 3 : 22 2/8 : 7 2/8. +Pot 3 : 29 : 17 4/8. + +Pot 4 : 35 4/8 : 10 4/8. +Pot 4 : 33 4/8 : 27. + +Pot 5 : 35 : 0. +Pot 5 : 38 : 18 3/8. +Pot 5 : 36 : 20 4/8. +Pot 5 : 37 4/8 : 34. +Pot 5 : 32 4/8 : 0. + +Total : 399.38 : 199.00. + +The twelve crossed plants now averaged 33.28, and the ten +self-fertilised 19.9 inches in height, or as 100 to 60; so that they +differed somewhat less than before. + +The plants in Pots 3 and 5 were placed under a net in the greenhouse, +two of the crossed plants in the latter pot being pulled up on account +of the death of two of the self-fertilised; so that altogether six +crossed and six self-fertilised plants were left to fertilise themselves +spontaneously. The pots were rather small, and the plants did not +produce many capsules. The small size of the self-fertilised plants will +largely account for the fewness of the capsules which they produced. The +six crossed plants bore 105, and the six self-fertilised only 30 +capsules; or as 100 to 29. + +The self-fertilised seeds thus obtained from the crossed and +self-fertilised plants, after germinating on sand, were planted on the +opposite sides of four small pots, and treated as before. But many of +the plants were unhealthy, and their heights were so unequal--some on +both sides being five times as tall as the others--that the averages +deduced from the measurements in Table 5/73 are not in the least +trustworthy. Nevertheless I have felt bound to give them, as they are +opposed to my general conclusions. + +The seven self-fertilised plants from the crossed plants here average +15.73, and the seven self-fertilised from the self-fertilised 21 inches +in height; or as 100 to 133. Strictly analogous experiments with Viola +tricolor and Lathyrus odoratus gave a very different result. + +TABLE 5/73. Nemophila insignis. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Self-fertilised Plants from Crossed Plants. + +Column 3: Self-fertilised Plants from Self-fertilised Plants. + +Pot 1 : 27 : 27 4/8. +Pot 1 : 14 : 34 2/8. + +Pot 2 : 17 6/8 : 23. +Pot 2 : 24 4/8 : 32. + +Pot 3 : 16 : 7. + +Pot 4 : 5 3/8 : 7 2/8. +Pot 4 : 5 4/8 : 16. + +Total : 110.13 : 147.00. + +23. BORAGINACEAE.--Borago officinalis. + +This plant is frequented by a greater number of bees than any other one +which I have observed. It is strongly proterandrous (H. Muller +'Befruchtung' etc. page 267), and the flowers can hardly fail to be +cross-fertilised; but should this not occur, they are capable of +self-fertilisation to a limited extent, as some pollen long remains +within the anthers, and is apt to fall on the mature stigma. In the year +1863 I covered up a plant, and examined thirty-five flowers, of which +only twelve yielded any seeds; whereas of thirty-five flowers on an +exposed plant growing close by, all with the exception of two yielded +seeds. The covered-up plant, however, produced altogether twenty-five +spontaneously self-fertilised seeds; the exposed plant producing +fifty-five seeds, the product, no doubt, of cross-fertilisation. + +In the year 1868 eighteen flowers on a protected plant were crossed with +pollen from a distinct plant, but only seven of these produced fruit; +and I suspect that I applied pollen to many of the stigmas before they +were mature. These fruits contained on an average 2 seeds, with a +maximum in one of three seeds. Twenty-four spontaneously self-fertilised +fruits were produced by the same plant, and these contained on an +average 1.2 seeds, with a maximum of two in one fruit. So that the +fruits from the artificially crossed flowers yielded seeds compared with +those from the spontaneously self-fertilised flowers, in the ratio of +100 to 60. But the self-fertilised seeds, as often occurs when few are +produced, were heavier than the crossed seeds in the ratio of 100 to 90. + +These two lots of seeds were sown on opposite sides of two large pots; +but I succeeded in raising only four pairs of equal age. When the +seedlings on both sides were about 8 inches in height they were equal. +When in full flower they were measured, as follows:-- + +TABLE 5/74. Borago officinalis. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 19 : 13 4/8. +Pot 1 : 21 : 18 6/8. +Pot 1 : 16 4/8 : 20 2/8. + +Pot 2 : 26 2/8 : 32 2/8. + +Total : 82.75 : 84.75. + +The average height of the four crossed plants is here 20.68, and that of +the four self-fertilised 21.18 inches; or as 100 to 102. The +self-fertilised plants thus exceeded the crossed in height by a little; +but this was entirely due to the tallness of one of the self-fertilised. +The crossed plants in both pots flowered before the self-fertilised. +Therefore I believe if more plants had been raised, the result would +have been different. I regret that I did not attend to the fertility of +the two lots. + +24. NOLANACEAE.--Nolana prostrata. + +In some of the flowers the stamens are considerably shorter than the +pistil, in others equal to it in length. I suspected, therefore, but +erroneously as it proved, that this plant was dimorphic, like Primula, +Linum, etc., and in the year 1862 twelve plants, covered by a net in the +greenhouse, were subjected to trial. The spontaneously self-fertilised +flowers yielded 64 grains weight of seeds, but the product of fourteen +artificially crossed flowers is here included, which falsely increases +the weight of the self-fertilised seeds. Nine uncovered plants, the +flowers of which were eagerly visited by bees for their pollen and were +no doubt intercrossed by them, produced 79 grains weight of seeds: +therefore twelve plants thus treated would have yielded 105 grains. Thus +the seeds produced by the flowers on an equal number of plants, when +crossed by bees, and spontaneously self-fertilised (the product of +fourteen artificially crossed flowers being, however, included in the +latter) were in weight as 100 to 61. + +In the summer of 1867 the trial was repeated; thirty flowers were +crossed with pollen from a distinct plant and produced twenty-seven +capsules, each containing five seeds. Thirty-two flowers were fertilised +with their own pollen, and produced only six capsules, each with five +seeds. So that the crossed and self-fertilised capsules contained the +same number of seeds, though many more capsules were produced by the +cross-fertilised than by the self-fertilised flowers, in the ratio of +100 to 21. + +An equal number of seeds of both lots were weighed, and the crossed +seeds were to the self-fertilised in weight as 100 to 82. Therefore a +cross increases the number of capsules produced and the weight of the +seeds, but not the number of seeds in each capsule. + +These two lots of seeds, after germinating on sand, were planted on the +opposite sides of three pots. The seedlings when from 6 to 7 inches in +height were equal. The plants were measured when fully grown, but their +heights were so unequal in the several pots, that the result cannot be +fully trusted. + +TABLE 5/75. Nolana prostrata. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 8 4/8 : 4 2/8. +Pot 1 : 6 4/8 : 7 4/8. + +Pot 2 : 10 4/8 : 14 4/8. +Pot 2 : 18 : 18. + +Pot 3 : 20 2/8 : 22 6/8. + +Total : 63.75 : 67.00. + +The five crossed plants average 12.75, and the five self-fertilised 13.4 +inches in height; or as 100 to 105. + + + +CHAPTER VI. + +SOLANACEAE, PRIMULACEAE, POLYGONEAE, ETC. + +Petunia violacea, crossed and self-fertilised plants compared for four +generations. +Effects of a cross with a fresh stock. +Uniform colour of the flowers on the self-fertilised plants of the +fourth generation. +Nicotiana tabacum, crossed and self-fertilised plants of equal height. +Great effects of a cross with a distinct sub-variety on the height, but +not on the fertility, of the offspring. +Cyclamen persicum, crossed seedlings greatly superior to the self-fertilised. +Anagallis collina. +Primula veris. +Equal-styled variety of Primula veris, fertility of, greatly increased +by a cross with a fresh stock. +Fagopyrum esculentum. +Beta vulgaris. +Canna warscewiczi, crossed and self-fertilised plants of equal height. +Zea mays. +Phalaris canariensis. + +25. SOLANACEAE. Petunia violacea. + +DINGY PURPLE VARIETY. + +The flowers of this plant are so seldom visited during the day by +insects in this country, that I have never seen an instance; but my +gardener, on whom I can rely, once saw some humble-bees at work. Mr. +Meehan says, that in the United States bees bore through the corolla for +the nectar, and adds that their "fertilisation is carried on by +night-moths." (6/1. 'Proceedings of the Academy of Natural Science of +Philadelphia' August 2, 1870 page 90.) + +In France M. Naudin, after castrating a large number of flowers whilst +in bud, left them exposed to the visits of insects, and about a quarter +produced capsules (6/2. 'Annales des Sc. Nat.' 4th series Bot. Tome 9 +cah. 5); but I am convinced that a much larger proportion of flowers in +my garden are cross-fertilised by insects, for protected flowers with +their own pollen placed on the stigma never yielded nearly a full +complement of seed; whilst those left uncovered produced fine capsules, +showing that pollen from other plants must have been brought to them, +probably by moths. Plants growing vigorously and flowering in pots in +the greenhouse, never yielded a single capsule; and this may be +attributed, at least in chief part, to the exclusion of moths. + +Six flowers on a plant covered by a net were crossed with pollen from a +distinct plant and produced six capsules, containing by weight 4.44 +grains of seed. Six other flowers were fertilised with their own pollen +and produced only three capsules, containing only 1.49 grains weight of +seed. From this it follows that an equal number of crossed and +self-fertilised capsules would have contained seeds by weight as 100 to +67. I should not have thought the proportional contents of so few +capsules worth giving, had not nearly the same result been confirmed by +several subsequent trials. + +Seeds of the two lots were placed on sand, and many of the +self-fertilised seeds germinated before the crossed, and were rejected. +Several pairs in an equal state of germination were planted on the +opposite sides of Pots 1 and 2; but only the tallest plant on each side +was measured. Seeds were also sown thickly on the two sides of a large +pot (3), the seedlings being afterwards thinned, so that an equal number +was left on each side; the three tallest on each side being measured. +The pots were kept in the greenhouse, and the plants were trained up +sticks. For some time the young crossed plants had no advantage in +height over the self-fertilised; but their leaves were larger. When +fully grown and in flower the plants were measured, as follows:-- + +TABLE 6/76. Petunia violacea (first generation). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 30 : 20 4/8. + +Pot 2 : 34 4/8 : 27 4/8. + +Pot 3 : 34 : 28 4/8. +Pot 3 : 30 4/8 : 27 4/8. +Pot 3 : 25 : 26. + +Total : 154 : 130. + +The five tallest crossed plants here average 30.8, and the five tallest +self-fertilised 26 inches in height, or as 100 to 84. + +Three capsules were obtained by crossing flowers on the above crossed +plants, and three other capsules by again self-fertilising flowers on +the self-fertilised plants. One of the latter capsules appeared as fine +as any one of the crossed capsules; but the other two contained many +imperfect seeds. From these two lots of seeds the plants of the +following generation were raised. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION. + +As in the last generation, many of the self-fertilised seeds germinated +before the crossed. + +Seeds in an equal state of germination were planted on the opposite +sides of three pots. The crossed seedlings soon greatly exceeded in +height the self-fertilised. In Pot 1, when the tallest crossed plant was +10 1/2 inches high, the tallest self-fertilised was only 3 1/2 inches; +in Pot 2 the excess in height of the crossed was not quite so great. The +plants were treated as in the last generation, and when fully grown +measured as before. In Pot 3 both the crossed plants were killed at an +early age by some animal, so that the self-fertilised had no +competitors. Nevertheless these two self-fertilised plants were +measured, and are included in Table 6/77. The crossed plants flowered +long before their self-fertilised opponents in Pots 1 and 2, and before +those growing separately in Pot 3. + +TABLE 6/77. Petunia violacea (Second generation). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 57 2/8 : 13 4/8. +Pot 1 : 36 2/8 : 8. + +Pot 2 : 44 4/8 : 33 2/8. +Pot 2 : 24 : 28. + +Pot 3 : 0 : 46 2/8. +Pot 3 : 0 : 28 4/8. + +Total : 162.0 : 157.5. + +The four crossed plants average 40.5, and the six self-fertilised 26.25 +inches in height; or as 100 to 65. But this great inequality is in part +accidental, owing to some of the self-fertilised plants being very +short, and to one of the crossed being very tall. + +Twelve flowers on these crossed plants were again crossed, and eleven +capsules were produced; of these, five were poor and six good; the +latter contained by weight 3.75 grains of seeds. Twelve flowers on the +self-fertilised plants were again fertilised with their own pollen and +produced no less than twelve capsules, and the six finest of these +contained by weight 2.57 grains of seeds. It should however be observed +that these latter capsules were produced by the plants in Pot 3, which +were not exposed to any competition. The seeds in the six fine crossed +capsules to those in the six finest self-fertilised capsules were in +weight as 100 to 68. From these seeds the plants of the next generation +were raised. + +CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION. + +TABLE 6/78. Petunia violacea (third generation; plants very young). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 1 4/8 : 5 6/8. +Pot 1 : 1 : 4 4/8. + +Pot 2 : 5 7/8 : 8 3/8. +Pot 2 : 5 6/8 : 6 7/8. + +Pot 3 : 4 : 5 5/8. + +Pot 4 : 1 4/8 : 5 3/8. + +Total : 19.63 : 36.50. + +The above seeds were placed on sand, and after germinating were planted +in pairs on the opposite sides of four pots; and all the remaining seeds +were thickly sown on the two sides of a fifth large pot. The result was +surprising, for the self-fertilised seedlings very early in life beat +the crossed, and at one time were nearly double their height. At first +the case appeared like that of Mimulus, in which after the third +generation a tall and highly self-fertile variety appeared. But as in +the two succeeding generations the crossed plants resumed their former +superiority over the self-fertilised, the case must be looked at as an +anomaly. The sole conjecture which I can form is that the crossed seeds +had not been sufficiently ripened, and thus produced weakly plants, as +occurred with Iberis. When the crossed plants were between 3 and 4 +inches in height, the six finest in four of the pots were measured to +the summits of their stems, and at the same time the six finest of the +self-fertilised plants. The measurements are given in Table 6/78, and it +may be here seen that all the self-fertilised plants exceed their +opponents in height, whereas when subsequently measured the excess of +the self-fertilised depended chiefly on the unusual tallness of two of +the plants in Pot 2. The crossed plants here average 3.27, and the +self-fertilised 6.08 inches in height; or as 100 to 186. + +When fully grown they were again measured, as follows:-- + +TABLE 6/79. Petunia violacea (third generation; plants fully grown). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 41 4/8 : 40 6/8. +Pot 1 : 48 : 39. +Pot 1 : 36 : 48. + +Pot 2 : 36 : 47. +Pot 2 : 21 : 80 2/8. +Pot 2 : 36 2/8 : 86 2/8. + +Pot 3 : 52 : 46. + +Pot 4 : 57 : 43 6/8. + +Total : 327.75 : 431.00. + +The eight crossed plants now averaged 40.96, and the eight +self-fertilised plants 53.87 inches in height, or as 100 to 131; and +this excess chiefly depended, as already stated, on the unusual tallness +of two of the self-fertilised plants in Pot 2. The self-fertilised had +therefore lost some of their former great superiority over the crossed +plants. In three of the pots the self-fertilised plants flowered first; +but in Pot 3 at the same time with the crossed. + +The case is rendered the more strange, because the crossed plants in the +fifth pot (not included in the two last tables), in which all the +remaining seeds had been thickly sown, were from the first finer plants +than the self-fertilised, and had larger leaves. At the period when the +two tallest crossed plants in this pot were 6 4/8 and 4 5/8 inches high, +the two tallest self-fertilised were only 4 inches. When the two crossed +plants were 12 and 10 inches high, the two self-fertilised were only 8 +inches. These latter plants, as well as many others on the same side of +this pot never grew any higher, whereas several of the crossed plants +grew to the height of two feet! On account of this great superiority of +the crossed plants, the plants on neither side of this pot have been +included in the two last tables. + +Thirty flowers on the crossed plants in Pots 1 and 4 (Table 6/79) were +again crossed, and produced seventeen capsules. Thirty flowers on the +self-fertilised plants in the same two pots were again self-fertilised, +but produced only seven capsules. The contents of each capsule of both +lots were placed in separate watch-glasses, and the seeds from the +crossed appeared to the eye to be at least double the number of those +from the self-fertilised capsules. + +In order to ascertain whether the fertility of the self-fertilised +plants had been lessened by the plants having been self-fertilised for +the three previous generations, thirty flowers on the crossed plants +were fertilised with their own pollen. These yielded only five capsules, +and their seeds being placed in separate watch-glasses did not seem more +numerous than those from the capsules on the self-fertilised plants +self-fertilised for the fourth time. So that as far as can be judged +from so few capsules, the self-fertility of the self-fertilised plants +had not decreased in comparison with that of the plants which had been +intercrossed during the three previous generations. It should, however, +be remembered that both lots of plants had been subjected in each +generation to almost exactly similar conditions. + +Seeds from the crossed plants again crossed, and from the +self-fertilised again self-fertilised, produced by the plants in Pot 1 +(Table 6/79), in which the three self-fertilised plants were on an +average only a little taller than the crossed, were used in the +following experiment. They were kept separate from two similar lots of +seeds produced by the two plants in Pot 4 in the same table, in which +the crossed plant was much taller than its self-fertilised opponent. + +CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM +THE PLANTS IN POT 1, TABLE 6/79). + +Crossed and self-fertilised seeds from plants of the last generation in +Pot 1 in Table 6/79, were placed on sand, and after germinating, were +planted in pairs on the opposite sides of four pots. The seedlings when +in full flower were measured to the base of the calyx. The remaining +seeds were sown crowded on the two sides of Pot 5; and the four tallest +plants on each side of this pot were measured in the same manner. + +TABLE 6/80. Petunia violacea (fourth generation; raised from plants of +the third generation in Pot 1, table 6/79). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 29 2/8 : 30 2/8. +Pot 1 : 36 2/8 : 34 6/8. +Pot 1 : 49 : 31 3/8. + +Pot 2 : 33 3/8 : 31 5/8. +Pot 2 : 37 3/8 : 38 2/8. +Pot 2 : 56 4/8 : 38 4/8. + +Pot 3 : 46 : 45 1/8. +Pot 3 : 67 2/8 : 45. +Pot 3 : 54 3/8 : 23 2/8. + +Pot 4 : 51 6/8 : 34. +Pot 4 : 51 7/8 : 0. + +Pot 5 : 49 4/8 : 22 3/8. +Pot 5 : 46 3/8 : 24 2/8. +Pot 5 : 40 : 24 6/8. +Pot 5 : 53 : 30. +Crowded plants. + +Total : 701.88 : 453.50. + +The fifteen crossed plants average 46.79, and the fourteen (one having +died) self-fertilised plants 32.39 inches in height; or as 100 to 69. So +that the crossed plants in this generation had recovered their wonted +superiority over the self-fertilised plants; though the parents of the +latter in Pot 1, Table 6/79, were a little taller than their crossed +opponents. + +CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM +THE PLANTS IN POT 4, TABLE 6/79). + +Two similar lots of seeds, obtained from the plants in Pot 4 in Table +6/79, in which the single crossed plant was at first shorter, but +ultimately much taller than its self-fertilised opponent, were treated +in every way like their brethren of the same generation in the last +experiment. We have in Table 6/81 the measurements of the present +plants. Although the crossed plants greatly exceeded in height the +self-fertilised; yet in three out of the five pots a self-fertilised +plant flowered before any one of the crossed; in a fourth pot +simultaneously; and in a fifth (namely Pot 2) a crossed plant flowered +first. + +TABLE 6/81. Petunia violacea (fourth generation; raised from plants of +the third generation in Pot 4, Table 6/79). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 46 : 30 2/8. +Pot 1 : 46 : 28. + +Pot 2 : 50 6/8 : 25. +Pot 2 : 40 2/8 : 31 3/8. +Pot 2 : 37 3/8 : 22 4/8. + +Pot 3 : 54 2/8 : 22 5/8. +Pot 3 : 61 1/8 : 26 6/8. +Pot 3 : 45 : 32. + +Pot 4 : 30 : 24 4/8. +Pot 4 : 29 1/8 : 26. + +Pot 5 : 37 4/8 : 40 2/8. +Pot 5 : 63 : 18 5/8. +Pot 5 : 41 2/8 : 17 4/8. +Crowded plants. + +Total : 581.63 : 349.36. + +The thirteen crossed plants here average 44.74, and the thirteen +self-fertilised plants 26.87 inches in height; or as 100 to 60. The +crossed parents of these were much taller, relatively to the +self-fertilised parents, than in the last case; and apparently they +transmitted some of this superiority to their crossed offspring. It is +unfortunate that I did not turn these plants out of doors, so as to +observe their relative fertility, for I compared the pollen from some of +the crossed and self-fertilised plants in Pot 1, Table 6/81, and there +was a marked difference in its state; that of the crossed plants +contained hardly any bad and empty grains, whilst such abounded in the +pollen of the self-fertilised plants. + +THE EFFECTS OF A CROSS WITH A FRESH STOCK. + +I procured from a garden in Westerham, whence my plants originally came, +a fresh plant differing in no respect from mine except in the colour of +the flowers, which was a fine purple. But this plant must have been +exposed during at least four generations to very different conditions +from those to which my plants had been subjected, as these had been +grown in pots in the greenhouse. Eight flowers on the self-fertilised +plants in Table 6/81, of the last or fourth self-fertilised generation, +were fertilised with pollen from this fresh stock; all eight produced +capsules containing together by weight 5.01 grains of seeds. The plants +raised from these seeds may be called the Westerham-crossed. + +Eight flowers on the crossed plants of the last or fourth generation in +Table 6/81 were again crossed with pollen from one of the other crossed +plants, and produced five capsules, containing by weight 2.07 grains of +seeds. The plants raised from these seeds may be called the +INTERCROSSED; and these form the fifth intercrossed generation. + +Eight flowers on the self-fertilised plants of the same generation in +Table 6/81 were again self-fertilised, and produced seven capsules, +containing by weight 2.1 grains of seeds. The SELF-FERTILISED plants +raised from these seeds form the fifth self-fertilised generation. These +latter plants and the intercrossed are comparable in all respects with +the crossed and self-fertilised plants of the four previous generations. + +From the foregoing data it is easy to calculate that: + +Ten Westerham-crossed capsules would have contained 6.26 grains weight +of seed. + +Ten intercrossed capsules would have contained 4.14 grains weight of +seed. + +Ten self-fertilised capsules would have contained 3.00 grains weight of +seed. + +We thus get the following ratios:-- + +Seeds from the Westerham-crossed capsules to those from the capsules of +the fifth self-fertilised generation, in weight as 100 to 48. + +Seeds from the Westerham-crossed capsules to those from the capsules of +the fifth intercrossed generation, in weight as 100 to 66. + +Seeds from the intercrossed capsules to those from the self-fertilised +capsules, in weight as 100 to 72. + +So that a cross with pollen from a fresh stock greatly increased the +productiveness of the flowers on plants which had been self-fertilised +for the four previous generations, in comparison not only with the +flowers on the same plants self-fertilised for the fifth time, but with +the flowers on the crossed plants crossed with pollen from another plant +of the same old stock for the fifth time. + +These three lots of seeds were placed on sand, and were planted in an +equal state of germination in seven pots, each made tripartite by three +superficial partitions. Some of the remaining seeds, whether or not in a +state of germination, were thickly sown in an eighth pot. The pots were +kept in the greenhouse, and the plants trained up sticks. They were +first measured to the tops of their stems when coming into flower; and +the twenty-two Westerham-crossed plants then averaged 25.51 inches; the +twenty-three intercrossed plants 30.38; and the twenty-three +self-fertilised plants 23.40 inches in height. We thus get the following +ratios:-- + +The Westerham-crossed plants in height to the self-fertilised as 100 to +91. + +The Westerham-crossed plants in height to the intercrossed as 100 to +119. + +The intercrossed plants in height to the self-fertilised as 100 to 77. + +These plants were again measured when their growth appeared on a casual +inspection to be complete. But in this I was mistaken, for after cutting +them down, I found that the summits of the stems of the +Westerham-crossed plants were still growing vigorously; whilst the +intercrossed had almost, and the self-fertilised had quite completed +their growth. Therefore I do not doubt, if the three lots had been left +to grow for another month, that the ratios would have been somewhat +different from those deduced from the measurements in Table 6/82. + +TABLE 6/82. Petunia violacea. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Westerham-Crossed Plants (from self-fertilised Plants of +fourth generation crossed by a fresh stock). + +Column 3: Intercrossed Plants (Plants of one and the same stock +intercrossed for five generations). + +Column 4: Self-fertilised Plants (self-fertilised for five generations). + +Pot 1 : 64 5/8 : 57 2/8 : 43 6/8. +Pot 1 : 24 : 64 : 56 3/8. +Pot 1 : 51 4/8 : 58 6/8 : 31 5/8. + +Pot 2 : 48 7/8 : 59 7/8 : 41 5/8. +Pot 2 : 54 4/8 : 58 2/8 : 41 2/8. +Pot 2 : 58 1/8 : 53 : 18 2/8. + +Pot 3 : 62 : 52 2/8 : 46 6/8. +Pot 3 : 53 2/8 : 54 6/8 : 45. +Pot 3 : 62 7/8 : 61 6/8 : 19 4/8. + +Pot 4 : 44 4/8 : 58 7/8 : 37 5/8. +Pot 4 : 49 2/8 : 65 2/8 : 33 2/8. +Pot 4 : .. : 59 6/8 : 32 2/8. + +Pot 5 : 43 1/8 : 35 6/8 : 41 6/8. +Pot 5 : 53 7/8 : 34 6/8 : 26 4/8. +Pot 5 : 53 2/8 : 54 6/8 : 0. + +Pot 6 : 37 4/8 : 56 : 46 4/8. +Pot 6 : 61 : 63 5/8 : 29 6/8. +Pot 6 : 0 : 57 7/8 : 14 4/8. + +Pot 7 : 59 6/8 : 51 : 43. +Pot 7 : 43 4/8 : 49 6/8 : 12 2/8. +Pot 7 : 50 5/8 : 0 : 0. + +Pot 8 : 37 7/8 : 38 5/8 : 21 6/8. +Pot 8 : 37 2/8 : 44 5/8 : 14 5/8. + +Total : 1051.25 : 1190.50 : 697.88. + +The twenty-one Westerham-crossed plants now averaged 50.05 inches; the +twenty-two intercrossed plants, 54.11 inches; and the twenty-one +self-fertilised plants, 33.23 inches in height. We thus get the +following ratios:-- + +The Westerham-crossed plants in height to the self-fertilised as 100 to +66. + +The Westerham-crossed plants in height to the intercrossed as 100 to +108. + +The intercrossed plants in height to the self-fertilised as 100 to 61. + +We here see that the Westerham-crossed (the offspring of plants +self-fertilised for four generations and then crossed with a fresh +stock) have gained greatly in height, since they were first measured, +relatively to the plants self-fertilised for five generations. They were +then as 100 to 91, and now as 100 to 66 in height. The intercrossed +plants (i.e., those which had been intercrossed for the last five +generations) likewise exceed in height the self-fertilised plants, as +occurred in all the previous generations with the exception of the +abnormal plants of the third generation. On the other hand, the +Westerham-crossed plants are exceeded in height by the intercrossed; and +this is a surprising fact, judging from most of the other strictly +analogous cases. But as the Westerham-crossed plants were still growing +vigorously, while the intercrossed had almost ceased to grow, there can +hardly be a doubt that if left to grow for another month they would have +beaten the intercrossed in height. That they were gaining on them is +clear, as when measured before they were as 100 to 119, and now as only +100 to 108 in height. The Westerham-crossed plants had also leaves of a +darker green, and looked altogether more vigorous than the intercrossed; +and what is much more important, they produced, as we shall presently +see, much heavier seed-capsules. So that in fact the offspring from the +self-fertilised plants of the fourth generation crossed by a fresh stock +were superior to the intercrossed, as well as to the self-fertilised +plants of the fifth generation--of which latter fact there could not be +the least doubt. + +These three lots of plants were cut down close to the ground and +weighed. The twenty-one Westerham-crossed plants weighed 32 ounces; the +twenty-two intercrossed plants, 34 ounces, and the twenty-one +self-fertilised plants 7 1/4 ounces. The following ratios are calculated +for an equal number of plants of each kind. But as the self-fertilised +plants were just beginning to wither, their relative weight is here +slightly too small; and as the Westerham-crossed were still growing +vigorously, their relative weight with time allowed would no doubt have +greatly increased. + +The Westerham-crossed plants in weight to the self-fertilised as 100 to +22. + +The Westerham-crossed plants in weight to the intercrossed as 100 to +101. + +The intercrossed plants in weight to the self-fertilised as 100 to 22.3. + +We here see, judging by weight instead of as before by height, that the +Westerham-crossed and the intercrossed have an immense advantage over +the self-fertilised. The Westerham-crossed are inferior to the +intercrossed by a mere trifle; but it is almost certain that if they had +been allowed to go on growing for another month, the former would have +completely beaten the latter. + +As I had an abundance of seeds of the same three lots, from which the +foregoing plants had been raised, these were sown in three long parallel +and adjoining rows in the open ground, so as to ascertain whether under +these circumstances the results would be nearly the same as before. Late +in the autumn (November 13) the ten tallest plants were carefully +selected out of each row, and their heights measured, with the following +result:-- + +TABLE 6/83. Petunia violacea (plants growing in the open ground). + +Heights of plants measured in inches. + +Column 1: Westerham-Crossed Plants (from self-fertilised Plants of the +fourth generation crossed by a fresh stock). + +Column 2: intercrossed Plants (Plants of one and the same stock +intercrossed for five generations). + +Column 3: self-fertilised Plants (self-fertilised for five generations). + + 34 2/8 : 38 : 27 3/8. + 36 2/8 : 36 2/8 : 23. + 35 2/8 : 39 5/8 : 25. + 32 4/8 : 37 : 24 1/8. + 37 : 36 : 22 4/8. + 36 4/8 : 41 3/8 : 23 3/8. + 40 7/8 : 37 2/8 : 21 5/8. + 37 2/8 : 40 : 23 4/8. + 38 2/8 : 41 2/8 : 21 3/8. + 38 5/8 : 36 : 21 2/8. + + 366.76 : 382.76 : 233.13. + +The ten Westerham-crossed plants here average 36.67 inches in height; +the ten intercrossed plants, 38.27 inches; and the ten self-fertilised, +23.31 inches. These three lots of plants were also weighed; the +Westerham-crossed plants weighed 28 ounces; the intercrossed plants, 41 +ounces; and the self-fertilised, 14.75 ounces. We thus get the following +ratios:-- + +The Westerham-crossed plants in height to the self-fertilised as 100 to +63. + +The Westerham-crossed plants in weight to the self-fertilised as 100 to +53. + +The Westerham-crossed plants in height to the intercrossed as 100 to +104. + +The Westerham-crossed plants in weight to the intercrossed as 100 to +146. + +The intercrossed plants in height to the self-fertilised as 100 to 61. + +The intercrossed plants in weight to the self-fertilised as 100 to 36. + +Here the relative heights of the three lots are nearly the same (within +three or four per cent) as with the plants in the pots. In weight there +is a much greater difference: the Westerham-crossed exceed the +self-fertilised by much less than they did before; but the +self-fertilised plants in the pots had become slightly withered, as +before stated, and were in consequence unfairly light. The +Westerham-crossed plants are here inferior in weight to the intercrossed +plants in a much higher degree than in the pots; and this appeared due +to their being much less branched, owing to their having germinated in +greater numbers and consequently being much crowded. Their leaves were +of a brighter green than those of the intercrossed and self-fertilised +plants. + +RELATIVE FERTILITY OF THE THREE LOTS OF PLANTS. + +None of the plants in pots in the greenhouse ever produced a capsule; +and this may be attributed in chief part to the exclusion of moths. +Therefore the fertility of the three lots could be judged of only by +that of the plants growing out of doors, which from being left uncovered +were probably cross-fertilised. The plants in the three rows were +exactly of the same age and had been subjected to closely similar +conditions, so that any difference in their fertility must be attributed +to their different origin; namely, to the one lot being derived from +plants self-fertilised for four generations and then crossed with a +fresh stock; to the second lot being derived from plants of the same old +stock intercrossed for five generations; and to the third lot being +derived from plants self-fertilised for five generations. All the +capsules, some nearly mature and some only half-grown, were gathered, +counted, and weighed from the ten finest plants in each of the three +rows, of which the measurements and weights have already been given. The +intercrossed plants, as we have seen, were taller and considerably +heavier than the plants of the other two lots, and they produced a +greater number of capsules than did even the Westerham-crossed plants; +and this may be attributed to the latter having grown more crowded and +being in consequence less branched. Therefore the average weight of an +equal number of capsules from each lot of plants seems to be the fairest +standard of comparison, as their weights will have been determined +chiefly by the number of the included seeds. As the intercrossed plants +were taller and heavier than the plants of the other two lots, it might +have been expected that they would have produced the finest or heaviest +capsules; but this was very far from being the case. + +The ten tallest Westerham-crossed plants produced 111 ripe and unripe +capsules, weighing 121.2 grains. Therefore 100 of such capsules would +have weighed 109.18 grains. + +The ten tallest intercrossed plants produced 129 capsules, weighing +76.45 grains. Therefore 100 of these capsules would have weighed 59.26 +grains. + +The ten tallest self-fertilised plants produced only 44 capsules, +weighing 22.35 grains. Therefore 100 of these capsules would have +weighed 50.79 grains. + +From these data we get the following ratios for the fertility of the +three lots, as deduced from the relative weights of an equal number of +capsules from the finest plants in each lot:-- + +Westerham-crossed plants to self-fertilised plants as 100 to 46. + +Westerham-crossed plants to intercrossed plants as 100 to 54. + +Intercrossed plants to self-fertilised plants as 100 to 86. + +We here see how potent the influence of a cross with pollen from a fresh +stock has been on the fertility of plants self-fertilised for four +generations, in comparison with plants of the old stock when either +intercrossed or self-fertilised for five generations; the flowers on all +these plants having been left to be freely crossed by insects or to +fertilise themselves. The Westerham-crossed plants were also much taller +and heavier plants than the self-fertilised, both in the pots and open +ground; but they were less tall and heavy than the intercrossed plants. +This latter result, however, would almost certainly have been reversed, +if the plants had been allowed to grow for another month, as the +Westerham-crossed were still growing vigorously, whilst the intercrossed +had almost ceased to grow. This case reminds us of the somewhat +analogous one of Eschscholtzia, in which plants raised from a cross with +a fresh stock did not grow higher than the self-fertilised or +intercrossed plants, but produced a greater number of seed-capsules, +which contained a far larger average number of seeds. + +COLOUR OF THE FLOWERS ON THE ABOVE THREE LOTS OF PLANTS. + +The original mother-plant, from which the five successive +self-fertilised generations were raised, bore dingy purple flowers. At +no time was any selection practised, and the plants were subjected in +each generation to extremely uniform conditions. The result was, as in +some previous cases, that the flowers on all the self-fertilised plants, +both in the pots and open ground, were absolutely uniform in tint; this +being a dull, rather peculiar flesh colour. This uniformity was very +striking in the long row of plants growing in the open ground, and these +first attracted my attention. I did not notice in which generation the +original colour began to change and to become uniform, but I have every +reason to believe that the change was gradual. The flowers on the +intercrossed plants were mostly of the same tint, but not nearly so +uniform as those on the self-fertilised plants, and many of them were +pale, approaching almost to white. The flowers on the plants from the +cross with the purple-flowered Westerham stock were, as might have been +expected, much more purple and not nearly so uniform in tint. The +self-fertilised plants were also remarkably uniform in height, as judged +by the eye; the intercrossed less so, whilst the Westerham-crossed +plants varied much in height. + +Nicotiana tabacum. + +This plant offers a curious case. Out of six trials with crossed and +self-fertilised plants, belonging to three successive generations, in +one alone did the crossed show any marked superiority in height over the +self-fertilised; in four of the trials they were approximately equal; +and in one (i.e., in the first generation) the self-fertilised plants +were greatly superior to the crossed. In no case did the capsules from +flowers fertilised with pollen from a distinct plant yield many more, +and sometimes they yielded much fewer seeds than the capsules from +self-fertilised flowers. But when the flowers of one variety were +crossed with pollen from a slightly different variety, which had grown +under somewhat different conditions,--that is, by a fresh stock,--the +seedlings derived from this cross exceeded in height and weight those +from the self-fertilised flowers in an extraordinary degree. + +Twelve flowers on some plants of the common tobacco, raised from +purchased seeds, were crossed with pollen from a distinct plant of the +same lot, and these produced ten capsules. Twelve flowers on the same +plants were fertilised with their own pollen, and produced eleven +capsules. The seeds in the ten crossed capsules weighed 31.7 grains, +whilst those in ten of the self-fertilised capsules weighed 47.67 +grains; or as 100 to 150. The much greater productiveness of the +self-fertilised than of the crossed capsules can hardly be attributed to +chance, as all the capsules of both lots were very fine and healthy +ones. + +The seeds were placed on sand, and several pairs in an equal state of +germination were planted on the opposite sides of three pots. The +remaining seeds were thickly sown on the two sides of Pot 4, so that the +plants in this pot were much crowded. The tallest plant on each side of +each pot was measured. Whilst the plants were quite young the four +tallest crossed plants averaged 7.87 inches, and the four tallest +self-fertilised 14.87 inches in height; or as 100 to 189. The heights at +this age are given in the two left columns of Table 6/84. + +When in full flower the tallest plants on each side were again measured, +see the two right hand columns in Table 6/84. But I should state that +the pots were not large enough, and the plants never grew to their +proper height. The four tallest crossed plants now averaged 18.5, and +the four tallest self-fertilised plants 32.75 inches in height; or as +100 to 178. In all four pots a self-fertilised plant flowered before any +one of the crossed. + +In Pot 4, in which the plants were extremely crowded, the two lots were +at first equal; and ultimately the tallest crossed plant exceeded by a +trifle the tallest self-fertilised plant. This recalled to my mind an +analogous case in the one generation of Petunia, in which the +self-fertilised plants were throughout their growth taller than the +crossed in all the pots except in the crowded one. Accordingly another +trial was made, and some of the same crossed and self-fertilised seeds +of tobacco were sown thickly on opposite sides of two additional pots; +the plants being left to grow up much crowded. When they were between 13 +and 14 inches in height there was no difference between the two sides, +nor was there any marked difference when the plants had grown as tall as +they could; for in one pot the tallest crossed plant was 26 1/2 inches +in height, and exceeded by 2 inches the tallest self-fertilised plant, +whilst in the other pot, the tallest crossed plant was shorter by 3 1/2 +inches than the tallest self-fertilised plant, which was 22 inches in +height. + +TABLE 6/84. Nicotiana tabacum (first generation). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants, May 20, 1868. + +Column 3: self-fertilised Plants, May 20, 1868. + +Column 4: Crossed Plants, December 6, 1868. + +Column 5: self-fertilised Plants, December 6, 1868. + +Pot 1 : 15 4/8 : 26 : 40 : 44. + +Pot 2 : 3 : 15 : 6 4/8 : 43. + +Pot 3 : 8 : 13 4/8 : 16 : 33. + +Pot 4 : 5 : 5 : 11 4/8 : 11. + +Total : 31.5 : 59.5 : 74.0 : 131.0. + +As the plants did not grow to their proper height in the above small +pots in Table 6/84, four crossed and four self-fertilised plants were +raised from the same seed, and were planted in pairs on the opposite +sides of four very large pots containing rich soil; so that they were +not exposed to at all severe mutual competition. When these plants were +in flower I neglected to measure them, but record in my notes that all +four self-fertilised plants exceeded in height the four crossed plants +by 2 or 3 inches. We have seen that the flowers on the original or +parent-plants which were crossed with pollen from a distinct plant +yielded much fewer seeds than those fertilised with their own pollen; +and the trial just given, as well as that in Table 6/84, show us clearly +that the plants raised from the crossed seeds were inferior in height to +those from the self-fertilised seeds; but only when not greatly crowded. +When crowded and thus subjected to very severe competition, the crossed +and self-fertilised plants were nearly equal in height. + +CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION. + +Twelve flowers on the crossed plants of the last generation growing in +the four large pots just mentioned, were crossed with pollen from a +crossed plant growing in one of the other pots; and twelve flowers on +the self-fertilised plants were fertilised with their own pollen. All +these flowers of both lots produced fine capsules. Ten of the crossed +capsules contained by weight 38.92 grains of seeds, and ten of the +self-fertilised capsules 37.74 grains; or as 100 to 97. Some of these +seeds in an equal state of germination were planted in pairs on the +opposite sides of five large pots. A good many of the crossed seeds +germinated before the self-fertilised, and were of course rejected. The +plants thus raised were measured when several of them were in full +flower. + +TABLE 6/85. Nicotiana tabacum (second generation). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 14 4/8 : 27 6/8. +Pot 1 : 78 4/8 : 8 6/8. +Pot 1 : 9 : 56. + +Pot 2 : 60 4/8 : 16 6/8. +Pot 2 : 44 6/8 : 7. +Pot 2 : 10 : 50 4/8. + +Pot 3 : 57 1/8 : 87 (A). +Pot 3 : 1 2/8 : 81 2/8 (B). + +Pot 4 : 6 6/8 : 19. +Pot 4 : 31 : 43 2/8. +Pot 4 : 69 4/8 : 4. + +Pot 5 : 99 4/8 : 9 4/8. +Pot 5 : 29 2/8 : 3. + +Total : 511.63 : 413.75. + +The thirteen crossed plants here average 39.35, and the thirteen +self-fertilised plants 31.82 inches in height; or as 100 to 81. But it +would be a very much fairer plan to exclude all the starved plants of +only 10 inches and under in height; and in this case the nine remaining +crossed plants average 53.84, and the seven remaining self-fertilised +plants 51.78 inches in height, or as 100 to 96; and this difference is +so small that the crossed and self-fertilised plants may be considered +as of equal heights. + +In addition to these plants, three crossed plants were planted +separately in three large pots, and three self-fertilised plants in +three other large pots, so that they were not exposed to any +competition; and now the self-fertilised plants exceeded the crossed in +height by a little, for the three crossed averaged 55.91, and the three +self-fertilised 59.16 inches; or as 100 to 106. + +CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION. + +TABLE 6/86. Nicotiana tabacum (third generation). Seedlings from the +self-fertilised plant A in pot 3, Table 6/85, of the last or second +generation. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: From Self-fertilised Plant, crossed by a Crossed Plant. + +Column 3: From Self-fertilised Plant again self-fertilised, forming the +third Self-fertilised generation. + +Pot 1 : 100 2/8 : 98. +Pot 1 : 91 : 79. + +Pot 2 : 110 2/8 : 59 1/8. +Pot 2 : 100 4/8 : 66 6/8. + +Pot 3 : 104 : 79 6/8. + +Pot 4 : 84 2/8 : 110 4/8. +Pot 4 : 76 4/8 : 64 1/8. + +Total : 666.75 : 557.25. + +As I wished to ascertain, firstly, whether those self-fertilised plants +of the last generation, which greatly exceeded in height their crossed +opponents, would transmit the same tendency to their offspring, and +secondly, whether they possessed the same sexual constitution, I +selected for experiment the two self-fertilised plants marked A and B in +Pot 3 in Table 6/85, as these two were of nearly equal height, and were +greatly superior to their crossed opponents. Four flowers on each plant +were fertilised with their own pollen, and four others on the same +plants were crossed with pollen from one of the crossed plants growing +in another pot. This plan differs from that before followed, in which +seedlings from crossed plants again crossed, have been compared with +seedlings from self-fertilised plants again self-fertilised. The seeds +from the crossed and self-fertilised capsules of the above two plants +were placed in separate watch-glasses and compared, but were not +weighed; and in both cases those from the crossed capsules seemed to be +rather less numerous than those from the self-fertilised capsules. These +seeds were planted in the usual manner, and the heights of the crossed +and self-fertilised seedlings, when fully grown, are given in Tables +6/86 and 6/87. + +The seven crossed plants in the first of these two tables average 95.25, +and the seven self-fertilised 79.6 inches in height; or as 100 to 83. In +half the pots a crossed plant, and in the other half a self-fertilised +plant flowered first. + +We now come to the seedlings raised from the other parent-plant B. + +TABLE 6/87. Nicotiana tabacum (third generation). Seedlings from the +self-fertilised plant B in pot 3, Table 6/85, of the last or second +generation. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: From Self-fertilised Plant, crossed by a Crossed Plant. + +Column 3: From Self-fertilised Plant again self-fertilised, forming the +third Self-fertilised generation. + +Pot 1 : 87 2/8 : 72 4/8. +Pot 1 : 49 : 14 2/8. + +Pot 2 : 98 4/8 : 73. +Pot 2 : 0 : 110 4/8. + +Pot 3 : 99 : 106 4/8. +Pot 3 : 15 2/8 : 73 6/8. + +Pot 4 : 97 6/8 : 48 6/8. + +Pot 5 : 48 6/8 : 81 2/8. +Pot 5 : 0 : 61 2/8. + +Total : 495.50 : 641.75. + +The seven crossed plants (for two of them died) here average 70.78 +inches, and the nine self-fertilised plants 71.3 inches in height; or as +100 to barely 101. In four out of these five pots, a self-fertilised +plant flowered before any one of the crossed plants. So that, +differently from the last case, the self-fertilised plants are in some +respects slightly superior to the crossed. + +If we now consider the crossed and self-fertilised plants of the three +generations, we find an extraordinary diversity in their relative +heights. In the first generation, the crossed plants were inferior to +the self-fertilised as 100 to 178; and the flowers on the original +parent-plants which were crossed with pollen from a distinct plant +yielded much fewer seeds than the self-fertilised flowers, in the +proportion of 100 to 150. But it is a strange fact that the +self-fertilised plants, which were subjected to very severe competition +with the crossed, had on two occasions no advantage over them. The +inferiority of the crossed plants of this first generation cannot be +attributed to the immaturity of the seeds, for I carefully examined +them; nor to the seeds being diseased or in any way injured in some one +capsule, for the contents of the ten crossed capsules were mingled +together and a few taken by chance for sowing. In the second generation +the crossed and self-fertilised plants were nearly equal in height. In +the third generation, crossed and self-fertilised seeds were obtained +from two plants of the previous generation, and the seedlings raised +from them differed remarkably in constitution; the crossed in the one +case exceeded the self-fertilised in height in the ratio of 100 to 83, +and in the other case were almost equal. This difference between the two +lots, raised at the same time from two plants growing in the same pot, +and treated in every respect alike, as well as the extraordinary +superiority of the self-fertilised over the crossed plants in the first +generation, considered together, make me believe that some individuals +of the present species differ to a certain extent from others in their +sexual affinities (to use the term employed by Gartner), like closely +allied species of the same genus. Consequently if two plants which thus +differ are crossed, the seedlings suffer and are beaten by those from +the self-fertilised flowers, in which the sexual elements are of the +same nature. It is known that with our domestic animals certain +individuals are sexually incompatible, and will not produce offspring, +although fertile with other individuals. (6/3. I have given evidence on +this head in my 'Variation of Animals and Plants under Domestication' +chapter 18 2nd edition volume 2 page 146.) But Kolreuter has recorded a +case which bears more closely on our present one, as it shows that in +the genus Nicotiana the varieties differ in their sexual affinities. +(6/4. 'Das Geschlecht der Pflanzen, Zweite Fortsetzung' 1764 pages +55-60.) He experimented on five varieties of the common tobacco, and +proved that they were varieties by showing that they were perfectly +fertile when reciprocally crossed; but one of these varieties, if used +either as the father or the mother, was more fertile than any of the +others when crossed with a widely distinct species, N. glutinosa. As the +different varieties thus differ in their sexual affinities, there is +nothing surprising in the individuals of the same variety differing in a +like manner to a slight degree. + +Taking the plants of the three generations altogether, the crossed show +no superiority over the self-fertilised, and I can account for this fact +only by supposing that with this species, which is perfectly +self-fertile without insect aid, most of the individuals are in the same +condition, as those of the same variety of the common pea and of a few +other exotic plants, which have been self-fertilised for many +generations. In such cases a cross between two individuals does no good; +nor does it in any case, unless the individuals differ in general +constitution, either from so-called spontaneous variation, or from their +progenitors having been subjected to different conditions. I believe +that this is the true explanation in the present instance, because, as +we shall immediately see, the offspring of plants, which did not profit +at all by being crossed with a plant of the same stock, profited to an +extraordinary degree by a cross with a slightly different sub-variety. + +THE EFFECTS OF A CROSS WITH A FRESH STOCK. + +I procured some seed of N. tabacum from Kew and raised some plants, +which formed a slightly different sub-variety from my former plants; as +the flowers were a shade pinker, the leaves a little more pointed, and +the plants not quite so tall. Therefore the advantage in height which +the seedlings gained by this cross cannot be attributed to direct +inheritance. Two of the plants of the third self-fertilised generation, +growing in Pots 2 and 5 in Table 6/87, which exceeded in height their +crossed opponents (as did their parents in a still higher degree) were +fertilised with pollen from the Kew plants, that is, by a fresh stock. +The seedlings thus raised may be called the Kew-crossed. Some other +flowers on the same two plants were fertilised with their own pollen, +and the seedlings thus raised from the fourth self-fertilised +generation. The crossed capsules produced by the plant in Pot 2, Table +6/87, were plainly less fine than the self-fertilised capsules on the +same plant. In Pot 5 the one finest capsule was also a self-fertilised +one; but the seeds produced by the two crossed capsules together +exceeded in number those produced by the two self-fertilised capsules on +the same plant. Therefore as far as the flowers on the parent-plants are +concerned, a cross with pollen from a fresh stock did little or no good; +and I did not expect that the offspring would have received any benefit, +but in this I was completely mistaken. + +The crossed and self-fertilised seeds from the two plants were placed on +bare sand, and very many of the crossed seeds of both sets germinated +before the self-fertilised seeds, and protruded their radicles at a +quicker rate. Hence many of the crossed seeds had to be rejected, before +pairs in an equal state of germination were obtained for planting on the +opposite sides of sixteen large pots. The two series of seedlings raised +from the parent-plants in the two Pots 2 and 5 were kept separate, and +when fully grown were measured to the tips of their highest leaves, as +shown in Table 6/88. But as there was no uniform difference in height +between the crossed and self-fertilised seedlings raised from the two +plants, their heights have been added together in calculating the +averages. I should state that by the accidental fall of a large bush in +the greenhouse, several plants in both the series were much injured. +These were at once measured together with their opponents and afterwards +thrown away. The others were left to grow to their full height, and were +measured when in flower. This accident accounts for the small height of +some of the pairs; but as all the pairs, whether only partly or fully +grown, were measured at the same time, the measurements are fair. + +The average height of the twenty-six crossed plants in the sixteen pots +of the two series is 63.29, and that of the twenty-six self-fertilised +plants is 41.67 inches; or as 100 to 66. The superiority of the crossed +plants was shown in another way, for in every one of the sixteen pots a +crossed plant flowered before a self-fertilised one, with the exception +of Pot 6 of the second series, in which the plants on the two sides +flowered simultaneously. + +TABLE 6/88. Nicotiana tabacum. Plants raised from two plants of the +third self-fertilised generation in Pots 2 and 5, in Table 6/87. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Kew-crossed Plants, pot 2, Table 6/87. + +Column 3: Plants of the fourth Self-fertilised generation, pot 2, Table +6/87. + +Column 4: Kew-crossed Plants, pot 5, Table 6/87. + +Column 5: Plants of the fourth Self-fertilised generation, pot 5, Table +6/87. + +Pot 1 : 84 6/8 : 68 4/8 : 77 6/8 : 56. +Pot 1 : 31 : 5 : 7 2/8 : 5 3/8. + +Pot 2 : 78 4/8 : 51 4/8 : 55 4/8 : 27 6/8. +Pot 2 : 48 : 70 : 18 : 7. + +Pot 3 : 77 3/8 : 12 6/8 : 76 2/8 : 60 6/8. +Pot 3 : 77 1/8 : 6 6/8. + +Pot 4 : 49 2/8 : 29 4/8 : 90 4/8 : 11 6/8. +Pot 4 : 15 6/8 : 32 : 22 2/8 : 4 1/8. + +Pot 5 : 89 : 85 : 94 2/8 : 28 4/8. +Pot 5 : 17 : 5 3/8. + +Pot 6 : 90 : 80 : 78 : 78 6/8. + +Pot 7 : 84 4/8 : 48 6/8 : 85 4/8 : 61 4/8. +Pot 7 : 76 4/8 : 56 4/8. + +Pot 8 : 83 4/8 : 84 4/8 : 65 5/8 : 78 3/8. +Pot 8 : : : 72 2/8 : 27 4/8. + +Total : 902.63 : 636.13 : 743.13 : 447.38. + +Some of the remaining seeds of both series, whether or not in a state of +germination, were thickly sown on the opposite sides of two very large +pots; and the six highest plants on each side of each pot were measured +after they had grown to nearly their full height. But their heights were +much less than in the former trials, owing to their extremely crowded +condition. Even whilst quite young, the crossed seedlings manifestly had +much broader and finer leaves than the self-fertilised seedlings. + +TABLE 6/89. Nicotiana tabacum. Plants of the same parentage as those in +Table 6/88, but grown extremely crowded in two large pots. + +Heights of plants measured in inches. + +Column 1: Kew-crossed Plants, from pot 2, Table 6/87. + +Column 2: Plants of the fourth Self-fertilised generation, from pot 2, +Table 6/87. + +Column 3: Kew-crossed Plants, from pot 5, Table 6/87. + +Column 4: Plants of the fourth Self-fertilised generation, from pot 5, +Table 6/87. + + 42 4/8 : 22 4/8 : 44 6/8 : 22 4/8. + 34 : 19 2/8 : 42 4/8 : 21. + 30 4/8 : 14 2/8 : 27 4/8 : 18. + 23 4/8 : 16 : 31 2/8 : 15 2/8. + 26 6/8 : 13 4/8 : 32 : 13 5/8. + 18 3/8 : 16 : 24 6/8 : 14 6/8. + +175.63 : 101.50 : 202.75 : 105.13. + +The twelve tallest crossed plants in the two pots belonging to the two +series average here 31.53, and the twelve tallest self-fertilised plants +17.21 inches in height; or as 100 to 54. The plants on both sides, when +fully grown, some time after they had been measured, were cut down close +to the ground and weighed. The twelve crossed plants weighed 21.25 +ounces; and the twelve self-fertilised plants only 7.83 ounces; or in +weight as 100 to 37. + +The rest of the crossed and self-fertilised seeds from the two +parent-plants (the same as in the last experiment) was sown on the 1st +of July in four long parallel and separate rows in good soil in the open +ground; so that the seedlings were not subjected to any mutual +competition. The summer was wet and unfavourable for their growth. +Whilst the seedlings were very small the two crossed rows had a clear +advantage over the two self-fertilised rows. When fully grown the twenty +tallest crossed plants and the twenty tallest self-fertilised plants +were selected and measured on the 11th of November to the extremities of +their leaves, as shown in Table 6/90. Of the twenty crossed plants, +twelve had flowered; whilst of the twenty self-fertilised plants one +alone had flowered. + +TABLE 6/90. Nicotiana tabacum. Plants raised from the same seeds as in +the last two experiments, but sown separately in the open ground, so as +not to compete together. + +Heights of plants measured in inches. + +Column 1: Kew-crossed Plants, from pot 2, Table 6/87. + +Column 2: Plants of the fourth Self-fertilised generation, from pot 2, +Table 6/87. + +Column 3: Kew-crossed Plants, from pot 5, Table 6/87. + +Column 4: Plants of the fourth Self-fertilised generation, from pot 5, +Table 6/87. + + 42 2/8 : 22 6/8 : 54 4/8 : 34 4/8. + 54 5/8 : 37 4/8 : 51 4/8 : 38 5/8. + 39 3/8 : 34 4/8 : 45 : 40 6/8. + 53 2/8 : 30 : 43 : 43 2/8. + 49 3/8 : 28 6/8 : 43 : 40. + 50 3/8 : 31 2/8 : 48 6/8 : 38 2/8. + 47 1/8 : 25 4/8 : 44 : 35 6/8. + 57 3/8 : 26 2/8 : 48 2/8 : 39 6/8. + 37 : 22 3/8 : 55 1/8 : 47 6/8. + 48 : 28 : 63 : 58 5/8. + +478.75 : 286.86 : 496.13 : 417.25 + +The twenty tallest crossed plants here average 48.74, and the twenty +tallest self-fertilised 35.2 inches in height; or as 100 to 72. These +plants after being measured were cut down close to the ground, and the +twenty crossed plants weighed 195.75 ounces, and the twenty +self-fertilised plants 123.25 ounces; or as 100 to 63. + +In Tables 6/88, 6/89 and 6/90, we have the measurements of fifty-six +plants derived from two plants of the third self-fertilised generation +crossed with pollen from a fresh stock, and of fifty-six plants of the +fourth self-fertilised generation derived from the same two plants. +These crossed and self-fertilised plants were treated in three different +ways, having been put, firstly, into moderately close competition with +one another in pots; secondly, having been subjected to unfavourable +conditions and to very severe competition from being greatly crowded in +two large pots; and thirdly, having been sown separately in open and +good ground, so as not to suffer from any mutual competition. In all +these cases the crossed plants in each lot were greatly superior to the +self-fertilised. This was shown in several ways,--by the earlier +germination of the crossed seeds, by the more rapid growth of the +seedlings whilst quite young, by the earlier flowering of the mature +plants, as well as by the greater height which they ultimately attained. +The superiority of the crossed plants was shown still more plainly when +the two lots were weighed; the weight of the crossed plants to that of +the self-fertilised in the two crowded pots being as 100 to 37. Better +evidence could hardly be desired of the immense advantage derived from a +cross with a fresh stock. + +26. PRIMULACEAE.--Cyclamen persicum. (6/5. Cyclamen repandum according +to Lecoq 'Geographie Botanique de l'Europe' tome 8 1858 page 150, is +proterandrous, and this I believe to be the case with Cyclamen +persicum.) + +Ten flowers crossed with pollen from plants known to be distinct +seedlings, yielded nine capsules, containing on an average 34.2 seeds, +with a maximum of seventy-seven in one. Ten flowers self-fertilised +yielded eight capsules, containing on an average only 13.1 seeds, with a +maximum of twenty-five in one. This gives a ratio of 100 to 38 for the +average number of seeds per capsule for the crossed and self-fertilised +flowers. The flowers hang downwards, and as the stigmas stand close +beneath the anthers, it might have been expected that pollen would have +fallen on them, and that they would have been spontaneously +self-fertilised; but these covered-up plants did not produce a single +capsule. On some other occasions uncovered plants in the same greenhouse +produced plenty of capsules, and I suppose that the flowers had been +visited by bees, which could hardly fail to carry pollen from plant to +plant. + +The seeds obtained in the manner just described were placed on sand, and +after germinating were planted in pairs,--three crossed and three +self-fertilised plants on the opposite sides of four pots. When the +leaves were 2 or 3 inches in length, including the foot-stalks, the +seedlings on both sides were equal. In the course of a month or two the +crossed plants began to show a slight superiority over the +self-fertilised, which steadily increased; and the crossed flowered in +all four pots some weeks before, and much more profusely than the +self-fertilised. The two tallest flower-stems on the crossed plants in +each pot were now measured, and the average height of the eight stems +was 9.49 inches. After a considerable interval of time the +self-fertilised plants flowered, and several of their flower-stems (but +I forgot to record how many) were roughly measured, and their average +height was a little under 7.5 inches; so that the flower-stems on the +crossed plants to those on the self-fertilised were at least as 100 to +79. The reason why I did not make more careful measurements of the +self-fertilised plants was, that they looked such poor specimens that I +determined to there them re-potted in larger pots and in the following +year to measure them carefully; but we shall see that this was partly +frustrated by so few flower-stems being then produced. + +These plants were left uncovered in the greenhouse; and the twelve +crossed plants produced forty capsules, whilst the twelve +self-fertilised plants produced only five; or as 100 to 12. But this +difference does not give a just idea of the relative fertility of the +two lots. I counted the seeds in one of the finest capsules on the +crossed plants, and it contained seventy-three; whilst the finest of the +five capsules produced by the self-fertilised plants contained only +thirty-five good seeds. In the other four capsules most of the seeds +were barely half as large as those in the crossed capsules. + +TABLE 6/91. Cyclamen persicum: 0 implies that no flower-stem was +produced. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 10 : 0. +Pot 1 : 9 2/8 : 0. +Pot 1 : 10 2/8 : 0. + +Pot 2 : 9 2/8 : 0. +Pot 2 : 10 : 0. +Pot 2 : 10 2/8 : 0. + +Pot 3 : 9 1/8 : 8. +Pot 3 : 9 5/8 : 6 7/8. +Pot 3 : 9 5/8 : 6 6/8. + +Pot 4 : 11 1/8 : 0. +Pot 4 : 10 5/8 : 7 7/8. +Pot 4 : 10 6/8 : 0. + +Total : 119.88 : 29.50. + +In the following year the crossed plants again bore many flowers before +the self-fertilised bore a single one. The three tallest flower-stems on +the crossed plants in each of the pots were measured, as shown in Table +6/91. In Pots 1 and 2 the self-fertilised plants did not produce a +single flower-stem; in Pot 4 only one; and in Pot 3 six, of which the +three tallest were measured. + +The average height of the twelve flower-stems on the crossed plants is +9.99, and that of the four flower-stems on the self-fertilised plants +7.37 inches; or as 100 to 74. The self-fertilised plants were miserable +specimens, whilst the crossed ones looked very vigorous. + +ANAGALLIS. + +Anagallis collina, var. grandiflora (pale red and blue-flowered +sub-varieties). + +Firstly, twenty-five flowers on some plants of the red variety were +crossed with pollen from a distinct plant of the same variety, and +produced ten capsules; thirty-one flowers were fertilised with their own +pollen, and produced eighteen capsules. These plants, which were grown +in pots in the greenhouse, were evidently in a very sterile condition, +and the seeds in both sets of capsules, especially in the +self-fertilised, although numerous, were of so poor a quality that it +was very difficult to determine which were good and which bad. But as +far as I could judge, the crossed capsules contained on an average 6.3 +good seeds, with a maximum in one of thirteen; whilst the +self-fertilised contained 6.05 such seeds, with a maximum in one of +fourteen. + +Secondly, eleven flowers on the red variety were castrated whilst young +and fertilised with pollen from the blue variety, and this cross +evidently much increased their fertility; for the eleven flowers yielded +seven capsules, which contained on an average twice as many good seeds +as before, namely, 12.7; with a maximum in two of the capsules of +seventeen seeds. Therefore these crossed capsules yielded seeds compared +with those in the foregoing self-fertilised capsules, as 100 to 48. +These seeds were also conspicuously larger than those from the cross +between two individuals of the same red variety, and germinated much +more freely. The flowers on most of the plants produced by the cross +between the two-coloured varieties (of which several were raised), took +after their mother, and were red-coloured. But on two of the plants the +flowers were plainly stained with blue, and to such a degree in one case +as to be almost intermediate in tint. + +The crossed seeds of the two foregoing kinds and the self-fertilised +were sown on the opposite sides of two large pots, and the seedlings +were measured when fully grown, as shown in Tables 6/92a and 6/92b. + +TABLE 6/92a. Anagallis collina: Red variety crossed by a distinct plant +of the red variety, and red variety self-fertilised. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 23 4/8 : 15 4/8. +Pot 1 : 21 : 15 4/8. +Pot 1 : 17 2/8 : 14. + +Total : 61.75 : 45.00. + +TABLE 6/92b. Anagallis collina: Red variety crossed by blue variety, and +red variety self-fertilised. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 2 : 30 4/8 : 24 4/8. +Pot 2 : 27 3/8 : 18 4/8. +Pot 2 : 25 : 11 6/8. + +Total : 82.88 : 54.75. + +Total of both lots: + : 144.63 : 99.75. + +As the plants of the two lots are few in number, they may be run +together for the general average; but I may first state that the height +of the seedlings from the cross between two individuals of the red +variety is to that of the self-fertilised plants of the red variety as +100 to 73; whereas the height of the crossed offspring from the two +varieties to the self-fertilised plants of the red variety is as 100 to +66. So that the cross between the two varieties is here seen to be the +most advantageous. The average height of all six crossed plants in the +two lots taken together is 48.20, and that of the six self-fertilised +plants 33.25; or as 100 to 69. + +These six crossed plants produced spontaneously twenty-six capsules, +whilst the six self-fertilised plants produced only two, or as 100 to 8. +There is therefore the same extraordinary difference in fertility +between the crossed and self-fertilised plants as in the last genus, +Cyclamen, which belongs to the same family of the Primulaceae. + +Primula veris. British flora. (var. officinalis, Linn.). + +THE COWSLIP. + +Most of the species in this genus are heterostyled or dimorphic; that +is, they present two forms,--one long-styled with short stamens, and the +other short-styled with long stamens. (6/6. See my paper 'On the Two +Forms or Dimorphic Condition in the Species of Primula' in 'Journal of +the Proceedings of the Linnean Society' volume 6 1862 page 77. A second +paper, to which I presently refer 'On the Hybrid-like Nature of the +Offspring from the Illegitimate Unions of Dimorphic and Trimorphic +Plants' was published in volume 10 1867 page 393 of the same journal.) +For complete fertilisation it is necessary that pollen from the one form +should be applied to the stigma of the other form; and this is effected +under nature by insects. Such unions, and the seedlings raised from +them, I have called legitimate. If one form is fertilised with pollen +from the same form, the full complement of seed is not produced; and in +the case of some heterostyled genera no seed at all is produced. Such +unions, and the seedlings raised from them, I have called illegitimate. +These seedlings are often dwarfed and more or less sterile, like +hybrids. I possessed some long-styled plants of Primula veris, which +during four successive generations had been produced from illegitimate +unions between long-styled plants; they were, moreover, in some degree +inter-related, and had been subjected all the time to similar conditions +in pots in the greenhouse. As long as they were cultivated in this +manner, they grew well and were healthy and fertile. Their fertility +even increased in the later generations, as if they were becoming +habituated to illegitimate fertilisation. Plants of the first +illegitimate generation when taken from the greenhouse and planted in +moderately good soil out of doors grew well and were healthy; but when +those of the two last illegitimate generations were thus treated they +became excessively sterile and dwarfed, and remained so during the +following year, by which time they ought to have become accustomed to +growing out of doors, so that they must have possessed a weak +constitution. + +Under these circumstances, it seemed advisable to ascertain what would +be the effect of legitimately crossing long-styled plants of the fourth +illegitimate generation with pollen taken from non-related short-styled +plants, growing under different conditions. Accordingly several flowers +on plants of the fourth illegitimate generation (i.e., +great-great-grandchildren of plants which had been legitimately +fertilised), growing vigorously in pots in the greenhouse, were +legitimately fertilised with pollen from an almost wild short-styled +cowslip, and these flowers yielded some fine capsules. Thirty other +flowers on the same illegitimate plants were fertilised with their own +pollen, and these yielded seventeen capsules, containing on an average +thirty-two seeds. This is a high degree of fertility; higher, I believe, +than that which generally obtains with illegitimately fertilised +long-styled plants growing out of doors, and higher than that of the +previous illegitimate generations, although their flowers were +fertilised with pollen taken from a distinct plant of the same form. + +These two lots of seeds were sown (for they will not germinate well when +placed on bare sand) on the opposite sides of four pots, and the +seedlings were thinned, so that an equal number were left on the two +sides. For some time there was no marked difference in height between +the two lots; and in Pot 3, Table 6/93, the self-fertilised plants were +rather the tallest. But by the time that they had thrown up young +flower-stems, the legitimately crossed plants revealed much the finest, +and had greener and larger leaves. The breadth of the largest leaf on +each plant was measured, and those on the crossed plants were on an +average a quarter of an inch (exactly .28 of an inch) broader than those +on the self-fertilised plants. The plants, from being too much crowded, +produced poor and short flower-stems. The two finest on each side were +measured; the eight on the legitimately crossed plants averaged 4.08, +and the eight on the illegitimately self-fertilised plants averaged 2.93 +inches in height; or as 100 to 72. + +These plants after they had flowered were turned out of their pots, and +planted in fairly good soil in the open ground. In the following year +(1870), when in full flower, the two tallest flower-stems on each side +were again measured, as shown in Table 6/93, which likewise gives the +number of flower-stems produced on both sides of all the pots. + +TABLE 6/93. Primula veris. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Height: Legitimately crossed Plants. + +Column 3: Number of Flower-stems produced: Legitimately crossed Plants. + +Column 4: Height: Illegitimately crossed Plants. + +Column 5: Number of Flower-stems produced: Illegitimately crossed +Plants. + +Pot 1 : 9 : 16 : 2 1/8 : 3. +Pot 1 : 8 : : 3 4/8. + +Pot 2 : 7 : 16 : 6 : 3. +Pot 2 : 6 4/8 : : 5 4/8. + +Pot 3 : 6 : 16 : 3 : 4. +Pot 3 : 6 2/8 : : 0 4/8. + +Pot 4 : 7 3/8 : 14 : 2 5/8 : 5. +Pot 4 : 6 1/8 : : 2 4/8. + +Total : 56.26 : 62 : 25.75 : 15. + +The average height of the eight tallest flower-stems on the crossed +plants is here 7.03 inches, and that of the eight tallest flower-stems +on the self-fertilised plants 3.21 inches; or as 100 to 46. We see, +also, that the crossed plants bore sixty-two flower-stems; that is, +above four times as many as those (namely fifteen) borne by the +self-fertilised plants. The flowers were left exposed to the visits of +insects, and as many plants of both forms grew close by, they must have +been legitimately and naturally fertilised. Under these circumstances +the crossed plants produced 324 capsules, whilst the self-fertilised +produced only 16; and these were all produced by a single plant in Pot +2, which was much finer than any other self-fertilised plant. Judging by +the number of capsules produced, the fertility of an equal number of +crossed and self-fertilised plants was as 100 to 5. + +In the succeeding year (1871) I did not count all the flower-stems on +these plants, but only those which produced capsules containing good +seeds. The season was unfavourable, and the crossed plants produced only +forty such flower-stems, bearing 168 good capsules, whilst the +self-fertilised plants produced only two such flower-stems, bearing only +6 capsules, half of which were very poor ones. So that the fertility of +the two lots, judging by the number of capsules, was as 100 to 3.5. + +In considering the great difference in height and the wonderful +difference in fertility between the two sets of plants, we should bear +in mind that this is the result of two distinct agencies. The +self-fertilised plants were the product of illegitimate fertilisation +during five successive generations, in all of which, excepting the last, +the plants had been fertilised with pollen taken from a distinct +individual belonging to the same form, but which was more or less +closely related. The plants had also been subjected in each generation +to closely similar conditions. This treatment alone, as I know from +other observations, would have greatly reduced the size and fertility of +the offspring. On the other hand, the crossed plants were the offspring +of long-styled plants of the fourth illegitimate generation legitimately +crossed with pollen from a short-styled plant, which, as well as its +progenitors, had been exposed to very different conditions; and this +latter circumstance alone would have given great vigour to the +offspring, as we may infer from the several analogous cases already +given. How much proportional weight ought to be attributed to these two +agencies,--the one tending to injure the self-fertilised offspring, and +the other to benefit the crossed offspring,--cannot be determined. But +we shall immediately see that the greater part of the benefit, as far as +increased fertility is concerned, must be attributed to the cross having +been made with a fresh stock. + +Primula veris. + +EQUAL-STYLED AND RED-FLOWERED VAR. + +I have described in my paper 'On the Illegitimate Unions of Dimorphic +and Trimorphic Plants' this remarkable variety, which was sent to me +from Edinburgh by Mr. J. Scott. It possessed a pistil proper to the +long-styled form, and stamens proper to the short-styled form; so that +it had lost the heterostyled or dimorphic character common to most of +the species of the genus, and may be compared with an hermaphrodite form +of a bisexual animal. Consequently the pollen and stigma of the same +flower are adapted for complete mutual fertilisation, instead of its +being necessary that pollen should be brought from one form to another, +as in the common cowslip. From the stigma and anthers standing nearly on +the same level, the flowers are perfectly self-fertile when insects are +excluded. Owing to the fortunate existence of this variety, it is +possible to fertilise its flowers in a legitimate manner with their own +pollen, and to cross other flowers in a legitimate manner with pollen +from another variety or fresh stock. Thus the offspring from both unions +can be compared quite fairly, free from any doubt from the injurious +effects of an illegitimate union. + +The plants on which I experimented had been raised during two successive +generations from spontaneously self-fertilised seeds produced by plants +under a net; and as the variety is highly self-fertile, its progenitors +in Edinburgh may have been self-fertilised during some previous +generations. Several flowers on two of my plants were legitimately +crossed with pollen from a short-styled common cowslip growing almost +wild in my orchard; so that the cross was between plants which had been +subjected to considerably different conditions. Several other flowers on +the same two plants were allowed to fertilise themselves under a net; +and this union, as already explained, is a legitimate one. + +The crossed and self-fertilised seeds thus obtained were sown thickly on +the opposite sides of three pots, and the seedlings thinned, so that an +equal number were left on the two sides. The seedlings during the first +year were nearly equal in height, excepting in Pot 3, Table 6/94, in +which the self-fertilised plants had a decided advantage. In the autumn +the plants were bedded out, in their pots; owing to this circumstance, +and to many plants growing in each pot, they did not flourish, and none +were very productive in seeds. But the conditions were perfectly equal +and fair for both sides. In the following spring I record in my notes +that in two of the pots the crossed plants are "incomparably the finest +in general appearance," and in all three pots they flowered before the +self-fertilised. When in full flower the tallest flower-stem on each +side of each pot was measured, and the number of the flower-stems on +both sides counted, as shown in Table 6/94. The plants were left +uncovered, and as other plants were growing close by, the flowers no +doubt were crossed by insects. When the capsules were ripe they were +gathered and counted, and the result is likewise shown in Table 6/94. + +TABLE 6/94. Primula veris (equal-styled, red-flowered variety). + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Height of tallest flower-stem: crossed Plants. + +Column 3: Number of Flower-stems: crossed Plants. + +Column 4: Number of good capsules: crossed Plants. + +Column 5: Height of tallest flower-stem: self-fertilised Plants. + +Column 6: Number of Flower-stems: self-fertilised Plants. + +Column 7: Number of good capsules: self-fertilised Plants. + +Pot 1 : 10 : 14 : 163 : 6 4/8 : 6 : 6. + +Pot 2 : 8 4/8 : 12 : * : 5 : 2 : 0. + *Several, not counted. + +Pot 3 : 7 4/8 : 7 : 43 : 10 4/8 : 5 : 26. + +Totals : 26.0 : 33 : 206 : 22.0 : 13 : 32. + +The average height of the three tallest flower-stems on the crossed +plants is 8.66 inches, and that of the three on the self-fertilised +plants 7.33 inches; or as 100 to 85. + +All the crossed plants together produced thirty-three flower-stems, +whilst the self-fertilised bore only thirteen. The number of the +capsules were counted only on the plants in Pots 1 and 3, for the +self-fertilised plants in Pot 2 produced none; therefore those on the +crossed plants on the opposite side were not counted. Capsules not +containing any good seeds were rejected. The crossed plants in the above +two pots produced 206, and the self-fertilised in the same pots only 32 +capsules; or as 100 to 15. Judging from the previous generations, the +extreme unproductiveness of the self-fertilised plants in this +experiment was wholly due to their having been subjected to unfavourable +conditions, and to severe competition with the crossed plants; for had +they grown separately in good soil, it is almost certain that they would +have produced a large number of capsules. The seeds were counted in +twenty capsules from the crossed plants, and they averaged 24.75; whilst +in twenty capsules from the self-fertilised plants the average was +17.65; or as 100 to 71. Moreover, the seeds from the self-fertilised +plants were not nearly so fine as those from the crossed plants. If we +consider together the number of capsules produced and the average number +of contained seeds, the fertility of the crossed plants to the +self-fertilised plants was as 100 to 11. We thus see what a great +effect, as far as fertility is concerned, was produced by a cross +between the two varieties, which had been long exposed to different +conditions, in comparison with self-fertilisation; the fertilisation +having been in both cases of the legitimate order. + +Primula sinensis. + +As the Chinese primrose is a heterostyled or dimorphic plant, like the +common cowslip, it might have been expected that the flowers of both +forms when illegitimately fertilised with their own pollen or with that +from flowers on another plant of the same form, would have yielded less +seed than the legitimately crossed flowers; and that the seedlings +raised from illegitimately self-fertilised seeds would have been +somewhat dwarfed and less fertile, in comparison with the seedlings from +legitimately crossed seeds. This holds good in relation to the fertility +of the flowers; but to my surprise there was no difference in growth +between the offspring from a legitimate union between two distinct +plants, and from an illegitimate union whether between the flowers on +the same plant, or between distinct plants of the same form. But I have +shown, in the paper before referred to, that in England this plant is in +an abnormal condition, such as, judging from analogous cases, would tend +to render a cross between two individuals of no benefit to the +offspring. Our plants have been commonly raised from self-fertilised +seeds; and the seedlings have generally been subjected to nearly uniform +conditions in pots in greenhouses. Moreover, many of the plants are now +varying and changing their character, so as to become in a greater or +less degree equal-styled, and in consequence highly self-fertile. From +the analogy of Primula veris there can hardly be a doubt that if a plant +of Primula sinensis could have been procured direct from China, and if +it had been crossed with one of our English varieties, the offspring +would have shown wonderful superiority in height and fertility (though +probably not in the beauty of their flowers) over our ordinary plants. + +My first experiment consisted in fertilising many flowers on long-styled +and short-styled plants with their own pollen, and other flowers on the +same plants with pollen taken from distinct plants belonging to the same +form; so that all the unions were illegitimate. There was no uniform and +marked difference in the number of seeds obtained from these two modes +of self-fertilisation, both of which were illegitimate. The two lots of +seeds from both forms were sown thickly on opposite sides of four pots, +and numerous plants thus raised. But there was no difference in their +growth, excepting in one pot, in which the offspring from the +illegitimate union of two long-styled plants exceeded in a decided +manner in height the offspring of flowers on the same plants fertilised +with their own pollen. But in all four pots the plants raised from the +union of distinct plants belonging to the same form, flowered before the +offspring from the self-fertilised flowers. + +Some long-styled and short-styled plants were now raised from purchased +seeds, and flowers on both forms were legitimately crossed with pollen +from a distinct plant; and other flowers on both forms were +illegitimately fertilised with pollen from the flowers on the same +plant. The seeds were sown on opposite sides of Pots 1 to 4 in Table +6/95; a single plant being left on each side. Several flowers on the +illegitimate long-styled and short-styled plants described in the last +paragraph, were also legitimately and illegitimately fertilised in the +manner just described, and their seeds were sown in Pots 5 to 8 in the +same table. As the two sets of seedlings did not differ in any essential +manner, their measurements are given in a single table. I should add +that the legitimate unions in both cases yielded, as might have been +expected, many more seeds than the illegitimate unions. The seedlings +whilst half-grown presented no difference in height on the two sides of +the several pots. When fully grown they were measured to the tips of +their longest leaves, and the result is given in Table 6/95. + +TABLE 6/95. Primula sinensis. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Plants from legitimately Crossed seeds. + +Column 3: Plants from illegitimately Self-fertilised seeds. + +Pot 1 : 8 2/8 : 8. +From short-styled mother. + +Pot 2 : 7 4/8 : 8 5/8. +From short-styled mother. + +Pot 3 : 9 5/8 : 9 3/8. +From long-styled mother. + +Pot 4 : 8 4/8 : 8 2/8. +From long-styled mother. + +Pot 5 : 9 3/8 : 9. +From illegitimate short-styled mother. + +Pot 6 : 9 7/8 : 9 4/8. +From illegitimate short-styled mother. + +Pot 7 : 8 4/8 : 9 4/8. +From illegitimate long-styled mother. + +Pot 8 : 10 4/8 : 10. +From illegitimate long-styled mother. + +Total : 72.13 : 72.25. + +In six out of the eight pots the legitimately crossed plants exceeded in +height by a trifle the illegitimately self-fertilised plants; but the +latter exceeded the former in two of the pots in a more strongly marked +manner. The average height of the eight legitimately crossed plants is +9.01, and that of the eight illegitimately self-fertilised 9.03 inches, +or as 100 to 100.2. The plants on the opposite sides produced, as far as +could be judged by the eye, an equal number of flowers. I did not count +the capsules or the seeds produced by them; but undoubtedly, judging +from many previous observations, the plants derived from the +legitimately crossed seeds would have been considerably more fertile +than those from the illegitimately self-fertilised seeds. The crossed +plants, as in the previous case, flowered before the self-fertilised +plants in all the pots except in Pot 2, in which the two sides flowered +simultaneously; and this early flowering may, perhaps, be considered as +an advantage. + +27. POLYGONEAE.--Fagopyrum esculentum. + +This plant was discovered by Hildebrand to be heterostyled, that is, to +present, like the species of Primula, a long-styled and a short-styled +form, which are adapted for reciprocal fertilisation. Therefore the +following comparison of the growth of the crossed and self-fertilised +seedlings is not fair, for we do not know whether the difference in +their heights may not be wholly due to the illegitimate fertilisation of +the self-fertilised flowers. + +I obtained seeds by legitimately crossing flowers on long-styled and +short-styled plants, and by fertilising other flowers on both forms with +pollen from the same plant. Rather more seeds were obtained by the +former than by the latter process; and the legitimately crossed seeds +were heavier than an equal number of the illegitimately self-fertilised +seeds, in the ratio of 100 to 82. Crossed and self-fertilised seeds from +the short-styled parents, after germinating on sand, were planted in +pairs on the opposite sides of a large pot; and two similar lots of +seeds from long-styled parents were planted in a like manner on the +opposite sides of two other pots. In all three pots the legitimately +crossed seedlings, when a few inches in height, were taller than the +self-fertilised; and in all three pots they flowered before them by one +or two days. When fully grown they were all cut down close to the +ground, and as I was pressed for time, they were placed in a long row, +the cut end of one plant touching the tip of another, and the total +length of the legitimately crossed plants was 47 feet 7 inches, and of +the illegitimately self-fertilised plants 32 feet 8 inches. Therefore +the average height of the fifteen crossed plants in all three pots was +38.06 inches, and that of the fifteen self-fertilised plants 26.13 +inches; or as 100 to 69. + +28. CHENOPODIACEAE.--Beta vulgaris. + +A single plant, no others growing in the same garden, was left to +fertilise itself, and the self-fertilised seeds were collected. Seeds +were also collected from a plant growing in the midst of a large bed in +another garden; and as the incoherent pollen is abundant, the seeds of +this plant will almost certainly have been the product of a crossed +between distinct plants by means of the wind. Some of the two lots of +seeds were sown on the opposite sides of two very large pots; and the +young seedlings were thinned, so that an equal but considerable number +was left on the two sides. These plants were thus subjected to very +severe competition, as well as to poor conditions. The remaining seeds +were sown out of doors in good soil in two long and not closely +adjoining rows, so that these seedlings were placed under favourable +conditions, and were not subjected to any mutual competition. The +self-fertilised seeds in the open ground came up very badly; and on +removing the soil in two or three places, it was found that many had +sprouted under ground and had then died. No such case had been observed +before. Owing to the large number of seedlings which thus perished, the +surviving self-fertilised plants grew thinly in the row, and thus had an +advantage over the crossed plants, which grew very thickly in the other +row. The young plants in the two rows were protected by a little straw +during the winter, and those in the two large pots were placed in the +greenhouse. + +There was no difference between the two lots in the pots until the +ensuing spring, when they had grown a little, and then some of the +crossed plants were finer and taller than any of the self-fertilised. +When in full flower their stems were measured, and the measurements are +given in Table 6/96. + +TABLE 6/96. Beta vulgaris. + +Heights of flower stems measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 34 6/8 : 36. +Pot 1 : 30 : 20 1/8. +Pot 1 : 33 6/8 : 32 2/8. +Pot 1 : 34 4/8 : 32. + +Pot 2 : 42 3/8 : 42 1/8. +Pot 2 : 33 1/8 : 26 4/8. +Pot 2 : 31 2/8 : 29 2/8. +Pot 2 : 33 : 20 2/8. + +Total : 272.75 : 238.50. + +The average height of the eight crossed plants is here 34.09, and that +of the eight self-fertilised plants 29.81; or as 100 to 87. + +With respect to the plants in the open ground, each long row was divided +into half, so as to diminish the chance of any accidental advantage in +one part of either row; and the four tallest plants in the two halves of +the two rows were carefully selected and measured. The eight tallest +crossed plants averaged 30.92, and the eight tallest self-fertilised +30.7 inches in height, or as 100 to 99; so that they were practically +equal. But we should bear in mind that the trial was not quite fair, as +the self-fertilised plants had a great advantage over the crossed in +being much less crowded in their own row, owing to the large number of +seeds which had perished under ground after sprouting. Nor were the lots +in the two rows subjected to any mutual competition. + +29. CANNACEAE.--Canna warscewiczi. + +In most or all the species belonging to this genus, the pollen is shed +before the flower expands, and adheres in a mass to the foliaceous +pistil close beneath the stigmatic surface. As the edge of this mass +generally touches the edge of the stigma, and as it was ascertained by +trials purposely made that a very few pollen-grains suffice for +fertilisation, the present species and probably all the others of the +genus are highly self-fertile. Exceptions occasionally occur in which, +from the stamen being slightly shorter than usual, the pollen is +deposited a little beneath the stigmatic surface, and such flowers drop +off unimpregnated unless they are artificially fertilised. Sometimes, +though rarely, the stamen is a little longer than usual, and then the +whole stigmatic surface gets thickly covered with pollen. As some pollen +is generally deposited in contact with the edge of the stigma, certain +authors have concluded that the flowers are invariably self-fertilised. +This is an extraordinary conclusion, for it implies that a great amount +of pollen is produced for no purpose. On this view, also, the large size +of the stigmatic surface is an unintelligible feature in the structure +of the flower, as well as the relative position of all the parts, which +is such that when insects visit the flowers to suck the copious nectar, +they cannot fail to carry pollen from one flower to another. (6/7. +Delpino has described 'Bot. Zeitung' 1867 page 277 and 'Scientific +Opinion' 1870 page 135, the structure of the flowers in this genus, but +he was mistaken in thinking that self-fertilisation is impossible, at +least in the case of the present species. Dr. Dickie and Professor +Faivre state that the flowers are fertilised in the bud, and that +self-fertilisation is inevitable. I presume that they were misled by the +pollen being deposited at a very early period on the pistil: see +'Journal of Linnean Society Botany' volume 10 page 55 and 'Variabilité +des Espèces' 1868 page 158.) + +According to Delpino, bees eagerly visit the flowers in North Italy, but +I have never seen any insect visiting the flowers of the present species +in my hothouse, although many plants grew there during several years. +Nevertheless these plants produced plenty of seed, as they likewise did +when covered by a net; they are therefore fully capable of +self-fertilisation, and have probably been self-fertilised in this +country for many generations. As they are cultivated in pots, and are +not exposed to competition with surrounding plants, they have also been +subjected for a considerable time to somewhat uniform conditions. This, +therefore, is a case exactly parallel with that of the common pea, in +which we have no right to expect much or any good from intercrossing +plants thus descended and thus treated; and no good did follow, +excepting that the cross-fertilised flowers yielded rather more seeds +than the self-fertilised. This species was one of the earlier ones on +which I experimented, and as I had not then raised any self-fertilised +plants for several successive generations under uniform conditions, I +did not know or even suspect that such treatment would interfere with +the advantages to be gained from a cross. I was therefore much surprised +at the crossed plants not growing more vigorously than the +self-fertilised, and a large number of plants were raised, +notwithstanding that the present species is an extremely troublesome one +to experiment on. The seeds, even those which have been long soaked in +water, will not germinate well on bare sand; and those that were sown in +pots (which plan I was forced to follow) germinated at very unequal +intervals of time; so that it was difficult to get pairs of the same +exact age, and many seedlings had to be pulled up and thrown away. My +experiments were continued during three successive generations; and in +each generation the self-fertilised plants were again self-fertilised, +their early progenitors in this country having probably been +self-fertilised for many previous generations. In each generation, also, +the crossed plants were fertilised with pollen from another crossed +plant. + +Of the flowers which were crossed in the three generations, taken +together, a rather larger proportion yielded capsules than did those +which were self-fertilised. The seeds were counted in forty-seven +capsules from the crossed flowers, and they contained on an average 9.95 +seeds; whereas forty-eight capsules from the self-fertilised flowers +contained on an average 8.45 seeds; or as 100 to 85. The seeds from the +crossed flowers were not heavier, on the contrary a little lighter, than +those from the self-fertilised flowers, as was thrice ascertained. On +one occasion I weighed 200 of the crossed and 106 of the self-fertilised +seeds, and the relative weight of an equal number was as 100 for the +crossed to 101.5 for the self-fertilised. With other plants, when the +seeds from the self-fertilised flowers were heavier than those from the +crossed flowers, this appeared to be due generally to fewer having been +produced by the self-fertilised flowers, and to their having been in +consequence better nourished. But in the present instance the seeds from +the crossed capsules were separated into two lots,--namely, those from +the capsules containing over fourteen seeds, and those from the capsules +containing under fourteen seeds, and the seeds from the more productive +capsules were the heavier of the two; so that the above explanation here +fails. + +As pollen is deposited at a very early age on the pistil, generally in +contact with the stigma, some flowers whilst still in bud were castrated +for my first experiment, and were afterwards fertilised with pollen from +a distinct plant. Other flowers were fertilised with their own pollen. +From the seeds thus obtained, I succeeded in rearing only three pairs of +plants of equal age. The three crossed plants averaged 32.79 inches, and +the three self-fertilised 32.08 inches in height; so that they were +nearly equal, the crossed having a slight advantage. As the same result +followed in all three generations, it would be superfluous to give the +heights of all the plants, and I will give only the averages. + +In order to raise crossed and self-fertilised plants of the second +generation, some flowers on the above crossed plants were crossed within +twenty-four hours after they had expanded with pollen from a distinct +plant; and this interval would probably not be too great to allow of +cross-fertilisation being effectual. Some flowers on the self-fertilised +plants of the last generation were also self-fertilised. From these two +lots of seeds, ten crossed and twelve self-fertilised plants of equal +ages were raised; and these were measured when fully grown. The crossed +averaged 36.98, and the self-fertilised averaged 37.42 inches in height; +so that here again the two lots were nearly equal; but the +self-fertilised had a slight advantage. + +In order to raise plants of the third generation, a better plan was +followed, and flowers on the crossed plants of the second generation +were selected in which the stamens were too short to reach the stigmas, +so that they could not possibly have been self-fertilised. These flowers +were crossed with pollen from a distinct plant. Flowers on the +self-fertilised plants of the second generation were again +self-fertilised. From the two lots of seeds thus obtained, twenty-one +crossed and nineteen self-fertilised plants of equal age, and forming +the third generation, were raised in fourteen large pots. They were +measured when fully grown, and by an odd chance the average height of +the two lots was exactly the same, namely, 35.96 inches; so that neither +side had the least advantage over the other. To test this result, all +the plants on both sides in ten out of the above fourteen pots were cut +down after they had flowered, and in the ensuing year the stems were +again measured; and now the crossed plants exceeded by a little (namely, +1.7 inches) the self-fertilised. They were again cut down, and on their +flowering for the third time, the self-fertilised plants had a slight +advantage (namely, 1.54 inches) over the crossed. Hence the result +arrived at with these plants during the previous trials was confirmed, +namely, that neither lot had any decided advantage over the other. It +may, however, be worth mentioning that the self-fertilised plants showed +some tendency to flower before the crossed plants: this occurred with +all three pairs of the first generation; and with the cut down plants of +the third generation, a self-fertilised plant flowered first in nine out +of the twelve pots, whilst in the remaining three pots a crossed plant +flowered first. + +If we consider all the plants of the three generations taken together, +the thirty-four crossed plants average 35.98, and the thirty-four +self-fertilised plants 36.39 inches in height; or as 100 to 101. We may +therefore conclude that the two lots possessed equal powers of growth; +and this I believe to be the result of long-continued +self-fertilisation, together with exposure to similar conditions in each +generation, so that all the individuals had acquired a closely similar +constitution. + +30. GRAMINACEAE.--Zea mays. + +This plant is monoecious, and was selected for trial on this account, no +other such plant having been experimented on. (6/8. Hildebrand remarks +that this species seems at first sight adapted to be fertilised by +pollen from the same plant, owing to the male flowers standing above the +female flowers; but practically it must generally be fertilised by +pollen from another plant, as the male flowers usually shed their pollen +before the female flowers are mature: 'Monatsbericht der K. Akad.' +Berlin October 1872 page 743.) It is also anemophilous, or is fertilised +by the wind; and of such plants only the common beet had been tried. +Some plants were raised in the greenhouse, and were crossed with pollen +taken from a distinct plant; and a single plant, growing quite +separately in a different part of the house, was allowed to fertilise +itself spontaneously. The seeds thus obtained were placed on damp sand, +and as they germinated in pairs of equal age were planted on the +opposite sides of four very large pots; nevertheless they were +considerably crowded. The pots were kept in the hothouse. The plants +were first measured to the tips of their leaves when only between 1 and +2 feet in height, as shown in Table 6/97. + +TABLE 6/97. Zea mays. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 23 4/8 : 17 3/8. +Pot 1 : 12 : 20 3/8. +Pot 1 : 21 : 20. + +Pot 2 : 22 : 20. +Pot 2 : 19 1/8 : 18 3/8. +Pot 2 : 21 4/8 : 18 5/8. + +Pot 3 : 22 1/8 : 18 5/8. +Pot 3 : 20 3/8 : 15 2/8. +Pot 3 : 18 2/8 : 16 4/8. +Pot 3 : 21 5/8 : 18. +Pot 3 : 23 2/8 : 16 2/8. + +Pot 4 : 21 : 18. +Pot 4 : 22 1/8 : 12 6/8. +Pot 4 : 23 : 15 4/8. +Pot 4 : 12 : 18. + +Total : 302.88 : 263.63. + +The fifteen crossed plants here average 20.19, and the fifteen +self-fertilised plants 17.57 inches in height; or as 100 to 87. Mr. +Galton made a graphical representation, in accordance with the method +described in the introductory chapter, of the above measurements, and +adds the words "very good" to the curves thus formed. + +Shortly afterwards one of the crossed plants in Pot 1 died; another +became much diseased and stunted; and the third never grew to its full +height. They seemed to have been all injured, probably by some larva +gnawing their roots. Therefore all the plants on both sides of this pot +were rejected in the subsequent measurements. When the plants were fully +grown they were again measured to the tips of the highest leaves, and +the eleven crossed plants now averaged 68.1, and the eleven +self-fertilised plants 62.34 inches in height; or as 100 to 91. In all +four pots a crossed plant flowered before any one of the +self-fertilised; but three of the plants did not flower at all. Those +that flowered were also measured to the summits of the male flowers: the +ten crossed plants averaged 66.51, and the nine self-fertilised plants +61.59 inches in height; or as 100 to 93. + +A large number of the same crossed and self-fertilised seeds were sown +in the middle of the summer in the open ground in two long rows. Very +much fewer of the self-fertilised than of the crossed plants produced +flowers; but those that did flower, flowered almost simultaneously. When +fully grown the ten tallest plants in each row were selected and +measured to the tips of their highest leaves, as well as to the summits +of their male flowers. The crossed averaged to the tips of their leaves +54 inches in height, and the self-fertilised 44.65, or as 100 to 83; and +to the summits of their male flowers, 53.96 and 43.45 inches; or as 100 +to 80. + +Phalaris canariensis. + +Hildebrand has shown in the paper referred to under the last species, +that this hermaphrodite grass is better adapted for cross-fertilisation +than for self-fertilisation. Several plants were raised in the +greenhouse close together, and their flowers were mutually intercrossed. +Pollen from a single plant growing quite separately was collected and +placed on the stigmas of the same plant. The seeds thus produced were +self-fertilised, for they were fertilised with pollen from the same +plant, but it will have been a mere chance whether with pollen from the +same flowers. Both lots of seeds, after germinating on sand, were +planted in pairs on the opposite sides of four pots, which were kept in +the greenhouse. When the plants were a little over a foot in height they +were measured, and the crossed plants averaged 13.38, and the +self-fertilised 12.29 inches in height; or as 100 to 92. + +When in full flower they were again measured to the extremities of their +culms, as shown in Table 6/98. + +TABLE 6/98. Phalaris canariensis. + +Heights of plants measured in inches. + +Column 1: Number (Name) of Pot. + +Column 2: Crossed Plants. + +Column 3: Self-fertilised Plants. + +Pot 1 : 42 2/8 : 41 2/8. +Pot 1 : 39 6/8 : 45 4/8. + +Pot 2 : 37 : 31 6/8. +Pot 2 : 49 4/8 : 37 2/8. +Pot 4 : 29 : 42 3/8. +Pot 2 : 37 : 34 7/8. + +Pot 3 : 37 6/8 : 28. +Pot 3 : 35 4/8 : 28. +Pot 3 : 43 : 34. + +Pot 4 : 40 2/8 : 35 1/8. +Pot 4 : 37 : 34 4/8. + +Total : 428.00 : 392.63. + +The eleven crossed plants now averaged 38.9, and the eleven +self-fertilised plants 35.69 inches in height; or as 100 to 92, which is +the same ratio as before. Differently to what occurred with the maize, +the crossed plants did not flower before the self-fertilised; and though +both lots flowered very poorly from having been kept in pots in the +greenhouse, yet the self-fertilised plants produced twenty-eight +flower-heads, whilst the crossed produced only twenty! + +Two long rows of the same seeds were sown out of doors, and care was +taken that they were sown in nearly equal number; but a far greater +number of the crossed than of the self-fertilised seeds yielded plants. +The self-fertilised plants were in consequence not so much crowded as +the crossed, and thus had an advantage over them. When in full flower, +the twelve tallest plants were carefully selected from both rows and +measured, as shown in Table 6/99. + +TABLE 6/99. Phalaris canariensis (growing in the open ground). + +Heights of plants measured in inches. + +Column 1: Crossed Plants, twelve tallest. + +Column 2: Self-fertilised Plants, twelve tallest. + + 34 1/8 : 35 2/8. + 35 7/8 : 31 1/8. + 36 : 33. + 35 5/8 : 32. + 35 5/8 : 31 5/8. + 36 1/8 : 36. + 36 6/8 : 33. + 38 6/8 : 32. + 36 2/8 : 35 1/8. + 35 5/8 : 33 5/8. + 34 1/8 : 34 2/8. + 34 5/8 : 35. + +Total : 429.5 : 402.0. + +The twelve crossed plants here average 35.78, and the twelve +self-fertilised 33.5 inches in height; or as 100 to 93. In this case the +crossed plants flowered rather before the self-fertilised, and thus +differed from those growing in the pots.] + + + +CHAPTER VII. + +SUMMARY OF THE HEIGHTS AND WEIGHTS OF THE CROSSED AND SELF-FERTILISED +PLANTS. + +Number of species and plants measured. +Tables given. +Preliminary remarks on the offspring of plants crossed by a fresh stock. +Thirteen cases specially considered. +The effects of crossing a self-fertilised plant either by another +self-fertilised plant or by an intercrossed plant of the old stock. +Summary of the results. +Preliminary remarks on the crossed and self-fertilised plants of the +same stock. +The twenty-six exceptional cases considered, in which the crossed plants +did not exceed greatly in height the self-fertilised. +Most of these cases shown not to be real exceptions to the rule that +cross-fertilisation is beneficial. +Summary of results. +Relative weights of the crossed and self-fertilised plants. + +The details which have been given under the head of each species are so +numerous and so intricate, that it is necessary to tabulate the results. +In Table 7/A, the number of plants of each kind which were raised from a +cross between two individuals of the same stock and from self-fertilised +seeds, together with their mean or average heights, are given. In the +right hand column, the mean height of the crossed to that of the +self-fertilised plants, the former being taken as 100, is shown. To make +this clear, it may be advisable to give an example. In the first +generation of Ipomoea, six plants derived from a cross between two +plants were measured, and their mean height is 86.00 inches; six plants +derived from flowers on the same parent-plant fertilised with their own +pollen were measured, and their mean height is 65.66 inches. From this +it follows, as shown in the right hand column, that if the mean height +of the crossed plants be taken as 100, that of the self-fertilised +plants is 76. The same plan is followed with all the other species. + +The crossed and self-fertilised plants were generally grown in pots in +competition with one another, and always under as closely similar +conditions as could be attained. They were, however, sometimes grown in +separate rows in the open ground. With several of the species, the +crossed plants were again crossed, and the self-fertilised plants again +self-fertilised, and thus successive generations were raised and +measured, as may be seen in Table 7/A. Owing to this manner of +proceeding, the crossed plants became in the later generations more or +less closely inter-related. + +In Table 7/B the relative weights of the crossed and self-fertilised +plants, after they had flowered and had been cut down, are given in the +few cases in which they were ascertained. The results are, I think, more +striking and of greater value as evidence of constitutional vigour than +those deduced from the relative heights of the plants. + +The most important table is Table 7/C, as it includes the relative +heights, weights, and fertility of plants raised from parents crossed by +a fresh stock (that is, by non-related plants grown under different +conditions), or by a distinct sub-variety, in comparison with +self-fertilised plants, or in a few cases with plants of the same old +stock intercrossed during several generations. The relative fertility of +the plants in this and the other tables will be more fully considered in +a future chapter. + +TABLE 7/A. Relative heights of plants from parents crossed with pollen +from other plants of the same stock, and self-fertilised. + +Heights of plants measured in inches. + +Column 1: Name of Plant. + +Column 2: Number of Crossed Plants measured. + +Column 3: Average Height of Crossed Plants. + +Column 4: Number of Self-fertilised Plants measured. + +Column 5: Average Height of Self-fertilised Plants. + +Column 6: x, where the ratio of the Average Height of the Crossed to the +Self-fertilised Plants is expressed as 100 to x. + +Ipomoea purpurea--first generation: + 6 : 86.00 : 6 : 65.66 : 76. + +Ipomoea purpurea--second generation: + 6 : 84.16 : 6 : 66.33 : 79. + +Ipomoea purpurea--third generation: + 6 : 77.41 : 6 : 52.83 : 68. + +Ipomoea purpurea--fourth generation: + 7 : 69.78 : 7 : 60.14 : 86. + +Ipomoea purpurea--fifth generation: + 6 : 82.54 : 6 : 62.33 : 75. + +Ipomoea purpurea--sixth generation: + 6 : 87.50 : 6 : 63.16 : 72. + +Ipomoea purpurea--seventh generation: + 9 : 83.94 : 9 : 68.25 : 81. + +Ipomoea purpurea--eighth generation: + 8 : 113.25 : 8 : 96.65 : 85. + +Ipomoea purpurea--ninth generation: + 14 : 81.39 : 14 : 64.07 : 79. + +Ipomoea purpurea--tenth generation: + 5 : 93.70 : 5 : 50.40 : 54. + +Ipomoea purpurea--Number and average height of all the plants of the ten +generations: + 73 : 85.84 : 73 : 66.02 : 77. + +Mimulus luteus--three first generations, before the new and taller +self-fertilised variety appeared: + 10 : 8.19 : 10 : 5.29 : 65. + +Digitalis purpurea: + 16 : 51.33 : 8 : 35.87 : 70. + +Calceolaria--(common greenhouse variety): + 1 : 19.50 : 1 : 15.00 : 77. + +Linaria vulgaris: + 3 : 7.08 : 3 : 5.75 : 81. + +Verbascum thapsus: + 6 : 65.34 : 6 : 56.50 : 86. + +Vandellia nummularifolia--crossed and self-fertilised plants, raised +from perfect flowers: + 20 : 4.30 : 20 : 4.27 : 99. + +Vandellia nummularifolia--crossed and self-fertilised plants, raised +from perfect flowers: second trial, plants crowded: + 24 : 3.60 : 24 : 3.38 : 94. + +Vandellia nummularifolia--crossed plants raised from perfect flowers, +and self-fertilised plants from cleistogene flowers: + 20 : 4.30 : 20 : 4.06 : 94. + +Gesneria pendulina: + 8 : 32.06 : 8 : 29.14 : 90. + +Salvia coccinea: + 6 : 27.85 : 6 : 21.16 : 76. + +Origanum vulgare: + 4 : 20.00 : 4 : 17.12 : 86. + +Thunbergia alata: + 6 : 60.00 : 6 : 65.00 : 108. + +Brassica oleracea: + 9 : 41.08 : 9 : 39.00 : 95. + +Iberis umbellata--the self-fertilised plants of the third generation: + 7 : 19.12 : 7 : 16.39 : 86. + +Papaver vagum: + 15 : 21.91 : 15 : 19.54 : 89. + +Eschscholtzia californica--English stock, first generation: + 4 : 29.68 : 4 : 25.56 : 86. + +Eschscholtzia californica--English stock, second generation: + 11 : 32.47 : 11 : 32.81 : 101. + +Eschscholtzia californica--Brazilian stock, first generation: + 14 : 44.64 : 14 : 45.12 : 101. + +Eschscholtzia californica--Brazilian stock, second generation: + 18 : 43.38 : 19 : 50.30 : 116. + +Eschscholtzia californica--average height and number of all the plants +of Eschscholtzia: + 47 : 40.03 : 48 : 42.72 : 107. + +Reseda lutea--grown in pots: + 24 : 17.17 : 24 : 14.61 : 85. + +Reseda lutea--grown in open ground : + 8 : 28.09 : 8 : 23.14 : 82. + +Reseda odorata--self-fertilised seeds from a highly self-fertile plant, +grown in pots: + 19 : 27.48 : 19 : 22.55 : 82. + +Reseda odorata--self-fertilised seeds from a highly self-fertile plant, +grown in open ground: + 8 : 25.76 : 8 : 27.09 : 105. + +Reseda odorata--self-fertilised seeds from a semi-self-fertile plant, +grown in pots: + 20 : 29.98 : 20 : 27.71 : 92. + +Reseda odorata--self-fertilised seeds from a semi-self-fertile plant, +grown in open ground: + 8 : 25.92 : 8 : 23.54 : 90. + +Viola tricolor: + 14 : 5.58 : 14 : 2.37 : 42. + +Adonis aestivalis: + 4 : 14.25 : 4 : 14.31 : 100. + +Delphinium consolida: + 6 : 14.95 : 6 : 12.50 : 84. + +Viscaria oculata: + 15 : 34.50 : 15 : 33.55 : 97. + +Dianthus caryophyllus--open ground, about : + 6?: 28? : 6?: 24? : 86. + +Dianthus caryophyllus--second generation, in pots, crowded: + 2 : 16.75 : 2 : 9.75 : 58. + +Dianthus caryophyllus--third generation, in pots: + 8 : 28.39 : 8 : 28.21 : 99. + +Dianthus caryophyllus--offspring from plants of the third +self-fertilised generation crossed by intercrossed plants of the third +generation, compared with plants of fourth self-fertilised generation: + 15 : 28.00 : 10 : 26.55 : 95. + +Dianthus caryophyllus--number and average height of all the plants of +Dianthus: + 31 : 27.37 : 26 : 25.18 : 92. + +Hibiscus africanus: + 4 : 13.25 : 4 : 14.43 : 109. + +Pelargonium zonale: + 7 : 22.35 : 7 : 16.62 : 74. + +Tropaeolum minus: + 8 : 58.43 : 8 : 46.00 : 79. + +Limnanthes douglasii: + 16 : 17.46 : 16 : 13.85 : 79. + +Lupinus luteus--second generation: + 8 : 30.78 : 8 : 25.21 : 82. + +Lupinus pilosus--plants of two generations: + 2 : 35.50 : 3 : 30.50 : 86. + +Phaseolus multiflorus: + 5 : 86.00 : 5 : 82.35 : 96. + +Pisum sativum: + 4 : 34.62 : 4 : 39.68 : 115. + +Sarothamnus scoparius--small seedlings: + 6 : 2.91 : 6 : 1.33 : 46. + +Sarothamnus scoparius--the three survivors on each side after three +years' growth: + : 18.91 : : 11.83 : 63. + +Ononis minutissima: + 2 : 19.81 : 2 : 17.37 : 88. + +Clarkia elegans: + 4 : 33.50 : 4 : 27.62 : 82. + +Bartonia aurea: + 8 : 24.62 : 8 : 26.31 : 107. + +Passiflora gracilis: + 2 : 49.00 : 2 : 51.00 : 104. + +Apium petroselinum: + * : : * : : 100. +*not measured. + +Scabiosa atro-purpurea: + 4 : 17.12 : 4 : 15.37 : 90. + +Lactuca sativa--plants of two generations: + 7 : 19.43 : 6 : 16.00 : 82. + +Specularia speculum: + 4 : 19.28 : 4 : 18.93 : 98. + +Lobelia ramosa--first generation: + 4 : 22.25 : 4 : 18.37 : 82. + +Lobelia ramosa--second generation: + 3 : 23.33 : 3 : 19.00 : 81. + +Lobelia fulgens--first generation: + 2 : 34.75 : 2 : 44.25 : 127. + +Lobelia fulgens--second generation: + 23 : 29.82 : 23 : 27.10 : 91. + +Nemophila insignis--half-grown: + 12 : 11.10 : 12 : 5.45 : 49. + +Nemophila insignis--the same fully-grown: + : 33.28 : : 19.90 : 60. + +Borago officinalis: + 4 : 20.68 : 4 : 21.18 : 102. + +Nolana prostrata: + 5 : 12.75 : 5 : 13.40 : 105. + +Petunia violacea--first generation: + 5 : 30.80 : 5 : 26.00 : 84. + +Petunia violacea--second generation: + 4 : 40.50 : 6 : 26.25 : 65. + +Petunia violacea--third generation: + 8 : 40.96 : 8 : 53.87 : 131. + +Petunia violacea--fourth generation: + 15 : 46.79 : 14 : 32.39 : 69. + +Petunia violacea--fourth generation, from a distinct parent: + 13 : 44.74 : 13 : 26.87 : 60. + +Petunia violacea--fifth generation: + 22 : 54.11 : 21 : 33.23 : 61. + +Petunia violacea--fifth generation, in open ground: + 10 : 38.27 : 10 : 23.31 : 61. + +Petunia violacea--Number and average height of all the plants in pots of +Petunia: + 67 : 46.53 : 67 : 33.12 : 71. + +Nicotiana tabacum--first generation: + 4 : 18.50 : 4 : 32.75 : 178. + +Nicotiana tabacum--second generation: + 9 : 53.84 : 7 : 51.78 : 96. + +Nicotiana tabacum--third generation: + 7 : 95.25 : 7 : 79.60 : 83. + +Nicotiana tabacum--third generation but raised from a distinct plant: + 7 : 70.78 : 9 : 71.30 : 101. + +Nicotiana tabacum--Number and average height of all the plants of +Nicotiana: + 27 : 63.73 : 27 : 61.31 : 96. + +Cyclamen persicum: + 8 : 9.49 : 8?: 7.50 : 79. + +Anagallis collina: + 6 : 42.20 : 6 : 33.35 : 69. + +Primula sinensis--a dimorphic species: + 8 : 9.01 : 8 : 9.03 : 100. + +Fagopyrum esculentum--a dimorphic species: + 15 : 38.06 : 15 : 26.13 : 69. + +Beta vulgaris--in pots: + 8 : 34.09 : 8 : 29.81 : 87. + +Beta vulgaris--in open ground: + 8 : 30.92 : 8 : 30.70 : 99. + +Canna warscewiczi--plants of three generations: + 34 : 35.98 : 34 : 36.39 : 101. + +Zea mays--in pots, whilst young, measured to tips of leaves: + 15 : 20.19 : 15 : 17.57 : 87. + +Zea mays--when full-grown, after the death of some, measured to tips of +leaves: + : 68.10 : : 62.34 : 91. + +Zea mays--when full-grown, after the death of some, measured to tips of +flowers: + : 66.51 : : 61.59 : 93. + +Zea mays--grown in open ground, measured to tips of leaves: + 10 : 54.00 : 10 : 44.55 : 83. + +Zea mays--grown in open ground, measured to tips of flowers: + : 53.96 : : 43.45 : 80. + +Phalaris canariensis--in pots. + 11 : 38.90 : 11 : 35.69 : 92. + +Phalaris canariensis--in open ground: + 12 : 35.78 : 12 : 33.50 : 93. + +TABLE 7/B.--Relative weights of plants from parents crossed with pollen +from distinct plants of the same stock, and self-fertilised. + +Column 1: Names of plants. + +Column 2: Number of crossed plants. + +Column 3: Number of self-fertilised plants. + +Column 4: x, where the ratio of the Weight of the Crossed to the +Self-fertilised Plants is expressed as 100 to x. + +Ipomoea purpurea--plants of the tenth generation: + 6 : 6 : 44. + +Vandellia nummularifolia--first generation: + 41 : 41 : 97. + +Brassica oleracea--first generation: + 9 : 9 : 37. + +Eschscholtzia californica--plants of the second generation: + 19 : 19 : 118. + +Reseda lutea--first generation, grown in pots: + 24 : 24 : 21. + +Reseda lutea--first generation, grown in open ground: + 8 : 8 : 40. + +Reseda odorata--first generation, descended from a highly self-fertile +plant, grown in pots: + 19 : 19 : 67. + +Reseda odorata--first generation, descended from a semi-self-fertile +plant, grown in pots: + 20 : 20 : 99. + +Dianthus caryophyllus--plants of the third generation: + 8 : 8 : 49. + +Petunia violacea--plants of the fifth generation, in pots: + 22 : 21 : 22. + +Petunia violacea--plants of the fifth generation, in open ground: + 10 : 10 : 36. + +TABLE 7/C.--Relative heights, weights, and fertility of plants from +parents crossed by a fresh stock, and from parents either +self-fertilised or intercrossed with plants of the same stock. + +Column 1: Names of the plants and nature of the experiments. + +Column 2: Number of plants from a cross with a fresh stock. + +Column 3: Average height in inches and weight. + +Column 4: Number of the plants from self-fertilised or intercrossed +parents of the same stock. + +Column 5: Average height in inches and weight. + +Column 4: x, where the ratio of the Height, Weight and Fertility of the +plants from the Cross with a fresh stock is expressed as 100 to x. + +Ipomoea purpurea--offspring of plants intercrossed for nine generations +and then crossed by a fresh stock, compared with plants of the tenth +intercrossed generation: + 19 : 84.03 : 19 : 65.78 : 78. + +Ipomoea purpurea--offspring of plants intercrossed for nine generations +and then crossed by a fresh stock, compared with plants of the tenth +intercrossed generation, in fertility: + .. : .. : .. : .. : 51. + +Mimulus luteus--offspring of plants self-fertilised for eight +generations and then crossed by a fresh stock, compared with plants of +the ninth self-fertilised generation: + 28 : 21.62 : 19 : 10.44 : 52. + +Mimulus luteus--offspring of plants self-fertilised for eight +generations and then crossed by a fresh stock, compared with plants of +the ninth self-fertilised generation, in fertility: + .. : .. : .. : .. : 3. + +Mimulus luteus--offspring of plants self-fertilised for eight +generations and then crossed by a fresh stock, compared with the +offspring of a plant self-fertilised for eight generations, and then +intercrossed with another self-fertilised plant of the same generation: + 28 : 21.62 : 27 : 12.20 : 56. + +Mimulus luteus--offspring of plants self-fertilised for eight +generations and then crossed by a fresh stock, compared with the +offspring of a plant self-fertilised for eight generations, and then +intercrossed with another self-fertilised plant of the same generation, +in fertility: + .. : .. : .. : .. : 4. + +Brassica oleracea--offspring of plants self-fertilised for two +generations and then crossed by a fresh stock, compared with plants of +the third self-fertilised generation, by weight: + 6 : : 6 : : 22. + +Iberis umbellata--offspring from English variety crossed by slightly +different Algerine variety, compared with the self-fertilised offspring +of the English variety: + 30 : 17.34 : 29 : 15.51 : 89. + +Iberis umbellata--offspring from English variety crossed by slightly +different Algerine variety, compared with the self-fertilised offspring +of the English variety, in fertility: + .. : .. : .. : .. : 75. + +Eschscholtzia californica--offspring of a Brazilian stock crossed by an +English stock, compared with plants of the Brazilian stock of the second +self-fertilised generation: + 19 : 45.92 : 19 : 50.30 : 109. + +Eschscholtzia californica--offspring of a Brazilian stock crossed by an +English stock, compared with plants of the Brazilian stock of the second +self-fertilised generation, in weight: + .. : .. : .. : .. : 118. + +Eschscholtzia californica--offspring of a Brazilian stock crossed by an +English stock, compared with plants of the Brazilian stock of the second +self-fertilised generation, in fertility: + .. : .. : .. : .. : 40. + +Eschscholtzia californica--offspring of a Brazilian stock crossed by an +English stock, compared with plants of the Brazilian stock of the second +intercrossed generation, in height: + 19 : 45.92 : 18 : 43.38 : 94. + +Eschscholtzia californica--offspring of a Brazilian stock crossed by an +English stock, compared with plants of the Brazilian stock of the second +intercrossed generation, in weight: + .. : .. : .. : .. : 100. + +Eschscholtzia californica--offspring of a Brazilian stock crossed by an +English stock, compared with plants of the Brazilian stock of the second +intercrossed generation, in fertility: + .. : .. : .. : .. : 45. + +Dianthus caryophyllus--offspring of plants self-fertilised for three +generations and then crossed by a fresh stock, compared with plants of +the fourth self-fertilised generation: + 16 : 32.82 : 10 : 26.55 : 81. + +Dianthus caryophyllus--offspring of plants self-fertilised for three +generations and then crossed by a fresh stock, compared with plants of +the fourth self-fertilised generation, in fertility: + .. : .. : .. : .. : 33. + +Dianthus caryophyllus--offspring of plants self-fertilised for three +generations and then crossed by a fresh stock, compared with the +offspring of plants self-fertilised for three generations and then +crossed by plants of the third intercrossed generation: + 16 : 32.82 : 15 : 28.00 : 85. + +Dianthus caryophyllus--offspring of plants self-fertilised for three +generations and then crossed by a fresh stock, compared with the +offspring of plants self-fertilised for three generations and then +crossed by plants of the third intercrossed generation, in fertility: + .. : .. : .. : .. : 45. + +Pisum sativum--offspring from a cross between two closely allied +varieties, compared with the self-fertilised offspring of one of the +varieties, or with intercrossed plants of the same stock: + ? : : ? : : 60 to 75. + +Lathyrus odoratus--offspring from two varieties, differing only in +colour of their flowers, compared with the self-fertilised offspring of +one of the varieties: in first generation: + 2 : 79.25 : 2 : 63.75 : 80. + +Lathyrus odoratus--offspring from two varieties, differing only in +colour of their flowers, compared with the self-fertilised offspring of +one of the varieties: in second generation: + 6 : 62.91 : 6 : 55.31 : 88. + +Petunia violacea--offspring of plants self-fertilised for four +generations and then crossed by a fresh stock, compared with plants of +the fifth self-fertilised generation, in height: + 21 : 50.05 : 21 : 33.23 : 66. + +Petunia violacea--offspring of plants self-fertilised for four +generations and then crossed by a fresh stock, compared with plants of +the fifth self-fertilised generation, in weight: + .. : .. : .. : .. : 23. + +Petunia violacea--offspring of plants self-fertilised for four +generations and then crossed by a fresh stock, compared with plants of +the fifth self-fertilised generation, grown in open ground, in height: + 10 : 36.67 : 10 : 23.31 : 63. + +Petunia violacea--offspring of plants self-fertilised for four +generations and then crossed by a fresh stock, compared with plants of +the fifth self-fertilised generation, grown in open ground, in weight: + .. : .. : .. : .. : 53. + +Petunia violacea--offspring of plants self-fertilised for four +generations and then crossed by a fresh stock, compared with plants of +the fifth self-fertilised generation, grown in open ground, in +fertility: + .. : .. : .. : .. : 46. + +Petunia violacea--offspring of plants self-fertilised for four +generations and then crossed by a fresh stock, compared with plants of +the fifth intercrossed generation, in height: + 21 : 50.05 : 22 : 54.11 : 108. + +Petunia violacea--offspring of plants self-fertilised for four +generations and then crossed by a fresh stock, compared with plants of +the fifth intercrossed generation, in weight: + .. : .. : .. : .. : 101. + +Petunia violacea--offspring of plants self-fertilised for four +generations and then crossed by a fresh stock, compared with plants of +the fifth intercrossed generation, grown in open ground, in height: + 10 : 36.67 : 10 : 38.27 : 104. + +Petunia violacea--offspring of plants self-fertilised for four +generations and then crossed by a fresh stock, compared with plants of +the fifth intercrossed generation, grown in open ground, in weight: + .. : .. : .. : .. : 146. + +Petunia violacea--offspring of plants self-fertilised for four +generations and then crossed by a fresh stock, compared with plants of +the fifth intercrossed generation, grown in open ground, in fertility: + .. : .. : .. : .. : 54. + +Nicotiana tabacum--offspring of plants self-fertilised for three +generations and then crossed by a slightly different variety, compared +with plants of the fourth self-fertilised generation, grown not much +crowded in pots, in height: + 26 : 63.29 : 26 : 41.67 : 66. + +Nicotiana tabacum--offspring of plants self-fertilised for three +generations and then crossed by a slightly different variety, compared +with plants of the fourth self-fertilised generation, grown much crowded +in pots, in height: + 12 : 31.53 : 12 : 17.21 : 54. + +Nicotiana tabacum--offspring of plants self-fertilised for three +generations and then crossed by a slightly different variety, compared +with plants of the fourth self-fertilised generation, grown much crowded +in pots, in weight: + .. : .. : .. : .. : 37. + +Nicotiana tabacum--offspring of plants self-fertilised for three +generations and then crossed by a slightly different variety, compared +with plants of the fourth self-fertilised generation, grown in open +ground, in height: + 20 : 48.74 : 20 : 35.20 : 72. + +Nicotiana tabacum--offspring of plants self-fertilised for three +generations and then crossed by a slightly different variety, compared +with plants of the fourth self-fertilised generation, grown in open +ground, in weight: + .. : .. : .. : .. : 63. + +Anagallis collina--offspring from a red variety crossed by a blue +variety, compared with the self-fertilised offspring of the red variety: + 3 : 27.62 : 3 : 18.21 : 66. + +Anagallis collina--offspring from a red variety crossed by a blue +variety, compared with the self-fertilised offspring of the red variety, +in fertility: + .. : .. : .. : .. : 6. + +Primula veris--offspring from long-styled plants of the third +illegitimate generation, crossed by a fresh stock, compared with plants +of the fourth illegitimate and self-fertilised generation: + 8 : 7.03 : 8 : 3.21 : 46. + +Primula veris--offspring from long-styled plants of the third +illegitimate generation, crossed by a fresh stock, compared with plants +of the fourth illegitimate and self-fertilised generation, in fertility: + .. : .. : .. : .. : 5. + +Primula veris--offspring from long-styled plants of the third +illegitimate generation, crossed by a fresh stock, compared with plants +of the fourth illegitimate and self-fertilised generation, in fertility +in following year: + .. : .. : .. : .. : 3.5. + +Primula veris--(equal-styled, red-flowered variety)--offspring from +plants self-fertilised for two generations and then crossed by a +different variety, compared with plants of the third self-fertilised +generation: + 3 : 8.66 : 3 : 7.33 : 85. + +Primula veris--(equal-styled, red-flowered variety)--offspring from +plants self-fertilised for two generations and then crossed by a +different variety, compared with plants of the third self-fertilised +generation, in fertility: + .. : .. : .. : .. : 11. + +In these three tables the measurements of fifty-seven species, belonging +to fifty-two genera and to thirty great natural families, are given. The +species are natives of various parts of the world. The number of crossed +plants, including those derived from a cross between plants of the same +stock and of two different stocks, amounts to 1,101; and the number of +self-fertilised plants (including a few in Table 7/C derived from a +cross between plants of the same old stock) is 1,076. Their growth was +observed from the germination of the seeds to maturity; and most of them +were measured twice and some thrice. The various precautions taken to +prevent either lot being unduly favoured, have been described in the +introductory chapter. Bearing all these circumstances in mind, it may be +admitted that we have a fair basis for judging of the comparative +effects of cross-fertilisation and of self-fertilisation on the growth +of the offspring. + +It will be the most convenient plan first to consider the results given +in Table 7/C, as an opportunity will thus be afforded of incidentally +discussing some important points. If the reader will look down the right +hand column of this table, he will see at a glance what an extraordinary +advantage in height, weight, and fertility the plants derived from a +cross with a fresh stock or with another sub-variety have over the +self-fertilised plants, as well as over the intercrossed plants of the +same old stock. There are only two exceptions to this rule, and these +are hardly real ones. In the case of Eschscholtzia, the advantage is +confined to fertility. In that of Petunia, though the plants derived +from a cross with a fresh stock had an immense superiority in height, +weight, and fertility over the self-fertilised plants, they were +conquered by the intercrossed plants of the same old stock in height and +weight, but not in fertility. It has, however, been shown that the +superiority of these intercrossed plants in height and weight was in all +probability not real; for if the two sets had been allowed to grow for +another month, it is almost certain that those from a cross with the +fresh stock would have been victorious in every way over the +intercrossed plants. + +Before we consider in detail the several cases given in Table 7/C, some +preliminary remarks must be made. There is the clearest evidence, as we +shall presently see, that the advantage of a cross depends wholly on the +plants differing somewhat in constitution; and that the disadvantages of +self-fertilisation depend on the two parents, which are combined in the +same hermaphrodite flower, having a closely similar constitution. A +certain amount of differentiation in the sexual elements seems +indispensable for the full fertility of the parents, and for the full +vigour of the offspring. All the individuals of the same species, even +those produced in a state of nature, differ somewhat, though often very +slightly, from one another in external characters and probably in +constitution. This obviously holds good between the varieties of the +same species, as far as external characters are concerned; and much +evidence could be advanced with respect to their generally differing +somewhat in constitution. There can hardly be a doubt that the +differences of all kinds between the individuals and varieties of the +same species depend largely, and as I believe exclusively, on their +progenitors having been subjected to different conditions; though the +conditions to which the individuals of the same species are exposed in a +state of nature often falsely appear to us the same. For instance, the +individuals growing together are necessarily exposed to the same +climate, and they seem to us at first sight to be subjected to +identically the same conditions; but this can hardly be the case, except +under the unusual contingency of each individual being surrounded by +other kinds of plants in exactly the same proportional numbers. For the +surrounding plants absorb different amounts of various substances from +the soil, and thus greatly affect the nourishment and even the life of +the individuals of any particular species. These will also be shaded and +otherwise affected by the nature of the surrounding plants. Moreover, +seeds often lie dormant in the ground, and those which germinate during +any one year will often have been matured during very different seasons. +Seeds are widely dispersed by various means, and some will occasionally +be brought from distant stations, where their parents have grown under +somewhat different conditions, and the plants produced from such seeds +will intercross with the old residents, thus mingling their +constitutional peculiarities in all sorts of proportions. + +Plants when first subjected to culture, even in their native country, +cannot fail to be exposed to greatly changed conditions of life, more +especially from growing in cleared ground, and from not having to +compete with many or any surrounding plants. They are thus enabled to +absorb whatever they require which the soil may contain. Fresh seeds are +often brought from distant gardens, where the parent-plants have been +subjected to different conditions. Cultivated plants like those in a +state of nature frequently intercross, and will thus mingle their +constitutional peculiarities. On the other hand, as long as the +individuals of any species are cultivated in the same garden, they will +apparently be subjected to more uniform conditions than plants in a +state of nature, as the individuals have not to compete with various +surrounding species. The seeds sown at the same time in a garden have +generally been matured during the same season and in the same place; and +in this respect they differ much from the seeds sown by the hand of +nature. Some exotic plants are not frequented by the native insects in +their new home, and therefore are not intercrossed; and this appears to +be a highly important factor in the individuals acquiring uniformity of +constitution. + +In my experiments the greatest care was taken that in each generation +all the crossed and self-fertilised plants should be subjected to the +same conditions. Not that the conditions were absolutely the same, for +the more vigorous individuals will have robbed the weaker ones of +nutriment, and likewise of water when the soil in the pots was becoming +dry; and both lots at one end of the pot will have received a little +more light than those at the other end. In the successive generations, +the plants were subjected to somewhat different conditions, for the +seasons necessarily varied, and they were sometimes raised at different +periods of the year. But as they were all kept under glass, they were +exposed to far less abrupt and great changes of temperature and moisture +than are plants growing out of doors. With respect to the intercrossed +plants, their first parents, which were not related, would almost +certainly have differed somewhat in constitution; and such +constitutional peculiarities would be variously mingled in each +succeeding intercrossed generation, being sometimes augmented, but more +commonly neutralised in a greater or less degree, and sometimes revived +through reversion; just as we know to be the case with the external +characters of crossed species and varieties. With the plants which were +self-fertilised during the successive generations, this latter important +source of some diversity of constitution will have been wholly +eliminated; and the sexual elements produced by the same flower must +have been developed under as nearly the same conditions as it is +possible to conceive. + +In Table 7/C the crossed plants are the offspring of a cross with a +fresh stock, or with a distinct variety; and they were put into +competition either with self-fertilised plants, or with intercrossed +plants of the same old stock. By the term fresh stock I mean a +non-related plant, the progenitors of which have been raised during some +generations in another garden, and have consequently been exposed to +somewhat different conditions. In the case of Nicotiana, Iberis, the red +variety of Primula, the common Pea, and perhaps Anagallis, the plants +which were crossed may be ranked as distinct varieties or sub-varieties +of the same species; but with Ipomoea, Mimulus, Dianthus, and Petunia, +the plants which were crossed differed exclusively in the tint of their +flowers; and as a large proportion of the plants raised from the same +lot of purchased seeds thus varied, the differences may be estimated as +merely individual. Having made these preliminary remarks, we will now +consider in detail the several cases given in Table 7/C, and they are +well worthy of full consideration. + +1. Ipomoea purpurea. + +Plants growing in the same pots, and subjected in each generation to the +same conditions, were intercrossed for nine consecutive generations. +These intercrossed plants thus became in the later generations more or +less closely inter-related. Flowers on the plants of the ninth +intercrossed generation were fertilised with pollen taken from a fresh +stock, and seedlings thus raised. Other flowers on the same intercrossed +plants were fertilised with pollen from another intercrossed plant, +producing seedlings of the tenth intercrossed generation. These two sets +of seedlings were grown in competition with one another, and differed +greatly in height and fertility. For the offspring from the cross with a +fresh stock exceeded in height the intercrossed plants in the ratio of +100 to 78; and this is nearly the same excess which the intercrossed had +over the self-fertilised plants in all ten generations taken together, +namely, as 100 to 77. The plants raised from the cross with a fresh +stock were also greatly superior in fertility to the intercrossed, +namely, in the ratio of 100 to 51, as judged by the relative weight of +the seed-capsules produced by an equal number of plants of the two sets, +both having been left to be naturally fertilised. It should be +especially observed that none of the plants of either lot were the +product of self-fertilisation. On the contrary, the intercrossed plants +had certainly been crossed for the last ten generations, and probably, +during all previous generations, as we may infer from the structure of +the flowers and from the frequency of the visits of humble-bees. And so +it will have been with the parent-plants of the fresh stock. The whole +great difference in height and fertility between the two lots must be +attributed to the one being the product of a cross with pollen from a +fresh stock, and the other of a cross between plants of the same old +stock. + +This species offers another interesting case. In the five first +generations in which intercrossed and self-fertilised plants were put +into competition with one another, every single intercrossed plant beat +its self-fertilised antagonist, except in one instance, in which they +were equal in height. But in the sixth generation a plant appeared, +named by me the Hero, remarkable for its tallness and increased +self-fertility, and which transmitted its characters to the next three +generations. The children of Hero were again self-fertilised, forming +the eighth self-fertilised generation, and were likewise intercrossed +one with another; but this cross between plants which had been subjected +to the same conditions and had been self-fertilised during the seven +previous generations, did not effect the least good; for the +intercrossed grandchildren were actually shorter than the +self-fertilised grandchildren, in the ratio of 100 to 107. We here see +that the mere act of crossing two distinct plants does not by itself +benefit the offspring. This case is almost the converse of that in the +last paragraph, on which the offspring profited so greatly by a cross +with a fresh stock. A similar trial was made with the descendants of +Hero in the following generation, and with the same result. But the +trial cannot be fully trusted, owing to the extremely unhealthy +condition of the plants. Subject to this same serious cause of doubt, +even a cross with a fresh stock did not benefit the great-grandchildren +of Hero; and if this were really the case, it is the greatest anomaly +observed by me in all my experiments. + +2. Mimulus luteus. + +During the three first generations the intercrossed plants taken +together exceeded in height the self-fertilised taken together, in the +ratio of 100 to 65, and in fertility in a still higher degree. In the +fourth generation a new variety, which grew taller and had whiter and +larger flowers than the old varieties, began to prevail, especially +amongst the self-fertilised plants. This variety transmitted its +characters with remarkable fidelity, so that all the plants in the later +self-fertilised generations belonged to it. These consequently exceeded +the intercrossed plants considerably in height. Thus in the seventh +generation the intercrossed plants were to the self-fertilised in height +as 100 to 137. It is a more remarkable fact that the self-fertilised +plants of the sixth generation had become much more fertile than the +intercrossed plants, judging by the number of capsules spontaneously +produced, in the ratio of 147 to 100. This variety, which as we have +seen appeared amongst the plants of the fourth self-fertilised +generation, resembles in almost all its constitutional peculiarities the +variety called Hero which appeared in the sixth self-fertilised +generation of Ipomoea. No other such case, with the partial exception of +that of Nicotiana, occurred in my experiments, carried on during eleven +years. + +Two plants of this variety of Mimulus, belonging to the sixth +self-fertilised generation, and growing in separate pots, were +intercrossed; and some flowers on the same plants were again +self-fertilised. From the seeds thus obtained, plants derived from a +cross between the self-fertilised plants, and others of the seventh +self-fertilised generation, were raised. But this cross did not do the +least good, the intercrossed plants being inferior in height to the +self-fertilised, in the ratio of 100 to 110. This case is exactly +parallel with that given under Ipomoea, of the grandchildren of Hero, +and apparently of its great-grandchildren; for the seedlings raised by +intercrossing these plants were not in any way superior to those of the +corresponding generation raised from the self-fertilised flowers. +Therefore in these several cases the crossing of plants, which had been +self-fertilised for several generations and which had been cultivated +all the time under as nearly as possible the same conditions, was not in +the least beneficial. + +Another experiment was now tried. Firstly, plants of the eighth +self-fertilised generation were again self-fertilised, producing plants +of the ninth self-fertilised generation. Secondly, two of the plants of +the eighth self-fertilised generation were intercrossed one with +another, as in the experiment above referred to; but this was now +effected on plants which had been subjected to two additional +generations of self-fertilisation. Thirdly, the same plants of the +eighth self-fertilised generation were crossed with pollen from plants +of a fresh stock brought from a distant garden. Numerous plants were +raised from these three sets of seeds, and grown in competition with one +another. The plants derived from a cross between the self-fertilised +plants exceeded in height by a little the self-fertilised, namely, as +100 to 92; and in fertility in a greater degree, namely, as 100 to 73. I +do not know whether this difference in the result, compared with that in +the previous case, can be accounted for by the increased deterioration +of the self-fertilised plants from two additional generations of +self-fertilisation, and the consequent advantage of any cross whatever, +along merely between the self-fertilised plants. But however this may +be, the effects of crossing the self-fertilised plants of the eighth +generation with a fresh stock were extremely striking; for the seedlings +thus raised were to the self-fertilised of the ninth generation as 100 +to 52 in height, and as 100 to 3 in fertility! They were also to the +intercrossed plants (derived from crossing two of the self-fertilised +plants of the eighth generation) in height as 100 to 56, and in +fertility as 100 to 4. Better evidence could hardly be desired of the +potent influence of a cross with a fresh stock on plants which had been +self-fertilised for eight generations, and had been cultivated all the +time under nearly uniform conditions, in comparison with plants +self-fertilised for nine generations continuously, or then once +intercrossed, namely in the last generation. + +3. Brassica oleracea. + +Some flowers on cabbage plants of the second self-fertilised generation +were crossed with pollen from a plant of the same variety brought from a +distant garden, and other flowers were again self-fertilised. Plants +derived from a cross with a fresh stock and plants of the third +self-fertilised generation were thus raised. The former were to the +self-fertilised in weight as 100 to 22; and this enormous difference +must be attributed in part to the beneficial effects of a cross with a +fresh stock, and in part to the deteriorating effects of +self-fertilisation continued during three generations. + +4. Iberis umbellata. + +Seedlings from a crimson English variety crossed by a pale-coloured +variety which had been grown for some generations in Algiers, were to +the self-fertilised seedlings from the crimson variety in height as 100 +to 89, and as 100 to 75 in fertility. I am surprised that this cross +with another variety did not produce a still more strongly marked +beneficial effect; for some intercrossed plants of the crimson English +variety, put into competition with plants of the same variety +self-fertilised during three generations, were in height as 100 to 86, +and in fertility as 100 to 75. The slightly greater difference in height +in this latter case, may possibly be attributed to the deteriorating +effects of self-fertilisation carried on for two additional generations. + +5. Eschscholtzia californica. + +This plant offers an almost unique case, inasmuch as the good effects of +a cross are confined to the reproductive system. Intercrossed and +self-fertilised plants of the English stock did not differ in height +(nor in weight, as far as was ascertained) in any constant manner; the +self-fertilised plants usually having the advantage. So it was with the +offspring of plants of the Brazilian stock, tried in the same manner. +The parent-plants, however, of the English stock produced many more +seeds when fertilised with pollen from another plant than when +self-fertilised; and in Brazil the parent-plants were absolutely sterile +unless they were fertilised with pollen from another plant. Intercrossed +seedlings, raised in England from the Brazilian stock, compared with +self-fertilised seedlings of the corresponding second generation, +yielded seeds in number as 100 to 89; both lots of plants being left +freely exposed to the visits of insects. If we now turn to the effects +of crossing plants of the Brazilian stock with pollen from the English +stock,--so that plants which had been long exposed to very different +conditions were intercrossed,--we find that the offspring were, as +before, inferior in height and weight to the plants of the Brazilian +stock after two generations of self-fertilisation, but were superior to +them in the most marked manner in the number of seeds produced, namely, +as 100 to 40; both lots of plants being left freely exposed to the +visits of insects. + +In the case of Ipomoea, we have seen that the plants derived from a +cross with a fresh stock were superior in height as 100 to 78, and in +fertility as 100 to 51, to the plants of the old stock, although these +had been intercrossed during the last ten generations. With +Eschscholtzia we have a nearly parallel case, but only as far as +fertility is concerned, for the plants derived from a cross with a fresh +stock were superior in fertility in the ratio of 100 to 45 to the +Brazilian plants, which had been artificially intercrossed in England +for the two last generations, and which must have been naturally +intercrossed by insects during all previous generations in Brazil, where +otherwise they are quite sterile. + +6. Dianthus caryophyllus. + +Plants self-fertilised for three generations were crossed with pollen +from a fresh stock, and their offspring were grown in competition with +plants of the fourth self-fertilised generation. The crossed plants thus +obtained were to the self-fertilised in height as 100 to 81, and in +fertility (both lots being left to be naturally fertilised by insects) +as 100 to 33. + +These same crossed plants were also to the offspring from the plants of +the third generation crossed by the intercrossed plants of the +corresponding generation, in height as 100 to 85, and in fertility as +100 to 45. + +We thus see what a great advantage the offspring from a cross with a +fresh stock had, not only over the self-fertilised plants of the fourth +generation, but over the offspring from the self-fertilised plants of +the third generation, when crossed by the intercrossed plants of the old +stock. + +7. Pisum sativum. + +It has been shown under the head of this species, that the several +varieties in this country almost invariably fertilise themselves, owing +to insects rarely visiting the flowers; and as the plants have been long +cultivated under nearly similar conditions, we can understand why a +cross between two individuals of the same variety does not do the least +good to the offspring either in height or fertility. This case is almost +exactly parallel with that of Mimulus, or that of the Ipomoea named +Hero; for in these two instances, crossing plants which had been +self-fertilised for seven generations did not at all benefit the +offspring. On the other hand, a cross between two varieties of the pea +causes a marked superiority in the growth and vigour of the offspring, +over the self-fertilised plants of the same varieties, as shown by two +excellent observers. From my own observations (not made with great care) +the offspring from crossed varieties were to self-fertilised plants in +height, in one case as 100 to about 75, and in a second case as 100 to +60. + +8. Lathyrus odoratus. + +The sweet-pea is in the same state in regard to self-fertilisation as +the common pea; and we have seen that seedlings from a cross between two +varieties, which differed in no respect except in the colour of their +flowers, were to the self-fertilised seedlings from the same +mother-plant in height as 100 to 80; and in the second generation as 100 +to 88. Unfortunately I did not ascertain whether crossing two plants of +the same variety failed to produce any beneficial effect, but I venture +to predict such would be the result. + +9. Petunia violacea. + +The intercrossed plants of the same stock in four out of the five +successive generations plainly exceeded in height the self-fertilised +plants. The latter in the fourth generation were crossed by a fresh +stock, and the seedlings thus obtained were put into competition with +the self-fertilised plants of the fifth generation. The crossed plants +exceeded the self-fertilised in height in the ratio of 100 to 66, and in +weight as 100 to 23; but this difference, though so great, is not much +greater than that between the intercrossed plants of the same stock in +comparison with the self-fertilised plants of the corresponding +generation. This case, therefore, seems at first sight opposed to the +rule that a cross with a fresh stock is much more beneficial than a +cross between individuals of the same stock. But as with Eschscholtzia, +the reproductive system was here chiefly benefited; for the plants +raised from the cross with the fresh stock were to the self-fertilised +plants in fertility, both lots being naturally fertilised, as 100 to 46, +whereas the intercrossed plants of the same stock were to the +self-fertilised plants of the corresponding fifth generation in +fertility only as 100 to 86. + +Although at the time of measurement the plants raised from the cross +with the fresh stock did not exceed in height or weight the intercrossed +plants of the old stock (owing to the growth of the former not having +been completed, as explained under the head of this species), yet they +exceeded the intercrossed plants in fertility in the ratio of 100 to 54. +This fact is interesting, as it shows that plants self-fertilised for +four generations and then crossed by a fresh stock, yielded seedlings +which were nearly twice as fertile as those from plants of the same +stock which had been intercrossed for the five previous generations. We +here see, as with Eschscholtzia and Dianthus, that the mere act of +crossing, independently of the state of the crossed plants, has little +efficacy in giving increased fertility to the offspring. The same +conclusion holds good, as we have already seen, in the analogous cases +of Ipomoea, Mimulus, and Dianthus, with respect to height. + +10. Nicotiana tabacum. + +My plants were remarkably self-fertile, and the capsules from the +self-fertilised flowers apparently yielded more seeds than those which +were cross-fertilised. No insects were seen to visit the flowers in the +hothouse, and I suspect that the stock on which I experimented had been +raised under glass, and had been self-fertilised during several previous +generations; if so, we can understand why, in the course of three +generations, the crossed seedlings of the same stock did not uniformly +exceed in height the self-fertilised seedlings. But the case is +complicated by individual plants having different constitutions, so that +some of the crossed and self-fertilised seedlings raised at the same +time from the same parents behaved differently. However this may be, +plants raised from self-fertilised plants of the third generation +crossed by a slightly different sub-variety, exceeded greatly in height +and weight the self-fertilised plants of the fourth generation; and the +trial was made on a large scale. They exceeded them in height when grown +in pots, and not much crowded, in the ratio of 100 to 66; and when much +crowded, as 100 to 54. These crossed plants, when thus subjected to +severe competition, also exceeded the self-fertilised in weight in the +ratio of 100 to 37. So it was, but in a less degree (as may be seen in +Table 7/C), when the two lots were grown out of doors and not subjected +to any mutual competition. Nevertheless, strange as is the fact, the +flowers on the mother-plants of the third self-fertilised generation did +not yield more seed when they were crossed with pollen from plants of +the fresh stock than when they were self-fertilised. + +11. Anagallis collina. + +Plants raised from a red variety crossed by another plant of the same +variety were in height to the self-fertilised plants from the red +variety as 100 to 73. When the flowers on the red variety were +fertilised with pollen from a closely similar blue-flowered variety, +they yielded double the number of seeds to what they did when crossed by +pollen from another individual of the same red variety, and the seeds +were much finer. The plants raised from this cross between the two +varieties were to the self-fertilised seedlings from the red variety, in +height as 100 to 66, and in fertility as 100 to 6. + +12. Primula veris. + +Some flowers on long-styled plants of the third illegitimate generation +were legitimately crossed with pollen from a fresh stock, and others +were fertilised with their own pollen. From the seeds thus produced +crossed plants, and self-fertilised plants of the fourth illegitimate +generation, were raised. The former were to the latter in height as 100 +to 46, and in fertility during one year as 100 to 5, and as 100 to 3.5 +during the next year. In this case, however, we have no means of +distinguishing between the evil effects of illegitimate fertilisation +continued during four generations (that is, by pollen of the same form, +but taken from a distinct plant) and strict self-fertilisation. But it +is probable that these two processes do not differ so essentially as at +first appears to be the case. In the following experiment any doubt +arising from illegitimate fertilisation was completely eliminated. + +13. Primula veris. (Equal-styled, red-flowered variety.) + +Flowers on plants of the second self-fertilised generation were crossed +with pollen from a distinct variety or fresh stock, and others were +again self-fertilised. Crossed plants and plants of the third +self-fertilised generation, all of legitimate origin, were thus raised; +and the former was to the latter in height as 100 to 85, and in +fertility (as judged by the number of capsules produced, together with +the average number of seeds) as 100 to 11. + +SUMMARY OF THE MEASUREMENTS IN TABLE 7/C. + +This table includes the heights and often the weights of 292 plants +derived from a cross with a fresh stock, and of 305 plants, either of +self-fertilised origin, or derived from an intercross between plants of +the same stock. These 597 plants belong to thirteen species and twelve +genera. The various precautions which were taken to ensure a fair +comparison have already been stated. If we now look down the right hand +column, in which the mean height, weight, and fertility of the plants +derived from a cross with a fresh stock are represented by 100, we shall +see by the other figures how wonderfully superior they are both to the +self-fertilised and to the intercrossed plants of the same stock. With +respect to height and weight, there are only two exceptions to the rule, +namely, with Eschscholtzia and Petunia, and the latter is probably no +real exception. Nor do these two species offer an exception in regard to +fertility, for the plants derived from the cross with a fresh stock were +much more fertile than the self-fertilised plants. The difference +between the two sets of plants in the table is generally much greater in +fertility than in height or weight. On the other hand, with some of the +species, as with Nicotiana, there was no difference in fertility between +the two sets, although a great difference in height and weight. +Considering all the cases in this table, there can be no doubt that +plants profit immensely, though in different ways, by a cross with a +fresh stock or with a distinct sub-variety. It cannot be maintained that +the benefit thus derived is due merely to the plants of the fresh stock +being perfectly healthy, whilst those which had been long intercrossed +or self-fertilised had become unhealthy; for in most cases there was no +appearance of such unhealthiness, and we shall see under Table 7/A that +the intercrossed plants of the same stock are generally superior to a +certain extent to the self-fertilised,--both lots having been subjected +to exactly the same conditions and being equally healthy or unhealthy. + +We further learn from Table 7/C, that a cross between plants that have +been self-fertilised during several successive generations and kept all +the time under nearly uniform conditions, does not benefit the offspring +in the least or only in a very slight degree. Mimulus and the +descendants of Ipomoea named Hero offer instances of this rule. Again, +plants self-fertilised during several generations profit only to a small +extent by a cross with intercrossed plants of the same stock (as in the +case of Dianthus), in comparison with the effects of a cross by a fresh +stock. Plants of the same stock intercrossed during several generations +(as with Petunia) were inferior in a marked manner in fertility to those +derived from the corresponding self-fertilised plants crossed by a fresh +stock. Lastly, certain plants which are regularly intercrossed by +insects in a state of nature, and which were artificially crossed in +each succeeding generation in the course of my experiments, so that they +can never or most rarely have suffered any evil from self-fertilisation +(as with Eschscholtzia and Ipomoea), nevertheless profited greatly by a +cross with a fresh stock. These several cases taken together show us in +the clearest manner that it is not the mere crossing of any two +individuals which is beneficial to the offspring. The benefit thus +derived depends on the plants which are united differing in some manner, +and there can hardly be a doubt that it is in the constitution or nature +of the sexual elements. Anyhow, it is certain that the differences are +not of an external nature, for two plants which resemble each other as +closely as the individuals of the same species ever do, profit in the +plainest manner when intercrossed, if their progenitors have been +exposed during several generations to different conditions. But to this +latter subject I shall have to recur in a future chapter. + +TABLE 7/A. + +We will now turn to our first table, which relates to crossed and +self-fertilised plants of the same stock. These consist of fifty-four +species belonging to thirty natural orders. The total number of crossed +plants of which measurements are given is 796, and of self-fertilised +809; that is altogether 1,605 plants. Some of the species were +experimented on during several successive generations; and it should be +borne in mind that in such cases the crossed plants in each generation +were crossed with pollen from another crossed plant, and the flowers on +the self-fertilised plants were almost always fertilised with their own +pollen, though sometimes with pollen from other flowers on the same +plant. The crossed plants thus became more or less closely inter-related +in the later generations; and both lots were subjected in each +generation to almost absolutely the same conditions, and to nearly the +same conditions in the successive generations. It would have been a +better plan in some respects if I had always crossed some flowers either +on the self-fertilised or intercrossed plants of each generation with +pollen from a non-related plant, grown under different conditions, as +was done with the plants in Table 7/C; for by this procedure I should +have learnt how much the offspring became deteriorated through continued +self-fertilisation in the successive generations. As the case stands, +the self-fertilised plants of the successive generations in Table 7/A +were put into competition with and compared with intercrossed plants, +which were probably deteriorated in some degree by being more or less +inter-related and grown under similar conditions. Nevertheless, had I +always followed the plan in Table 7/C, I should not have discovered the +important fact that, although a cross between plants which are rather +closely related and which had been subjected to closely similar +conditions, gives during several generations some advantage to the +offspring, yet that after a time they may be intercrossed with no +advantage whatever to the offspring. Nor should I have learnt that the +self-fertilised plants of the later generations might be crossed with +intercrossed plants of the same stock with little or no advantage, +although they profited to an extraordinary degree by a cross with a +fresh stock. + +With respect to the greater number of the plants in Table 7/A, nothing +special need here be said; full particulars may be found under the head +of each species by the aid of the Index. The figures in the right-hand +column show the mean height of the self-fertilised plants, that of the +crossed plants with which they competed being represented by 100. No +notice is here taken of the few cases in which crossed and +self-fertilised plants were grown in the open ground, so as not to +compete together. The table includes, as we have seen, plants belonging +to fifty-four species, but as some of these were measured during several +successive generations, there are eighty-three cases in which crossed +and self-fertilised plants were compared. As in each generation the +number of plants which were measured (given in the table) was never very +large and sometimes small, whenever in the right hand column the mean +height of the crossed and self-fertilised plants is the same within five +per cent, their heights may be considered as practically equal. Of such +cases, that is, of self-fertilised plants of which the mean height is +expressed by figures between 95 and 105, there are eighteen, either in +some one or all the generations. There are eight cases in which the +self-fertilised plants exceed the crossed by above five per cent, as +shown by the figures in the right hand column being above 105. Lastly, +there are fifty-seven cases in which the crossed plants exceed the +self-fertilised in a ratio of at least 100 to 95, and generally in a +much higher degree. + +If the relative heights of the crossed and self-fertilised plants had +been due to mere chance, there would have been about as many cases of +self-fertilised plants exceeding the crossed in height by above five per +cent as of the crossed thus exceeding the self-fertilised; but we see +that of the latter there are fifty-seven cases, and of the former only +eight cases; so that the cases in which the crossed plants exceed in +height the self-fertilised in the above proportion are more than seven +times as numerous as those in which the self-fertilised exceed the +crossed in the same proportion. For our special purpose of comparing the +powers of growth of crossed and self-fertilised plants, it may be said +that in fifty-seven cases the crossed plants exceeded the +self-fertilised by more than five per cent, and that in twenty-six cases +(18 + 8) they did not thus exceed them. But we shall now show that in +several of these twenty-six cases the crossed plants had a decided +advantage over the self-fertilised in other respects, though not in +height; that in other cases the mean heights are not trustworthy, owing +to too few plants having been measured, or to their having grown +unequally from being unhealthy, or to both causes combined. +Nevertheless, as these cases are opposed to my general conclusion I have +felt bound to give them. Lastly, the cause of the crossed plants having +no advantage over the self-fertilised can be explained in some other +cases. Thus a very small residue is left in which the self-fertilised +plants appear, as far as my experiments serve, to be really equal or +superior to the crossed plants. + +We will now consider in some little detail the eighteen cases in which +the self-fertilised plants equalled in average height the crossed plants +within five per cent; and the eight cases in which the self-fertilised +plants exceeded in average height the crossed plants by above five per +cent; making altogether twenty-six cases in which the crossed plants +were not taller than the self-fertilised plants in any marked degree. + +[1. Dianthus caryophyllus (third generation). + +This plant was experimented on during four generations, in three of +which the crossed plants exceeded in height the self-fertilised +generally by much more than five per cent; and we have seen under Table +7/C that the offspring from the plants of the third self-fertilised +generation crossed by a fresh stock profited in height and fertility to +an extraordinary degree. But in this third generation the crossed plants +of the same stock were in height to the self-fertilised only as 100 to +99, that is, they were practically equal. Nevertheless, when the eight +crossed and eight self-fertilised plants were cut down and weighed, the +former were to the latter in weight as 100 to 49! There can therefore be +not the least doubt that the crossed plants of this species are greatly +superior in vigour and luxuriance to the self-fertilised; and what was +the cause of the self-fertilised plants of the third generation, though +so light and thin, growing up so as almost to equal the crossed in +height, I cannot explain. + +2. Lobelia fulgens (first generation). + +The crossed plants of this generation were much inferior in height to +the self-fertilised, in the proportion of 100 to 127. Although only two +pairs were measured, which is obviously much too few to be trusted, yet +from other evidence given under the head of this species, it is certain +that the self-fertilised plants were very much more vigorous than the +crossed. As I used pollen of unequal maturity for crossing and +self-fertilising the parent-plants, it is possible that the great +difference in the growth of their offspring may have been due to this +cause. In the next generation this source of error was avoided, and many +more plants were raised, and now the average height of the twenty-three +crossed plants was to that of the twenty-three self-fertilised plants as +100 to 91. We can therefore hardly doubt that a cross is beneficial to +this species. + +3. Petunia violacea (third generation). + +Eight crossed plants were to eight self-fertilised of the third +generation in average height as 100 to 131; and at an early age the +crossed were inferior even in a still higher degree. But it is a +remarkable fact that in one pot in which plants of both lots grew +extremely crowded, the crossed were thrice as tall as the +self-fertilised. As in the two preceding and two succeeding generations, +as well as with plants raised by a crossed with a fresh stock, the +crossed greatly exceeded the self-fertilised in height, weight, and +fertility (when these two latter points were attended to), the present +case must be looked at as an anomaly not affecting the general rule. The +most probable explanation is that the seeds from which the crossed +plants of the third generation were raised were not well ripened; for I +have observed an analogous case with Iberis. Self-fertilised seedlings +of this latter plant, which were known to have been produced from seeds +not well matured, grew from the first much more quickly than the crossed +plants, which were raised from better matured seeds; so that having thus +once got a great start they were enabled ever afterwards to retain their +advantage. Some of these same seeds of the Iberis were sown on the +opposite sides of pots filled with burnt earth and pure sand, not +containing any organic matter; and now the young crossed seedlings grew +during their short life to double the height of the self-fertilised, in +the same manner as occurred with the above two sets of seedlings of +Petunia which were much crowded and thus exposed to very unfavourable +conditions. We have seen also in the eighth generation of Ipomoea that +the self-fertilised seedlings raised from unhealthy parents grew at +first very much more quickly than the crossed seedlings, so that they +were for a long time much taller, though ultimately beaten by them. + +4, 5, 6. Eschscholtzia californica. + +Four sets of measurements are given in Table 7/A. In one of these the +crossed plants exceed the self-fertilised in average height, so that +this is not one of the exceptions here to be considered. In two other +cases the crossed equalled the self-fertilised in height within five per +cent; and in the fourth case the self-fertilised exceeded the crossed by +above this limit. We have seen in Table 7/C that the whole advantage of +a cross by a fresh stock is confined to fertility, and so it was with +the intercrossed plants of the same stock compared with the +self-fertilised, for the former were in fertility to the latter as 100 +to 89. The intercrossed plants thus have at least one important +advantage over the self-fertilised. Moreover, the flowers on the +parent-plants when fertilised with pollen from another individual of the +same stock yield far more seeds than when self-fertilised; the flowers +in this latter case being often quite sterile. We may therefore conclude +that a cross does some good, though it does not give to the crossed +seedlings increased powers of growth. + +7. Viscaria oculata. + +The average height of the fifteen intercrossed plants to that of the +fifteen self-fertilised plants was only as 100 to 97; but the former +produced many more capsules than the latter, in the ratio of 100 to 77. +Moreover, the flowers on the parent-plants which were crossed and +self-fertilised, yielded seeds on one occasion in the proportion of 100 +to 38, and on a second occasion in the proportion of 100 to 58. So that +there can be no doubt about the beneficial effects of a cross, although +the mean height of the crossed plants was only three per cent above that +of the self-fertilised plants. + +8. Specularia speculum. + +Only the four tallest of the crossed and the four tallest of the +self-fertilised plants, growing in four pots, were measured; and the +former were to the latter in height as 100 to 98. In all four pots a +crossed plant flowered before any one of the self-fertilised plants, and +this is usually a safe indication of some real superiority in the +crossed plants. The flowers on the parent-plants which were crossed with +pollen from another plant yielded seeds compared with the +self-fertilised flowers in the ratio of 100 to 72. We may therefore draw +the same conclusion as in the last case with respect to a cross being +decidedly beneficial. + +9. Borago officinalis. + +Only four crossed and four self-fertilised plants were raised and +measured, and the former were to the latter in height as 100 to 102. So +small a number of measurements ought never to be trusted; and in the +present instance the advantage of the self-fertilised over the crossed +plants depended almost entirely on one of the self-fertilised plants +having grown to an unusual height. All four crossed plants flowered +before their self-fertilised opponents. The cross-fertilised flowers on +the parent-plants in comparison with the self-fertilised flowers yielded +seeds in the proportion of 100 to 60. So that here again we may draw the +same conclusion as in the two last cases. + +10. Passiflora gracilis. + +Only two crossed and two self-fertilised plants were raised; and the +former were to the latter in height as 100 to 104. On the other hand, +fruits from the cross-fertilised flowers on the parent-plants contained +seeds in number, compared with those from the self-fertilised flowers, +in the proportion of 100 to 85. + +11. Phaseolus multiflorus. + +The five crossed plants were to the five self-fertilised in height as +100 to 96. Although the crossed plants were thus only four per cent +taller than the self-fertilised, they flowered in both pots before them. +It is therefore probable that they had some real advantage over the +self-fertilised plants. + +12. Adonis aestivalis. + +The four crossed plants were almost exactly equal in height to the four +self-fertilised plants, but as so few plants were measured, and as these +were all "miserably unhealthy," nothing can be inferred with safety with +respect to their relative heights. + +13. Bartonia aurea. + +The eight crossed plants were to the eight self-fertilised in height as +100 to 107. This number of plants, considering the care with which they +were raised and compared, ought to have given a trustworthy result. But +from some unknown cause they grew very unequally, and they became so +unhealthy that only three of the crossed and three of the +self-fertilised plants set any seeds, and these few in number. Under +these circumstances the mean height of neither lot can be trusted, and +the experiment is valueless. The cross-fertilised flowers on the +parent-plants yielded rather more seeds than the self-fertilised +flowers. + +14. Thunbergia alata. + +The six crossed plants were to the six self-fertilised in height as 100 +to 108. Here the self-fertilised plants seem to have a decided +advantage; but both lots grew unequally, some of the plants in both +being more than twice as tall as others. The parent-plants also were in +an odd semi-sterile condition. Under these circumstances the superiority +of the self-fertilised plants cannot be fully trusted. + +15. Nolana prostrata. + +The five crossed plants were to the five self-fertilised in height as +100 to 105; so that the latter seem here to have a small but decided +advantage. On the other hand, the flowers on the parent-plants which +were cross-fertilised produced very many more capsules than the +self-fertilised flowers, in the ratio of 100 to 21; and the seeds which +the former contained were heavier than an equal number from the +self-fertilised capsules in the ratio of 100 to 82. + +16. Hibiscus africanus. + +Only four pairs were raised, and the crossed were to the self-fertilised +in height as 100 to 109. Excepting that too few plants were measured, I +know of nothing else to cause distrust in the result. The +cross-fertilised flowers on the parent-plants were, on the other hand, +rather more productive than the self-fertilised flowers. + +17. Apium petroselinum. + +A few plants (number not recorded) derived from flowers believed to have +been crossed by insects and a few self-fertilised plants were grown on +the opposite sides of four pots. They attained to a nearly equal height, +the crossed having a very slight advantage. + +18. Vandellia nummularifolia. + +Twenty crossed plants raised from the seeds of perfect flowers were to +twenty self-fertilised plants, likewise raised from the seeds of perfect +flowers, in height as 100 to 99. The experiment was repeated, with the +sole difference that the plants were allowed to grow more crowded; and +now the twenty-four tallest of the crossed plants were to the +twenty-four tallest self-fertilised plants in height as 100 to 94, and +in weight as 100 to 97. Moreover, a larger number of the crossed than of +the self-fertilised plants grew to a moderate height. The +above-mentioned twenty crossed plants were also grown in competition +with twenty self-fertilised plants raised from the closed or cleistogene +flowers, and their heights were as 100 to 94. Therefore had it not been +for the first trial, in which the crossed plants were to the +self-fertilised in height only as 100 to 99, this species might have +been classed with those in which the crossed plants exceed the +self-fertilised by above five per cent. On the other hand, the crossed +plants in the second trial bore fewer capsules; and these contained +fewer seeds, than did the self-fertilised plants, all the capsules +having been produced by cleistogene flowers. The whole case therefore +must be left doubtful. + +19. Pisum sativum (common pea). + +Four-plants derived from a cross between individuals of the same variety +were in height to four self-fertilised plants belonging to the same +variety as 100 to 115. Although this cross did no good, we have seen +under Table 7/C that a cross between distinct varieties adds greatly to +the height and vigour of the offspring; and it was there explained that +the fact of a cross between the individuals of the same variety not +being beneficial, is almost certainly due to their having been +self-fertilised for many generations, and in each generation grown under +nearly similar conditions. + +20, 21, 22. Canna warscewiczi. + +Plants belonging to three generations were observed, and in all of three +the crossed were approximately equal to the self-fertilised; the average +height of the thirty-four crossed plants being to that of the same +number of self-fertilised plants as 100 to 101. Therefore the crossed +plants had no advantage over the self-fertilised; and it is probable +that the same explanation here holds good as in the case of Pisum +sativum; for the flowers of this Canna are perfectly self-fertile, and +were never seen to be visited by insects in the hothouse, so as to be +crossed by them. This plant, moreover, has been cultivated under glass +for several generations in pots, and therefore under nearly uniform +conditions. The capsules produced by the cross-fertilised flowers on the +above thirty-four crossed plants contained more seeds than did the +capsules produced by the self-fertilised flowers on the self-fertilised +plants, in the proportion of 100 to 85; so that in this respect crossing +was beneficial. + +23. Primula sinensis. + +The offspring of plants, some of which were legitimately and others +illegitimately fertilised with pollen from a distinct plant, were almost +exactly of the same height as the offspring of self-fertilised plants; +but the former with rare exceptions flowered before the latter. I have +shown in my paper on dimorphic plants that this species is commonly +raised in England from self-fertilised seed, and the plants from having +been cultivated in pots have been subjected to nearly uniform +conditions. Moreover, many of them are now varying and changing their +character, so as to become in a greater or less degree equal-styled, and +in consequence highly self-fertile. Therefore I believe that the cause +of the crossed plants not exceeding in height the self-fertilised is the +same as in the two previous cases of Pisum sativum and Canna. + +24, 25, 26. Nicotiana tabacum. + +Four sets of measurements were made; in one, the self-fertilised plants +greatly exceeded in height the crossed, in two others they were +approximately equal to the crossed, and in the fourth were beaten by +them; but this latter case does not here concern us. The individual +plants differ in constitution, so that the descendants of some profit by +their parents having been intercrossed, whilst others do not. Taking all +three generations together, the twenty-seven crossed plants were in +height to the twenty-seven self-fertilised plants as 100 to 96. This +excess of height in the crossed plants, is so small compared with that +displayed by the offspring from the same mother-plants when crossed by a +slightly different variety, that we may suspect (as explained under +Table 7/C) that most of the individuals belonging to the variety which +served as the mother-plants in my experiments, had acquired a nearly +similar constitution, so as not to profit by being mutually +intercrossed.] + +Reviewing these twenty-six cases, in which the crossed plants either do +not exceed the self-fertilised by above five per cent in height, or are +inferior to them, we may conclude that much the greater number of the +cases do not form real exceptions to the rule,--that a cross between two +plants, unless these have been self-fertilised and exposed to nearly the +same conditions for many generations, gives a great advantage of some +kind to the offspring. Of the twenty-six cases, at least two, namely, +those of Adonis and Bartonia, may be wholly excluded, as the trials were +worthless from the extreme unhealthiness of the plants. Inn twelve other +cases (three trials with Eschscholtzia here included) the crossed plants +either were superior in height to the self-fertilised in all the other +generations excepting the one in question, or they showed their +superiority in some different manner, as in weight, fertility, or in +flowering first; or again, the cross-fertilised flowers on the +mother-plant were much more productive of seed than the self-fertilised. + +Deducting these fourteen cases, there remain twelve in which the crossed +plants show no well-marked advantage over the self-fertilised. On the +other hand, we have seen that there are fifty-seven cases in which the +crossed plants exceed the self-fertilised in height by at least five per +cent, and generally in a much higher degree. But even in the twelve +cases just referred to, the want of any advantage on the crossed side is +far from certain: with Thunbergia the parent-plants were in an odd +semi-sterile condition, and the offspring grew very unequally; with +Hibiscus and Apium much too few plants were raised for the measurements +to be trusted, and the cross-fertilised flowers of Hibiscus produced +rather more seed than did the self-fertilised; with Vandellia the +crossed plants were a little taller and heavier than the +self-fertilised, but as they were less fertile the case must be left +doubtful. Lastly, with Pisum, Primula, the three generations of Canna, +and the three of Nicotiana (which together complete the twelve cases), a +cross between two plants certainly did no good or very little good to +the offspring; but we have reason to believe that this is the result of +these plants having been self-fertilised and cultivated under nearly +uniform conditions for several generations. The same result followed +with the experimental plants of Ipomoea and Mimulus, and to a certain +extent with some other species, which had been intentionally treated by +me in this manner; yet we know that these species in their normal +condition profit greatly by being intercrossed. There is, therefore, not +a single case in Table 7/A which affords decisive evidence against the +rule that a cross between plants, the progenitors of which have been +subjected to somewhat diversified conditions, is beneficial to the +offspring. This is a surprising conclusion, for from the analogy of +domesticated animals it could not have been anticipated, that the good +effects of crossing or the evil effects of self-fertilisation would have +been perceptible until the plants had been thus treated for several +generations. + +The results given in Table 7/A may be looked at under another point of +view. Hitherto each generation has been considered as a separate case, +of which there are eighty-three; and this no doubt is the more correct +method of comparing the crossed and self-fertilised plants. + +But in those cases in which plants of the same species were observed +during several generations, a general average of their heights in all +the generations together may be made; and such averages are given in +Table 7/A; for instance, under Ipomoea the general average for the +plants of all ten generations is as 100 for the crossed, to 77 for the +self-fertilised plants. This having been done in each case in which more +than one generation was raised, it is easy to calculate the average of +the average heights of the crossed and self-fertilised plants of all the +species included in Table 7/A. It should however be observed that as +only a few plants of some species, whilst a considerable number of +others, were measured, the value of the mean or average heights of the +several species is very different. Subject to this source of error, it +may be worth while to give the mean of the mean heights of the +fifty-four species in Table 7/A; and the result is, calling the mean of +the mean heights of the crossed plants 100, that of the self-fertilised +plants is 87. But it is a better plan to divide the fifty-four species +into three groups, as was done with the previously given eighty-three +cases. The first group consists of species of which the mean heights of +the self-fertilised plants are within five per cent of 100; so that the +crossed and self-fertilised plants are approximately equal; and of such +species there are twelve about which nothing need be said, the mean of +the mean heights of the self-fertilised being of course very nearly 100, +or exactly 99.58. The second group consists of the species, thirty-seven +in number, of which the mean heights of the crossed plants exceed that +of the self-fertilised plants by more than five per cent; and the mean +of their mean heights is to that of the self-fertilised plants as 100 to +78. The third group consists of the species, only five in number, of +which the mean heights of the self-fertilised plants exceed that of the +crossed by more than five per cent; and here the mean of the mean +heights of the crossed plants is to that of the self-fertilised as 100 +to 109. Therefore if we exclude the species which are approximately +equal, there are thirty-seven species in which the mean of the mean +heights of the crossed plants exceeds that of the self-fertilised by +twenty-two per cent; whereas there are only five species in which the +mean of the mean heights of the self-fertilised plants exceeds that of +the crossed, and this only by nine per cent. + +The truth of the conclusion--that the good effects of a cross depend on +the plants having been subjected to different conditions or to their +belonging to different varieties, in both of which cases they would +almost certainly differ somewhat in constitution--is supported by a +comparison of the Tables 7/A and 7/C. The latter table gives the results +of crossing plants with a fresh stock or with a distinct variety; and +the superiority of the crossed offspring over the self-fertilised is +here much more general and much more strongly marked than in Table 7/A, +in which plants of the same stock were crossed. We have just seen that +the mean of the mean heights of the crossed plants of the whole +fifty-four species in Table 7/A is to that of the self-fertilised plants +as 100 to 87; whereas the mean of the mean heights of the plants crossed +by a fresh stock is to that of the self-fertilised in Table 7/C as 100 +to 74. So that the crossed plants beat the self-fertilised plants by +thirteen per cent in Table 7/A, and by twenty-six per cent, or double as +much, in Table 7/C, which includes the results of the cross by a fresh +stock. + +TABLE 7/B. + +A few words must be added on the weights of the crossed plants of the +same stock, in comparison with the self-fertilised. Eleven cases are +given in Table 7/B, relating to eight species. The number of plants +which were weighed is shown in the two left columns, and their relative +weights in the right column, that of the crossed plants being taken as +100. A few other cases have already been recorded in Table 7/C in +reference to plants crossed by a fresh stock. I regret that more trials +of this kind were not made, as the evidence of the superiority of the +crossed over the self-fertilised plants is thus shown in a more +conclusive manner than by their relative heights. But this plan was not +thought of until a rather late period, and there were difficulties +either way, as the seeds had to be collected when ripe, by which time +the plants had often begun to wither. In only one out of the eleven +cases in Table 7/B, that of Eschscholtzia, do the self-fertilised plants +exceed the crossed in weight; and we have already seen they are likewise +superior to them in height, though inferior in fertility, the whole +advantage of a cross being here confined to the reproductive system. +With Vandellia the crossed plants were a little heavier, as they were +also a little taller than the self-fertilised; but as a greater number +of more productive capsules were produced by the cleistogene flowers on +the self-fertilised plants than by those on the crossed plants, the case +must be left, as remarked under Table 7/A, altogether doubtful. The +crossed and self-fertilised offspring from a partially self-sterile +plant of Reseda odorata were almost equal in weight, though not in +height. In the remaining eight cases, the crossed plants show a +wonderful superiority over the self-fertilised, being more than double +their weight, except in one case, and here the ratio is as high as 100 +to 67. The results thus deduced from the weights of the plants confirm +in a striking manner the former evidence of the beneficial effects of a +cross between two plants of the same stock; and in the few cases in +which plants derived from a cross with a fresh stock were weighed, the +results are similar or even more striking. + + + +CHAPTER VIII. + +DIFFERENCE BETWEEN CROSSED AND SELF-FERTILISED PLANTS IN CONSTITUTIONAL +VIGOUR AND IN OTHER RESPECTS. + +Greater constitutional vigour of crossed plants. +The effects of great crowding. +Competition with other kinds of plants. +Self-fertilised plants more liable to premature death. +Crossed plants generally flower before the self-fertilised. +Negative effects of intercrossing flowers on the same plant. +Cases described. +Transmission of the good effects of a cross to later generations. +Effects of crossing plants of closely related parentage. +Uniform colour of the flowers on plants self-fertilised during several +generations and cultivated under similar conditions. + +GREATER CONSTITUTIONAL VIGOUR OF CROSSED PLANTS. + +As in almost all my experiments an equal number of crossed and +self-fertilised seeds, or more commonly seedlings just beginning to +sprout, were planted on the opposite sides of the same pots, they had to +compete with one another; and the greater height, weight, and fertility +of the crossed plants may be attributed to their possessing greater +innate constitutional vigour. Generally the plants of the two lots +whilst very young were of equal height; but afterwards the crossed +gained insensibly on their opponents, and this shows that they possessed +some inherent superiority, though not displayed at a very early period +in life. There were, however, some conspicuous exceptions to the rule of +the two lots being at first equal in height; thus the crossed seedlings +of the broom (Sarothamnus scoparius) when under three inches in height +were more than twice as tall as the self-fertilised plants. + +After the crossed or the self-fertilised plants had once grown decidedly +taller than their opponents, a still increasing advantage would tend to +follow from the stronger plants robbing the weaker ones of nourishment +and overshadowing them. This was evidently the case with the crossed +plants of Viola tricolor, which ultimately quite overwhelmed the +self-fertilised. But that the crossed plants have an inherent +superiority, independently of competition, was sometimes well shown when +both lots were planted separately, not far distant from one another, in +good soil in the open ground. This was likewise shown in several cases, +even with plants growing in close competition with one another, by one +of the self-fertilised plants exceeding for a time its crossed opponent, +which had been injured by some accident or was at first sickly, but +being ultimately conquered by it. The plants of the eighth generation of +Ipomoea were raised from small seeds produced by unhealthy parents, and +the self-fertilised plants grew at first very rapidly, so that when the +plants of both lots were about three feet in height, the mean height of +the crossed to that of the self-fertilised was as 100 to 122; when they +were about six feet high the two lots were very nearly equal, but +ultimately when between eight and nine feet in height, the crossed +plants asserted their usually superiority, and were to the +self-fertilised in height as 100 to 85. + +The constitutional superiority of the crossed over the self-fertilised +plants was proved in another way in the third generation of Mimulus, by +self-fertilised seeds being sown on one side of a pot, and after a +certain interval of time crossed seeds on the opposite side. The +self-fertilised seedlings thus had (for I ascertained that the seeds +germinated simultaneously) a clear advantage over the crossed in the +start for the race. Nevertheless they were easily beaten (as may be seen +under the head of Mimulus) when the crossed seeds were sown two whole +days after the self-fertilised. But when the interval was four days, the +two lots were nearly equal throughout life. Even in this latter case the +crossed plants still possessed an inherent advantage, for after both +lots had grown to their full height they were cut down, and without +being disturbed were transferred to a larger pot, and when in the +ensuing year they had again grown to their full height they were +measured; and now the tallest crossed plants were to the tallest +self-fertilised plants in height as 100 to 75, and in fertility (i.e., +by weight of seeds produced by an equal number of capsules from both +lots) as 100 to 34. + +My usual method of proceeding, namely, to plant several pairs of crossed +and self-fertilised seeds in an equal state of germination on the +opposite sides of the same pots, so that the plants were subjected to +moderately severe mutual competition, was I think the best that could +have been followed, and was a fair test of what occurs in a state of +nature. For plants sown by nature generally come up crowded, and are +almost always exposed to very severe competition with one another and +with other kinds of plants. This latter consideration led me to make +some trials, chiefly but not exclusively with Ipomoea and Mimulus, by +sowing crossed and self-fertilised seeds on the opposite sides of large +pots in which other plants had long been growing, or in the midst of +other plants out of doors. The seedlings were thus subjected to very +severe competition with plants of other kinds; and in all such cases, +the crossed seedlings exhibited a great superiority in their power of +growth over the self-fertilised. + +After the germinating seedlings had been planted in pairs on the +opposite sides of several pots, the remaining seeds, whether or not in a +state of germination, were in most cases sown very thickly on the two +sides of an additional large pot; so that the seedlings came up +extremely crowded, and were subjected to extremely severe competition +and unfavourable conditions. In such cases the crossed plants almost +invariably showed a greater superiority over the self-fertilised, than +did the plants which grew in pairs in the pots. + +Sometimes crossed and self-fertilised seeds were sown in separate rows +in the open ground, which was kept clear of weeds; so that the seedlings +were not subjected to any competition with other kinds of plants. Those +however in each row had to struggle with the adjoining ones in the same +row. When fully grown, several of the tallest plants in each row were +selected, measured, and compared. The result was in several cases (but +not so invariably as might have been expected) that the crossed plants +did not exceed in height the self-fertilised in nearly so great a degree +as when grown in pairs in the pots. Thus with the plants of Digitalis, +which competed together in pots, the crossed were to the self-fertilised +in height as 100 to 70; whilst those which were grown separately were +only as 100 to 85. Nearly the same result was observed with Brassica. +With Nicotiana the crossed were to the self-fertilised plants in height, +when grown extremely crowded together in pots, as 100 to 54; when grown +much less crowded in pots as 100 to 66, and when grow in the open +ground, so as to be subjected to but little competition, as 100 to 72. +On the other hand with Zea, there was a greater difference in height +between the crossed and self-fertilised plants growing out of doors, +than between the pairs which grew in pots in the hothouse; but this may +be attributed to the self-fertilised plants being more tender, so that +they suffered more than the crossed, when both lots were exposed to a +cold and wet summer. Lastly, with one out of two series of Reseda +odorata, grown out of doors in rows, as well as with Beta vulgaris, the +crossed plants did not at all exceed the self-fertilised in height, or +exceeded them by a mere trifle. + +The innate power of the crossed plants to resist unfavourable conditions +far better than did the self-fertilised plants, was shown on two +occasions in a curious manner, namely, with Iberis and in the third +generation of Petunia, by the great superiority in height of the crossed +over the self-fertilised seedlings, when both sets were grown under +extremely unfavourable conditions; whereas owing to special +circumstances exactly the reverse occurred with the plants raised from +the same seeds and grown in pairs in pots. A nearly analogous case was +observed on two other occasions with plants of the first generation of +Nicotiana. + +The crossed plants always withstood the injurious effects of being +suddenly removed into the open air after having been kept in the +greenhouse better than did the self-fertilised. On several occasions +they also resisted much better cold and intemperate weather. This was +manifestly the case with some crossed and self-fertilised plants of +Ipomoea, which were suddenly moved from the hothouse to the coldest part +of a cool greenhouse. The offspring of plants of the eighth +self-fertilised generation of Mimulus crossed by a fresh stock, survived +a frost which killed every single self-fertilised and intercrossed plant +of the same old stock. Nearly the same result followed with some crossed +and self-fertilised plants of Viola tricolor. Even the tips of the +shoots of the crossed plants of Sarothamnus scoparius were not touched +by a very severe winter; whereas all the self-fertilised plants were +killed halfway down to the ground, so that they were not able to flower +during the next summer. Young crossed seedlings of Nicotiana withstood a +cold and wet summer much better than the self-fertilised seedlings. I +have met with only one exception to the rule of crossed plants being +hardier than the self-fertilised: three long rows of Eschscholtzia +plants, consisting of crossed seedlings from a fresh stock, of +intercrossed seedlings of the same stock, and of self-fertilised ones, +were left unprotected during a severe winter, and all perished except +two of the self-fertilised. But this case is not so anomalous as it at +first appears, for it should be remembered that the self-fertilised +plants of Eschscholtzia always grow taller and are heavier than the +crossed; the whole benefit of a cross with this species being confined +to increased fertility. + +Independently of any external cause which could be detected, the +self-fertilised plants were more liable to premature death than were the +crossed; and this seems to me a curious fact. Whilst the seedlings were +very young, if one died its antagonist was pulled up and thrown away, +and I believe that many more of the self-fertilised died at this early +age than of the crossed; but I neglected to keep any record. With Beta +vulgaris, however, it is certain that a large number of the +self-fertilised seeds perished after germinating beneath the ground, +whereas the crossed seeds sown at the same time did not thus suffer. +When a plant died at a somewhat more advanced age the fact was recorded; +and I find in my notes that out of several hundred plants, only seven of +the crossed died, whilst of the self-fertilised at least twenty-nine +were thus lost, that is more than four times as many. Mr. Galton, after +examining some of my tables, remarks: "It is very evident that the +columns with the self-fertilised plants include the larger number of +exceptionally small plants;" and the frequent presence of such puny +plants no doubt stands in close relation with their liability to +premature death. The self-fertilised plants of Petunia completed their +growth and began to wither sooner than did the intercrossed plants; and +these latter considerably before the offspring from a cross with a fresh +stock. + +PERIOD OF FLOWERING. + +In some cases, as with Digitalis, Dianthus, and Reseda, a larger number +of the crossed than of the self-fertilised plants threw up flower-stems; +but this probably was merely the result of their greater power of +growth; for in the first generation of Lobelia fulgens, in which the +self-fertilised plants greatly exceeded in height the crossed plants, +some of the latter failed to throw up flower-stems. With a large number +of species, the crossed plants exhibited a well-marked tendency to +flower before the self-fertilised ones growing in the same pots. It +should however be remarked that no record was kept of the flowering of +many of the species; and when a record was kept, the flowering of the +first plant in each pot was alone observed, although two or more pairs +grew in the same pot. I will now give three lists,--one of the species +in which the first plant that flowered was a crossed one,--a second in +which the first that flowered was a self-fertilised plant,--and a third +of those which flowered at the same time. + +[SPECIES, OF WHICH THE FIRST PLANTS THAT FLOWERED WERE OF CROSSED +PARENTAGE. + +Ipomoea purpurea. + +I record in my notes that in all ten generations many of the crossed +plants flowered before the self-fertilised; but no details were kept. + +Mimulus luteus (First Generation). + +Ten flowers on the crossed plants were fully expanded before one on the +self-fertilised. + +Mimulus luteus (Second and Third Generation). + +In both these generations a crossed plant flowered before one of the +self-fertilised in all three pots. + +Mimulus luteus (Fifth Generation). + +In all three pots a crossed plant flowered first; yet the +self-fertilised plants, which belonged to the new tall variety, were in +height to the crossed as 126 to 100. + +Mimulus luteus. + +Plants derived from a cross with a fresh stock as well as the +intercrossed plants of the old stock, flowered before the +self-fertilised plants in nine out of the ten pots. + +Salvia coccinea. + +A crossed plant flowered before any one of the self-fertilised in all +three pots. + +Origanum vulgare. + +During two successive seasons several crossed plants flowered before the +self-fertilised. + +Brassica oleracea (First Generation). + +All the crossed plants growing in pots and in the open ground flowered +first. + +Brassica oleracea (Second Generation). + +A crossed plant in three out of the four pots flowered before any one of +the self-fertilised. + +Iberis umbellata. + +In both pots a crossed plant flowered first. + +Eschscholtzia californica. + +Plants derived from the Brazilian stock crossed by the English stock +flowered in five out of the nine pots first; in four of them a +self-fertilised plant flowered first; and not in one pot did an +intercrossed plant of the old stock flower first. + +Viola tricolor. + +A crossed plant in five out of the six pots flowered before any one of +the self-fertilised. + +Dianthus caryophyllus (First Generation). + +In two large beds of plants, four of the crossed plants flowered before +any one of the self-fertilised. + +Dianthus caryophyllus (Second Generation). + +In both pots a crossed plant flowered first. + +Dianthus caryophyllus (Third Generation). + +In three out of the four pots a crossed plant flowered first; yet the +crossed were to the self-fertilised in height only as 100 to 99, but in +weight as 100 to 49. + +Dianthus caryophyllus. + +Plants derived from a cross with a fresh stock, and the intercrossed +plants of the old stock, both flowered before the self-fertilised in +nine out of the ten pots. + +Hibiscus africanus. + +In three out of the four pots a crossed plant flowered before any one of +the self-fertilised; yet the latter were to the crossed in height as 109 +to 100. + +Tropaeolum minus. + +A crossed plant flowered before any one of the self-fertilised in three +out of the four pots, and simultaneously in the fourth pot. + +Limnanthes douglasii. + +A crossed plant flowered before any one of the self-fertilised in four +out of the five pots. + +Phaseolus multiflorus. + +In both pots a crossed plant flowered first. + +Specularia speculum. + +In all four pots a crossed plant flowered first. + +Lobelia ramosa (First Generation). + +In all four pots a crossed plant flowered before any one of the +self-fertilised. + +Lobelia ramosa (Second Generation). + +In all four pots a crossed plant flowered some days before any one of +the self-fertilised. + +Nemophila insignis. + +In four out of the five pots a crossed plant flowered first. + +Borago officinalis. + +In both pots a crossed plant flowered first. + +Petunia violacea (Second Generation). + +In all three pots a crossed plant flowered first. + +Nicotiana tabacum. + +A plant derived from a cross with a fresh stock flowered before any one +of the self-fertilised plants of the fourth generation, in fifteen out +of the sixteen pots. + +Cyclamen persicum. + +During two successive seasons a crossed plant flowered some weeks before +any one of the self-fertilised in all four pots. + +Primula veris (equal-styled var.) + +In all three pots a crossed plant flowered first. + +Primula sinensis. + +In all four pots plants derived from an illegitimate cross between +distinct plants flowered before any one of the self-fertilised plants. + +Primula sinensis. + +A legitimately crossed plant flowered before any one of the +self-fertilised plants in seven out of the eight pots. + +Fagopyrum esculentum. + +A legitimately crossed plant flowered from one to two days before any +one of the self-fertilised plants in all three pots. + +Zea mays. + +In all four pots a crossed plant flowered first. + +Phalaris canariensis. + +The crossed plants flowered before the self-fertilised in the open +ground, but simultaneously in the pots. + +SPECIES OF WHICH THE FIRST PLANTS THAT FLOWERED WERE OF SELF-FERTILISED +PARENTAGE. + +Eschscholtzia californica (First Generation). + +The crossed plants were at first taller than the self-fertilised, but on +their second growth during the following year the self-fertilised +exceeded the crossed in height, and now they flowered first in three out +of the four pots. + +Lupinus luteus. + +Although the crossed plants were to the self-fertilised in height as 100 +to 82; yet in all three pots the self-fertilised plants flowered first. + +Clarkia elegans. + +Although the crossed plants were, as in the last case, to the +self-fertilised in height as 100 to 82, yet in the two pots the +self-fertilised flowered first. + +Lobelia fulgens (First Generation). + +The crossed plants were to the self-fertilised in height only as 100 to +127, and the latter flowered much before the crossed. + +Petunia violacea (Third Generation). + +The crossed plants were to the self-fertilised in height as 100 to 131, +and in three out of the four pots a self-fertilised plant flowered +first; in the fourth pot simultaneously. + +Petunia violacea (Fourth generation). + +Although the crossed plants were to the self-fertilised in height as 100 +to 69, yet in three out of the five pots a self-fertilised plant +flowered first; in the fourth pot simultaneously, and only in the fifth +did a crossed plant flower first. + +Nicotiana tabacum (First Generation). + +The crossed plants were to the self-fertilised in height only as 100 to +178, and a self-fertilised plant flowered first in all four pots. + +Nicotiana tabacum (Third Generation). + +The crossed plants were to the self-fertilised in height as 100 to 101, +and in four out of the five pots a self-fertilised plant flowered first. + +Canna warscewiczi. + +In the three generations taken together the crossed were to the +self-fertilised in height as 100 to 101; in the first generation the +self-fertilised plants showed some tendency to flower first, and in the +third generation they flowered first in nine out of the twelve pots. + +SPECIES IN WHICH THE CROSSED AND SELF-FERTILISED PLANTS FLOWERED ALMOST +SIMULTANEOUSLY. + +Mimulus luteus (Sixth Generation). + +The crossed plants were inferior in height and vigour to the +self-fertilised plants, which all belonged to the new white-flowered +tall variety, yet in only half the pots did the self-fertilised plants +flower first, and in the other half the crossed plants. + +Viscaria oculata. + +The crossed plants were only a little taller than the self-fertilised +(namely, as 100 to 97), but considerably more fertile, yet both lots +flowered almost simultaneously. + +Lathyrus odoratus (Second Generation). + +Although the crossed plants were to the self-fertilised in height as 100 +to 88, yet there was no marked difference in their period of flowering. + +Lobelia fulgens (Second Generation). + +Although the crossed plants were to the self-fertilised in height as 100 +to 91, yet they flowered simultaneously. + +Nicotiana tabacum (Third Generation). + +Although the crossed plants were to the self-fertilised in height as 100 +to 83, yet in half the pots a self-fertilised plant flowered first, and +in the other half a crossed plant.] + +These three lists include fifty-eight cases, in which the period of +flowering of the crossed and self-fertilised plants was recorded. In +forty-four of them a crossed plant flowered first either in a majority +of the pots or in all; in nine instances a self-fertilised plant +flowered first, and in five the two lots flowered simultaneously. One of +the most striking cases is that of Cyclamen, in which the crossed plants +flowered some weeks before the self-fertilised in all four pots during +two seasons. In the second generation of Lobelia ramosa, a crossed plant +flowered in all four pots some days before any one of the +self-fertilised. Plants derived from a cross with a fresh stock +generally showed a very strongly marked tendency to flower before the +self-fertilised and the intercrossed plants of the old stock; all three +lots growing in the same pots. Thus with Mimulus and Dianthus, in only +one pot out of ten, and in Nicotiana in only one pot out of sixteen, did +a self-fertilised plant flower before the plants of the two crossed +kinds,--these latter flowering almost simultaneously. + +A consideration of the two first lists, especially of the second one, +shows that a tendency to flower first is generally connected with +greater power of growth, that is, with greater height. But there are +some remarkable exceptions to this rule, proving that some other cause +comes into play. Thus the crossed plants both of Lupinus luteus and +Clarkia elegans were to the self-fertilised plants in height as 100 to +82, and yet the latter flowered first. In the third generation of +Nicotiana, and in all three generations of Canna, the crossed and +self-fertilised plants were of nearly equal height, yet the +self-fertilised tended to flower first. On the other hand, with Primula +sinensis, plants raised from a cross between two distinct individuals, +whether these were legitimately or illegitimately crossed, flowered +before the illegitimately self-fertilised plants, although all the +plants were of nearly equal height in both cases. So it was with respect +to height and flowering with Phaseolus, Specularia, and Borago. The +crossed plants of Hibiscus were inferior in height to the +self-fertilised, in the ratio of 100 to 109, and yet they flowered +before the self-fertilised in three out of the four pots. On the whole, +there can be no doubt that the crossed plants exhibit a tendency to +flower before the self-fertilised, almost though not quite so strongly +marked as to grow to a greater height, to weigh more, and to be more +fertile. + +A few other cases not included in the above three lists deserve notice. +In all three pots of Viola tricolor, naturally crossed plants the +offspring of crossed plants flowered before naturally crossed plants the +offspring of self-fertilised plants. Flowers on two plants, both of +self-fertilised parentage, of the sixth generation of Mimulus luteus +were intercrossed, and other flowers on the same plants were fertilised +with their own pollen; intercrossed seedlings and seedlings of the +seventh self-fertilised generation were thus raised, and the latter +flowered before the intercrossed in three out of the five pots. Flowers +on a plant both of Mimulus luteus and of Ipomoea purpurea were crossed +with pollen from other flowers on the same plant, and other flowers were +fertilised with their own pollen; intercrossed seedlings of this +peculiar kind, and others strictly self-fertilised being thus raised. In +the case of the Mimulus the self-fertilised plants flowered first in +seven out of the eight pots, and in the case of the Ipomoea in eight out +of the ten pots; so that an intercross between the flowers on the same +plant was very far from giving to the offspring thus raised, any +advantage over the strictly self-fertilised plants in their period of +flowering. + +EFFECTS OF CROSSING FLOWERS ON THE SAME PLANT. + +In the discussion on the results of a cross with a fresh stock, given +under Table 7/C in the last chapter, it was shown that the mere act of +crossing by itself does no good; but that the advantages thus derived +depend on the plants which are crossed, either consisting of distinct +varieties which will almost certainly differ somewhat in constitution, +or on the progenitors of the plants which are crossed, though identical +in every external character, having been subjected to somewhat different +conditions and having thus acquired some slight difference in +constitution. All the flowers produced by the same plant have been +developed from the same seed; those which expand at the same time have +been exposed to exactly the same climatic influences; and the stems have +all been nourished by the same roots. Therefore in accordance with the +conclusion just referred to, no good ought to result from crossing +flowers on the same plant. (8/1. It is, however, possible that the +stamens which differ in length or construction in the same flower may +produce pollen differing in nature, and in this manner a cross might be +made effective between the several flowers on the same plant. Mr. Macnab +states in a communication to M. Verlot 'La Production des Varietes' 1865 +page 42, that seedlings raised from the shorter and longer stamens of +rhododendron differ in character; but the shorter stamens apparently are +becoming rudimentary, and the seedlings are dwarfs, so that the result +may be simply due to a want of fertilising power in the pollen, as in +the case of the dwarfed plants of Mirabilis raised by Naudin by the use +of too few pollen-grains. Analogous statements have been made with +respect to the stamens of Pelargonium. With some of the Melastomaceae, +seedlings raised by me from flowers fertilised by pollen from the +shorter stamens, certainly differed in appearance from those raised from +the longer stamens, with differently coloured anthers; but here, again, +there is some reason for believing that the shorter stamens are tending +towards abortion. In the very different case of trimorphic heterostyled +plants, the two sets of stamens in the same flower have widely different +fertilising powers.) In opposition to this conclusion is the fact that a +bud is in one sense a distinct individual, and is capable of +occasionally or even not rarely assuming new external characters, as +well as new constitutional peculiarities. Plants raised from buds which +have thus varied may be propagated for a great length of time by grafts, +cuttings, etc., and sometimes even by seminal generation. (8/2. I have +given numerous cases of such bud-variations in my 'Variation of Animals +and Plants under Domestication' chapter 11 2nd edition volume 1 page +448.) There exist also numerous species in which the flowers on the same +plant differ from one another,--as in the sexual organs of monoecious +and polygamous plants,--in the structure of the circumferential flowers +in many Compositae, Umbelliferae, etc.,--in the structure of the central +flower in some plants,--in the two kinds of flowers produced by +cleistogene species,--and in several other such cases. These instances +clearly prove that the flowers on the same plant have often varied +independently of one another in many important respects, such variations +having been fixed, like those on distinct plants during the development +of species. + +It was therefore necessary to ascertain by experiment what would be the +effect of intercrossing flowers on the same plant, in comparison with +fertilising them with their own pollen or crossing them with pollen from +a distinct plant. Trials were carefully made on five genera belonging to +four families; and in only one case, namely, Digitalis, did the +offspring from a cross between the flowers on the same plant receive any +benefit, and the benefit here was small compared with that derived from +a cross between distinct plants. In the chapter on Fertility, when we +consider the effects of cross-fertilisation and self-fertilisation on +the productiveness of the parent-plants we shall arrive at nearly the +same result, namely, that a cross between the flowers on the same plant +does not at all increase the number of the seeds, or only occasionally +and to a slight degree. I will now give an abstract of the results of +the five trials which were made. + +1. Digitalis purpurea. + +Seedlings raised from intercrossed flowers on the same plant, and others +from flowers fertilised with their own pollen, were grown in the usual +manner in competition with one another on the opposite sides of ten +pots. In this and the four following cases, the details may be found +under the head of each species. In eight pots, in which the plants did +not grow much crowded, the flower-stems on sixteen intercrossed plants +were in height to those on sixteen self-fertilised plants, as 100 to 94. +In the two other pots on which the plants grew much crowded, the +flower-stems on nine intercrossed plants were in height to those on nine +self-fertilised plants, as 100 to 90. That the intercrossed plants in +these two latter pots had a real advantage over their self-fertilised +opponents, was well shown by their relative weights when cut down, which +was as 100 to 78. The mean height of the flower-stems on the twenty-five +intercrossed plants in the ten pots taken together, was to that of the +flower-stems on the twenty-five self-fertilised plants, as 100 to 92. +Thus the intercrossed plants were certainly superior to the +self-fertilised in some degree; but their superiority was small compared +with that of the offspring from a cross between distinct plants over the +self-fertilised, this being in the ratio of 100 to 70 in height. Nor +does this latter ratio show at all fairly the great superiority of the +plants derived from a cross between distinct individuals over the +self-fertilised, as the former produced more than twice as many +flower-stems as the latter, and were much less liable to premature +death. + +2. Ipomoea purpurea. + +Thirty-one intercrossed plants raised from a cross between flowers on +the same plants were grown in ten pots in competition with the same +number of self-fertilised plants, and the former were to the latter in +height as 100 to 105. So that the self-fertilised plants were a little +taller than the intercrossed; and in eight out of the ten pots a +self-fertilised plant flowered before any one of the crossed plants in +the same pots. The plants which were not greatly crowded in nine of the +pots (and these offer the fairest standard of comparison) were cut down +and weighed; and the weight of the twenty-seven intercrossed plants was +to that of the twenty-seven self-fertilised as 100 to 124; so that by +this test the superiority of the self-fertilised was strongly marked. To +this subject of the superiority of the self-fertilised plants in certain +cases, I shall have to recur in a future chapter. If we now turn to the +offspring from a cross between distinct plants when put into competition +with self-fertilised plants, we find that the mean height of +seventy-three such crossed plants, in the course of ten generations, was +to that of the same number of self-fertilised plants as 100 to 77; and +in the case of the plants of the tenth generation in weight as 100 to +44. Thus the contrast between the effects of crossing flowers on the +same plant, and of crossing flowers on distinct plants, is wonderfully +great. + +3. Mimulus luteus. + +Twenty-two plants raised by crossing flowers on the same plant were +grown in competition with the same number of self-fertilised plants; and +the former were to the latter in height as 100 to 105, and in weight as +100 to 103. Moreover, in seven out of the eight pots a self-fertilised +plant flowered before any of the intercrossed plants. So that here again +the self-fertilised exhibit a slight superiority over the intercrossed +plants. For the sake of comparison, I may add that seedlings raised +during three generations from a cross between distinct plants were to +the self-fertilised plants in height as 100 to 65. + +4. Pelargonium zonale. + +Two plants growing in separate pots, which had been propagated by +cuttings from the same plant, and therefore formed in fact parts of the +same individual, were intercrossed, and other flowers on one of these +plants were self-fertilised; but the seedlings obtained by the two +processes did not differ in height. When, on the other hand, flowers on +one of the above plants were crossed with pollen taken from a distinct +seedling, and other flowers were self-fertilised, the crossed offspring +thus obtained were to the self-fertilised in height as 100 to 74. + +5. Origanum vulgare. + +A plant which had been long cultivated in my kitchen garden, had spread +by stolons so as to form a large bed or clump. Seedlings raised by +intercrossing flowers on these plants, which strictly consisted of the +same plant, and other seedlings raised from self-fertilised flowers, +were carefully compared from their earliest youth to maturity; and they +did not differ at all in height or in constitutional vigour. Some +flowers on these seedlings were then crossed with pollen taken from a +distinct seedling, and other flowers were self-fertilised; two fresh +lots of seedlings being thus raised, which were the grandchildren of the +plant that had spread by stolons and formed a large clump in my garden. +These differed much in height, the crossed plants being to the +self-fertilised as 100 to 86. They differed, also, to a wonderful degree +in constitutional vigour. The crossed plants flowered first, and +produced exactly twice as many flower-stems; and they afterwards +increased by stolons to such an extent as almost to overwhelm the +self-fertilised plants. + +Reviewing these five cases, we see that in four of them, the effect of a +cross between flowers on the same plant (even on offsets of the same +plant growing on separate roots, as with the Pelargonium and Origanum) +does not differ from that of the strictest self-fertilisation. Indeed, +in two of the cases the self-fertilised plants were superior to such +intercrossed plants. With Digitalis a cross between the flowers on the +same plant certainly did do some good, yet very slight compared with +that from a cross between distinct plants. On the whole the results here +arrived at, if we bear in mind that the flower-buds are to a certain +extent distinct individuals and occasionally vary independently of one +another, agree well with our general conclusion, that the advantages of +a cross depend on the progenitors of the crossed plants possessing +somewhat different constitutions, either from having been exposed to +different conditions, or to their having varied from unknown causes in a +manner which we in our ignorance are forced to speak of as spontaneous. +Hereafter I shall have to recur to this subject of the inefficiency of a +cross between the flowers on the same plant, when we consider the part +which insects play in the cross-fertilisation of flowers. + +ON THE TRANSMISSION OF THE GOOD EFFECTS FROM A CROSS AND OF THE EVIL +EFFECTS FROM SELF-FERTILISATION. + +We have seen that seedlings from a cross between distinct plants almost +always exceed their self-fertilised opponents in height, weight, and +constitutional vigour, and, as will hereafter be shown, often in +fertility. To ascertain whether this superiority would be transmitted +beyond the first generation, seedlings were raised on three occasions +from crossed and self-fertilised plants, both sets being fertilised in +the same manner, and therefore not as in the many cases given in Tables +7/A, 7/B, 7/C, in which the crossed plants were again crossed and the +self-fertilised again self-fertilised. + +Firstly, seedlings were raised from self-fertilised seeds produced under +a net by crossed and self-fertilised plants of Nemophila insignis; and +the latter were to the former in height as 133 to 100. But these +seedlings became very unhealthy early in life, and grew so unequally +that some of them in both lots were five times as tall as the others. +Therefore this experiment was quite worthless; but I have felt bound to +give it, as opposed to my general conclusion. I should state that in +this and the two following trials, both sets of plants were grown on the +opposite sides of the same pots, and treated in all respects alike. The +details of the experiments may be found under the head of each species. + +Secondly, a crossed and a self-fertilised plant of Heartsease (Viola +tricolor) grew near together in the open ground and near to other plants +of heartsease; and as both produced an abundance of very fine capsules, +the flowers on both were certainly cross-fertilised by insects. Seeds +were collected from both plants, and seedlings raised from them. Those +from the crossed plants flowered in all three pots before those from the +self-fertilised plants; and when fully grown the former were to the +latter in height as 100 to 82. As both sets of plants were the product +of cross-fertilisation, the difference in their growth and period of +flowering was clearly due to their parents having been of crossed and +self-fertilised parentage; and it is equally clear that they transmitted +different constitutional powers to their offspring, the grandchildren of +the plants which were originally crossed and self-fertilised. + +Thirdly, the Sweet Pea (Lathyrus odoratus) habitually fertilises itself +in this country. As I possessed plants, the parents and grandparents of +which had been artificially crossed and other plants descended from the +same parents which had been self-fertilised for many previous +generations, these two lots of plants were allowed to fertilise +themselves under a net, and their self-fertilised seeds saved. The +seedlings thus raised were grown in competition with each other in the +usual manner, and differed in their powers of growth. Those from the +self-fertilised plants which had been crossed during the two previous +generations were to those from the plants self-fertilised during many +previous generations in height as 100 to 90. These two lots of seeds +were likewise tried by being sown under very unfavourable conditions in +poor exhausted soil, and the plants whose grandparents and +great-grandparents had been crossed showed in an unmistakable manner +their superior constitutional vigour. In this case, as in that of the +heartsease, there could be no doubt that the advantage derived from a +cross between two plants was not confined to the offspring of the first +generation. That constitutional vigour due to cross-parentage is +transmitted for many generations may also be inferred as highly +probable, from some of Andrew Knight's varieties of the common pea, +which were raised by crossing distinct varieties, after which time they +no doubt fertilised themselves in each succeeding generation. These +varieties lasted for upwards of sixty years, "but their glory is now +departed." (8/3. See the evidence on this head in my 'Variation under +Domestication' chapter 9 volume 1 2nd edition page 397.) On the other +hand, most of the varieties of the common pea, which there is no reason +to suppose owe their origin to a cross, have had a much shorter +existence. Some also of Mr. Laxton's varieties produced by artificial +crosses have retained their astonishing vigour and luxuriance for a +considerable number of generations; but as Mr. Laxton informs me, his +experience does not extend beyond twelve generations, within which +period he has never perceived any diminution of vigour in his plants. + +An allied point may be here noticed. As the force of inheritance is +strong with plants (of which abundant evidence could be given), it is +almost certain that seedlings from the same capsule or from the same +plant would tend to inherit nearly the same constitution; and as the +advantage from a cross depends on the plants which are crossed differing +somewhat in constitution, it may be inferred as probable that under +similar conditions a cross between the nearest relations would not +benefit the offspring so much as one between non-related plants. In +support of this conclusion we have some evidence, as Fritz Muller has +shown by his valuable experiments on hybrid Abutilons, that the union of +brothers and sisters, parents and children, and of other near relations +is highly injurious to the fertility of the offspring. In one case, +moreover, seedlings from such near relations possessed very weak +constitutions. (8/4. 'Jenaische Zeitschrift fur Naturw.' B. 7 pages 22 +and 45 1872 and 1873 pages 441-450.) This same observer also found three +plants of a Bignonia growing near together. (8/5. 'Botanische Zeitung' +1868 page 626.) He fertilised twenty-nine flowers on one of them with +their own pollen, and they did not set a single capsule. Thirty flowers +were then fertilised with pollen from a distinct plant, one of the three +growing together, and they yielded only two capsules. Lastly, five +flowers were fertilised with pollen from a fourth plant growing at a +distance, and all five produced capsules. It seems therefore probable, +as Fritz Muller suggests, that the three plants growing near together +were seedlings from the same parent, and that from being closely related +they had little power of fertilising one another. (8/6. Some remarkable +cases are given in my 'Variation under Domestication' chapter 17 2nd +edition volume 2 page 121, of hybrids of Gladiolus and Cistus, any one +of which could be fertilised by pollen from any other, but not by its +own pollen.) + +Lastly, the fact of the intercrossed plants in Table 7/A not exceeding +in height the self-fertilised plants in a greater and greater degree in +the later generations, is probably the result of their having become +more and more closely inter-related. + +UNIFORM COLOUR OF THE FLOWERS ON PLANTS, SELF-FERTILISED AND GROWN UNDER +SIMILAR CONDITIONS FOR SEVERAL GENERATIONS. + +At the commencement of my experiments, the parent-plants of Mimulus +luteus, Ipomoea purpurea, Dianthus caryophyllus, and Petunia violacea, +raised from purchased seeds, varied greatly in the colour of their +flowers. This occurs with many plants which have been long cultivated as +an ornament for the flower-garden, and which have been propagated by +seeds. The colour of the flowers was a point to which I did not at first +in the least attend, and no selection whatever was practised. +Nevertheless, the flowers produced by the self-fertilised plants of the +above four species became absolutely uniform in tint, or very nearly so, +after they had been grown for some generations under closely similar +conditions. The intercrossed plants, which were more or less closely +inter-related in the later generations, and which had been likewise +cultivated all the time under similar conditions, became more uniform in +the colour of their flowers than were the original parent-plants, but +much less so than the self-fertilised plants. When self-fertilised +plants of one of the later generations were crossed with a fresh stock, +and seedlings thus raised, these presented a wonderful contrast in the +diversified tints of their flowers compared with those of the +self-fertilised seedlings. As such cases of flowers becoming uniformly +coloured without any aid from selection seem to me curious, I will give +a full abstract of my observations. + +Mimulus luteus. + +A tall variety, bearing large, almost white flowers blotched with +crimson, appeared amongst the intercrossed and self-fertilised plants of +the third and fourth generations. This variety increased so rapidly, +that in the sixth generation of self-fertilised plants every single one +consisted of it. So it was with all the many plants which were raised, +up to the last or ninth self-fertilised generation. Although this +variety first appeared amongst the intercrossed plants, yet from their +offspring being intercrossed in each succeeding generation, it never +prevailed amongst them; and the flowers on the several intercrossed +plants of the ninth generation differed considerably in colour. On the +other hand, the uniformity in colour of the flowers on the plants of all +the later self-fertilised generations was quite surprising; on a casual +inspection they might have been said to be quite alike, but the crimson +blotches were not of exactly the same shape, or in exactly the same +position. Both my gardener and myself believe that this variety did not +appear amongst the parent-plants, raised from purchased seeds, but from +its appearance amongst both the crossed and self-fertilised plants of +the third and fourth generations; and from what I have seen of the +variation of this species on other occasions, it is probable that it +would occasionally appear under any circumstances. We learn, however, +from the present case that under the peculiar conditions to which my +plants were subjected, this particular variety, remarkable for its +colouring, largeness of the corolla, and increased height of the whole +plant, prevailed in the sixth and all the succeeding self-fertilised +generations to the complete exclusion of every other variety. + +Ipomoea purpurea. + +My attention was first drawn to the present subject by observing that +the flowers on all the plants of the seventh self-fertilised generation +were of a uniform, remarkably rich, dark purple tint. The many plants +which were raised during the three succeeding generations, up to the +last or tenth, all produced flowers coloured in the same manner. They +were absolutely uniform in tint, like those of a constant species living +in a state of nature; and the self-fertilised plants might have been +distinguished with certainty, as my gardener remarked, without the aid +of labels, from the intercrossed plants of the later generations. These, +however, had more uniformly coloured flowers than those which were first +raised from the purchased seeds. This dark purple variety did not +appear, as far as my gardener and myself could recollect, before the +fifth or sixth self-fertilised generation. However this may have been, +it became, through continued self-fertilisation and the cultivation of +the plants under uniform conditions, perfectly constant, to the +exclusion of every other variety. + +Dianthus caryophyllus. + +The self-fertilised plants of the third generation all bore flowers of +exactly the same pale rose-colour; and in this respect they differed +quite remarkably from the plants growing in a large bed close by and +raised from seeds purchased from the same nursery garden. In this case +it is not improbable that some of the parent-plants which were first +self-fertilised may have borne flowers thus coloured; but as several +plants were self-fertilised in the first generation, it is extremely +improbable that all bore flowers of exactly the same tint as those of +the self-fertilised plants of the third generation. The intercrossed +plants of the third generation likewise produced flowers almost, though +not quite so uniform in tint as those of the self-fertilised plants. + +Petunia violacea. + +In this case I happened to record in my notes that the flowers on the +parent-plant which was first self-fertilised were of a "dingy purple +colour." In the fifth self-fertilised generation, every one of the +twenty-one self-fertilised plants growing in pots, and all the many +plants in a long row out of doors, produced flowers of absolutely the +same tint, namely, of a dull, rather peculiar and ugly flesh colour; +therefore, considerably unlike those on the parent-plant. I believe that +this change of colour supervened quite gradually; but I kept no record, +as the point did not interest me until I was struck with the uniform +tint of the flowers on the self-fertilised plants of the fifth +generation. The flowers on the intercrossed plants of the corresponding +generation were mostly of the same dull flesh colour, but not nearly so +uniform as those on the self-fertilised plants, some few being very +pale, almost white. The self-fertilised plants which grew in a long row +in the open ground were also remarkable for their uniformity in height, +as were the intercrossed plants in a less degree, both lots being +compared with a large number of plants raised at the same time under +similar conditions from the self-fertilised plants of the fourth +generation crossed by a fresh stock. I regret that I did not attend to +the uniformity in height of the self-fertilised seedlings in the later +generations of the other species. + +These few cases seem to me to possess much interest. We learn from them +that new and slight shades of colour may be quickly and firmly fixed, +independently of any selection, if the conditions are kept as nearly +uniform as is possible, and no intercrossing be permitted. With Mimulus, +not only a grotesque style of colouring, but a larger corolla and +increased height of the whole plant were thus fixed; whereas with most +plants which have been long cultivated for the flower-garden, no +character is more variable than that of colour, excepting perhaps that +of height. From the consideration of these cases we may infer that the +variability of cultivated plants in the above respects is due, firstly, +to their being subjected to somewhat diversified conditions, and, +secondly, to their being often intercrossed, as would follow from the +free access of insects. I do not see how this inference can be avoided, +as when the above plants were cultivated for several generations under +closely similar conditions, and were intercrossed in each generation, +the colour of their flowers tended in some degree to change and to +become uniform. When no intercrossing with other plants of the same +stock was allowed,--that is, when the flowers were fertilised with their +own pollen in each generation--their colour in the later generations +became as uniform as that of plants growing in a state of nature, +accompanied at least in one instance by much uniformity in the height of +the plants. But in saying that the diversified tints of the flowers on +cultivated plants treated in the ordinary manner are due to differences +in the soil, climate, etc., to which they are exposed, I do not wish to +imply that such variations are caused by these agencies in any more +direct manner than that in which the most diversified illnesses, as +colds, inflammation of the lungs or pleura, rheumatism, etc., may be +said to be caused by exposure to cold. In both cases the constitution of +the being which is acted on is of preponderant importance. + + + +CHAPTER IX. + +THE EFFECTS OF CROSS-FERTILISATION AND SELF-FERTILISATION ON THE +PRODUCTION OF SEEDS. + +Fertility of plants of crossed and self-fertilised parentage, both lots +being fertilised in the same manner. +Fertility of the parent-plants when first crossed and self-fertilised, +and of their crossed and self-fertilised offspring when again crossed +and self-fertilised. +Comparison of the fertility of flowers fertilised with their own pollen +and with that from other flowers on the same plant. +Self-sterile plants. +Causes of self-sterility. +The appearance of highly self-fertile varieties. +Self-fertilisation apparently in some respects beneficial, independently +of the assured production of seeds. +Relative weights and rates of germination of seeds from crossed and +self-fertilised flowers. + +The present chapter is devoted to the Fertility of plants, as influenced +by cross-fertilisation and self-fertilisation. The subject consists of +two distinct branches; firstly, the relative productiveness or fertility +of flowers crossed with pollen from a distinct plant and with their own +pollen, as shown by the proportional number of capsules which they +produce, together with the number of the contained seeds. Secondly, the +degree of innate fertility or sterility of the seedlings raised from +crossed and self-fertilised seeds; such seedlings being of the same age, +grown under the same conditions, and fertilised in the same manner. +These two branches of the subject correspond with the two which have to +be considered by any one treating of hybrid plants; namely, in the first +place the comparative productiveness of a species when fertilised with +pollen from a distinct species and with its own pollen; and in the +second place, the fertility of its hybrid offspring. These two classes +of cases do not always run parallel; thus some plants, as Gartner has +shown, can be crossed with great ease, but yield excessively sterile +hybrids; while others are crossed with extreme difficulty, but yield +fairly fertile hybrids. + +The natural order to follow in this chapter would have been first to +consider the effects on the fertility of the parent-plants of crossing +them, and of fertilising them with their own pollen; but as we have +discussed in the two last chapters the relative height, weight, and +constitutional vigour of crossed and self-fertilised plants--that is, of +plants raised from crossed and self-fertilised seeds--it will be +convenient here first to consider their relative fertility. The cases +observed by me are given in Table 9/D, in which plants of crossed and +self-fertilised parentage were left to fertilise themselves, being +either crossed by insects or spontaneously self-fertilised. It should be +observed that the results cannot be considered as fully trustworthy, for +the fertility of a plant is a most variable element, depending on its +age, health, nature of the soil, amount of water given, and temperature +to which it is exposed. The number of the capsules produced and the +number of the contained seeds, ought to have been ascertained on a large +number of crossed and self-fertilised plants of the same age and treated +in every respect alike. In these two latter respects my observations may +be trusted, but a sufficient number of capsules were counted only in a +few instances. The fertility, or as it may perhaps better be called the +productiveness, of a plant depends on the number of capsules produced, +and on the number of seeds which these contain. But from various causes, +chiefly from the want of time, I was often compelled to rely on the +number of the capsules alone. Nevertheless, in the more interesting +cases, the seeds were also counted or weighed. The average number of +seeds per capsule is a more valuable criterion of fertility than the +number of capsules produced. This latter circumstance depends partly on +the size of the plant; and we know that crossed plants are generally +taller and heavier than the self-fertilised; but the difference in this +respect is rarely sufficient to account for the difference in the number +of the capsules produced. It need hardly be added that in Table 9/D the +same number of crossed and self-fertilised plants are always compared. +Subject to the foregoing sources of doubt I will now give the table, in +which the parentage of the plants experimented on, and the manner of +determining their fertility are explained. Fuller details may be found +in the previous part of this work, under the head of each species. + +TABLE 9/D.--RELATIVE FERTILITY OF PLANTS OF CROSSED AND SELF-FERTILISED +PARENTAGE, BOTH SETS BEING FERTILISED IN THE SAME MANNER. FERTILITY +JUDGED OF BY VARIOUS STANDARDS. THAT OF THE CROSSED PLANTS TAKEN AS 100. + +Column 1: Name of plant and feature observed. + +Column 2: x, in the expression, as 100 to x. + +Ipomoea purpurea--first generation: seeds per capsule on crossed and +self-fertilised plants, not growing much crowded, spontaneously +self-fertilised under a net, in number: 99. + +Ipomoea purpurea--seeds per capsule on crossed and self-fertilised +plants from the same parents as in the last case, but growing much +crowded, spontaneously self-fertilised under a net, in number: 93. + +Ipomoea purpurea--productiveness of the same plants, as judged by the +number of capsules produced, and average number of seeds per capsule: +45. + +Ipomoea purpurea--third generation: seeds per capsule on crossed and +self-fertilised plants, spontaneously self-fertilised under a net, in +number: 94. + +Ipomoea purpurea--productiveness of the same plants, as judged by the +number of capsules produced, and the average number of seeds per +capsule: 35. + +Ipomoea purpurea--fifth generation: seeds per capsule on crossed and +self-fertilised plants, left uncovered in the hothouse, and +spontaneously fertilised: 89. + +Ipomoea purpurea--ninth generation: number of capsules on crossed plants +to those on self-fertilised plants, spontaneously self-fertilised under +a net: 26. + +Mimulus luteus--an equal number of capsules on plants descended from +self-fertilised plants of the 8th generation crossed by a fresh stock, +and on plants of the 9th self-fertilised generation, both sets having +been left uncovered and spontaneously fertilised, contained seeds, by +weight: 30. + +Mimulus luteus--productiveness of the same plants, as judged by the +number of capsules produced, and the average weight of seeds per +capsule: 3. + +Vandellia nummularifolia--seeds per capsule from cleistogene flowers on +the crossed and self-fertilised plants, in number: 106. + +Salvia coccinea--crossed plants, compared with self-fertilised plants, +produced flowers, in number: 57. + +Iberis umbellata--plants left uncovered in greenhouse; intercrossed +plants of the 3rd generation, compared with self-fertilised plants of +the 3rd generation, yielded seeds, in number: 75. + +Iberis umbellata--plants from a cross between two varieties, compared +with self-fertilised plants of the 3rd generation, yielded seeds, by +weight : 75. + +Papaver vagum--crossed and self-fertilised plants, left uncovered, +produced capsules, in number: 99. + +Eschscholtzia californica--Brazilian stock; plants left uncovered and +cross-fertilised by bees; capsules on intercrossed plants of the 2nd +generation, compared with capsules on self-fertilised plants of 2nd +generation, contained seeds, in number: 78. + +Eschscholtzia californica--productiveness of the same plants, as judged +by the number of capsules produced, and the average number of seeds per +capsule: 89. + +Eschscholtzia californica--plants left uncovered and cross-fertilised by +bees; capsules on plants derived from intercrossed plants of the 2nd +generation of the Brazilian stock crossed by English stock, compared +with capsules on self-fertilised plants of 2nd generation, contained +seeds, in number: 63. + +Eschscholtzia californica--productiveness of the same plants, as judged +by the number of capsules produced, and the average number of seeds per +capsule: 40. + +Reseda odorata--crossed and self-fertilised plants, left uncovered and +cross-fertilised by bees; produced capsules in number (about): 100. + +Viola tricolor--crossed and self-fertilised plants, left uncovered and +cross-fertilised by bees, produced capsules in number: 10. + +Delphinium consolida--crossed and self-fertilised plants, left uncovered +in the greenhouse, produced capsules in number: 56. + +Viscaria oculata--crossed and self-fertilised plants, left uncovered in +the greenhouse, produced capsules in number: 77. + +Dianthus caryophyllus--plants spontaneously self-fertilised under a net; +capsules on intercrossed and self-fertilised plants of the 3rd +generation contained seeds in number: 125. + +Dianthus caryophyllus--plants left uncovered and cross-fertilised by +insects: offspring from plants self-fertilised for three generations and +then crossed by an intercrossed plant of the same stock, compared with +plants of the 4th self-fertilised generation, produced seeds by weight: +73. + +Dianthus caryophyllus--plants left uncovered and cross-fertilised by +insects: offspring from plants self-fertilised for three generations and +then crossed by a fresh stock, compared with plants of the 4th +self-fertilised generation, produced seeds by weight: 33. + +Tropaeolum minus--crossed and self-fertilised plants, left uncovered in +the greenhouse, produced seeds in number: 64. + +Limnanthes douglasii--crossed and self-fertilised plants, left uncovered +in the greenhouse, produced capsules in number (about): 100. + +Lupinus luteus--crossed and self-fertilised plants of the 2nd +generation, left uncovered in the greenhouse, produced seeds in number +(judged from only a few pods): 88. + +Phaseolus multiflorus--crossed and self-fertilised plants, left +uncovered in the greenhouse, produced seeds in number (about): 100. + +Lathyrus odoratus--crossed and self-fertilised plants of the 2nd +generation, left uncovered in the greenhouse, but certainly +self-fertilised, produced pods in number: 91. + +Clarkia elegans--crossed and self-fertilised plants, left uncovered in +the greenhouse, produced capsules in number: 60. + +Nemophila insignis--crossed and self-fertilised plants, covered by a net +and spontaneously self-fertilised in the greenhouse, produced capsules +in number: 29. + +Petunia violacea--left uncovered and cross-fertilised by insects: plants +of the 5th intercrossed and self-fertilised generations produced seeds, +as judged by the weight of an equal number of capsules: 86. + +Petunia violacea--left uncovered as above: offspring of plants +self-fertilised for four generations and then crossed by a fresh stock, +compared with plants of the 5th self-fertilised generation, produced +seeds, as judged by the weight of an equal number of capsules: 46. + +Cyclamen persicum--crossed and self-fertilised plants, left uncovered in +the greenhouse, produced capsules in number: 12. + +Anagallis collina--crossed and self-fertilised plants, left uncovered in +the greenhouse, produced capsules in number: 8. + +Primula veris--left uncovered in open ground and cross-fertilised by +insects: offspring from plants of the 3rd illegitimate generation +crossed by a fresh stock, compared with plants of the 4th illegitimate +and self-fertilised generation, produced capsules in number: 5. + +Same plants in the following year: 3.5. + +Primula veris--(equal-styled variety): left uncovered in open ground and +cross-fertilised by insects: offspring from plants self-fertilised for +two generations and then crossed by another variety, compared with +plants of the 3rd self-fertilised generation, produced capsules in +number: 15. + +Primula veris--(equal-styled variety) same plants; average number of +seeds per capsule: 71. + +Primula veris--(equal-styled variety) productiveness of the same plants, +as judged by the number of capsules produced and the average number of +seeds per capsule: 11. + +This table includes thirty-three cases relating to twenty-three species, +and shows the degree of innate fertility of plants of crossed parentage +in comparison with those of self-fertilised parentage; both lots being +fertilised in the same manner. With several of the species, as with +Eschscholtzia, Reseda, Viola, Dianthus, Petunia, and Primula, both lots +were certainly cross-fertilised by insects, and so it probably was with +several of the others; but in some of the species, as with Nemophila, +and in some of the trials with Ipomoea and Dianthus, the plants were +covered up, and both lots were spontaneously self-fertilised. This also +was necessarily the case with the capsules produced by the cleistogene +flowers of Vandellia. + +The fertility of the crossed plants is represented in Table 9/D by 100, +and that of the self-fertilised by the other figures. There are five +cases in which the fertility of the self-fertilised plants is +approximately equal to that of the crossed; nevertheless, in four of +these cases the crossed plants were plainly taller, and in the fifth +somewhat taller than the self-fertilised. But I should state that in +some of these five cases the fertility of the two lots was not strictly +ascertained, as the capsules were not actually counted, from appearing +equal in number and from all apparently containing a full complement of +seeds. In only two instances in the table, namely, with Vandellia and in +the third generation of Dianthus, the capsules on the self-fertilised +plants contained more seed than those on the crossed plants. With +Dianthus the ratio between the number of seeds contained in the +self-fertilised and crossed capsules was as 125 to 100; both sets of +plants were left to fertilise themselves under a net; and it is almost +certain that the greater fertility of the self-fertilised plants was +here due merely to their having varied and become less strictly +dichogamous, so as to mature their anthers and stigmas more nearly at +the same time than is proper to the species. Excluding the seven cases +now referred to, there remain twenty-six in which the crossed plants +were manifestly much more fertile, sometimes to an extraordinary degree, +than the self-fertilised with which they grew in competition. The most +striking instances are those in which plants derived from a cross with a +fresh stock are compared with plants of one of the later self-fertilised +generations; yet there are some striking cases, as that of Viola, +between the intercrossed plants of the same stock and the +self-fertilised, even in the first generation. The results most to be +trusted are those in which the productiveness of the plants was +ascertained by the number of capsules produced by an equal number of +plants, together with the actual or average number of seeds in each +capsule. Of such cases there are twelve in the table, and the mean of +their mean fertility is as 100 for the crossed plants, to 59 for the +self-fertilised plants. The Primulaceae seem eminently liable to suffer +in fertility from self-fertilisation. + +The following short table, Table 9/E, includes four cases which have +already been partly given in the last table. + +TABLE 9/E.--INNATE FERTILITY OF PLANTS FROM A CROSS WITH A FRESH STOCK, +COMPARED WITH THAT OF INTERCROSSED PLANTS OF THE SAME STOCK, AND WITH +THAT OF SELF-FERTILISED PLANTS, ALL OF THE CORRESPONDING GENERATION. +FERTILITY JUDGED OF BY THE NUMBER OR WEIGHT OF SEEDS PRODUCED BY AN +EQUAL NUMBER OF PLANTS. + +Column 1: Name of plant and feature observed. + +Column 2: Plants from a cross with a fresh stock. + +Column 3: Intercrossed plants of the same stock. + +Column 4: Self-fertilised plants. + +Mimulus luteus--the intercrossed plants are derived from a cross between +two plants of the 8th self-fertilised generation. The self-fertilised +plants belong to the 9th generation: 100 : 4 : 3. + +Eschscholtzia californica--the intercrossed and self-fertilised plants +belong to the 2nd generation: 100 : 45 : 40. + +Dianthus caryophyllus--the intercrossed plants are derived from +self-fertilised of the 3rd generation, crossed by intercrossed plants of +the 3rd generation. The self-fertilised plants belong to the 4th +generation: 100 : 45 : 33. + +Petunia violacea--the intercrossed and self-fertilised plants belong to +the 5th generation: 100 : 54 : 46. + +NB.--In the above cases, excepting in that of Eschscholtzia, the plants +derived from a cross with a fresh stock belong on the mother-side to the +same stock with the intercrossed and self-fertilised plants, and to the +corresponding generation. + +These cases show us how greatly superior in innate fertility the +seedlings from plants self-fertilised or intercrossed for several +generations and then crossed by a fresh stock are, in comparison with +the seedlings from plants of the old stock, either intercrossed or +self-fertilised for the same number of generations. The three lots of +plants in each case were left freely exposed to the visits of insects, +and their flowers without doubt were cross-fertilised by them. + +Table 9/E further shows us that in all four cases the intercrossed +plants of the same stock still have a decided though small advantage in +fertility over the self-fertilised plants. + +With respect to the state of the reproductive organs in the +self-fertilised plants of Tables 9/D and 9/E, only a few observations +were made. In the seventh and eighth generation of Ipomoea, the anthers +in the flowers of the self-fertilised plants were plainly smaller than +those in the flowers of the intercrossed plants. The tendency to +sterility in these same plants was also shown by the first-formed +flowers, after they had been carefully fertilised, often dropping off, +in the same manner as frequently occurs with hybrids. The flowers +likewise tended to be monstrous. In the fourth generation of Petunia, +the pollen produced by the self-fertilised and intercrossed plants was +compared, and they were far more empty and shrivelled grains in the +former. + +RELATIVE FERTILITY OF FLOWERS CROSSED WITH POLLEN FROM A DISTINCT PLANT +AND WITH THEIR OWN POLLEN. THIS HEADING INCLUDES FLOWERS ON THE +PARENT-PLANTS, AND ON THE CROSSED AND SELF-FERTILISED SEEDLINGS OF THE +FIRST OR A SUCCEEDING GENERATION. + +I will first treat of the parent-plants, which were raised from seeds +purchased from nursery-gardens, or taken from plants growing in my +garden, or growing wild, and surrounded in every case by many +individuals of the same species. Plants thus circumstanced will commonly +have been intercrossed by insects; so that the seedlings which were +first experimented on will generally have been the product of a cross. +Consequently any difference in the fertility of their flowers, when +crossed and self-fertilised, will have been caused by the nature of the +pollen employed; that is, whether it was taken from a distinct plant or +from the same flower. The degrees of fertility shown in Table 9/F, were +determined in each case by the average number of seeds per capsule, +ascertained either by counting or weighing. + +Another element ought properly to have been taken into account, namely, +the proportion of flowers which yielded capsules when they were crossed +and self-fertilised; and as crossed flowers generally produce a larger +proportion of capsules, their superiority in fertility, if this element +had been taken into account, would have been much more strongly marked +than appears in Table 9/F. But had I thus acted, there would have been +greater liability to error, as pollen applied to the stigma at the wrong +time fails to produce any effect, independently of its greater or less +potency. A good illustration of the great difference in the results +which sometimes follows, if the number of capsules produced relatively +to the number of flowers fertilised be included in the calculation, was +afforded by Nolana prostrata. Thirty flowers on some plants of this +species were crossed and produced twenty-seven capsules, each containing +five seeds; thirty-two flowers on the same plants were self-fertilised +and produced only six capsules, each containing five seeds. As the +number of seeds per capsule is here the same, the fertility of the +crossed and self-fertilised flowers is given in Table 9/F as equal, or +as 100 to 100. But if the flowers which failed to produce capsules be +included, the crossed flowers yielded on an average 4.50 seeds, whilst +the self-fertilised flowers yielded only 0.94 seeds, so that their +relative fertility would have been as 100 to 21. I should here state +that it has been found convenient to reserve for separate discussion the +cases of flowers which are usually quite sterile with their own pollen. + +TABLE 9/f.--relative fertility of the flowers on the parent-plants used +in my experiments, when fertilised with pollen from a distinct plant and +with their own pollen. Fertility judged of by the average number of +seeds per capsule. Fertility of crossed flowers taken as 100. + +Column 1: Name of plant and feature observed. + +Column 2: x, in the expression 100 to x. + +Ipomoea purpurea--crossed and self-fertilised flowers yielded seeds as +(about): 100. + +Mimulus luteus--crossed and self-fertilised flowers yielded seeds as (by +weight): 79. + +Linaria vulgaris--crossed and self-fertilised flowers yielded seeds as: +14. + +Vandellia nummularifolia--crossed and self-fertilised flowers yielded +seeds as: 67? + +Gesneria pendulina--crossed and self-fertilised flowers yielded seeds as +(by weight): 100. + +Salvia coccinea--crossed and self-fertilised flowers yielded seeds as +(about): 100. + +Brassica oleracea--crossed and self-fertilised flowers yielded seeds as: +25. + +Eschscholtzia californica--(English stock) crossed and self-fertilised +flowers yielded seeds as (by weight): 71. + +Eschscholtzia californica--(Brazilian stock grown in England) crossed +and self-fertilised flowers yielded seeds (by weight) as (about): 15. + +Delphinium consolida--crossed and self-fertilised flowers +(self-fertilised capsules spontaneously produced, but result supported +by other evidence) yielded seeds as: 59. + +Viscaria oculata--crossed and self-fertilised flowers yielded seeds as +(by weight): 38. + +Viscaria oculata--crossed and self-fertilised flowers (crossed capsules +compared on following year with spontaneously self-fertilised capsules) +yielded seeds as : 58. + +Dianthus caryophyllus--crossed and self-fertilised flowers yielded seeds +as: 92. + +Tropaeolum minus--crossed and self-fertilised flowers yielded seeds as: +92. + +Tropaeolum tricolorum--crossed and self-fertilised flowers yielded seeds +as: 115. (9/1. Tropaeolum tricolorum and Cuphea purpurea have been +introduced into this table, although seedlings were not raised from +them; but of the Cuphea only six crossed and six self-fertilised +capsules, and of the Tropaeolum only six crossed and eleven +self-fertilised capsules, were compared. A larger proportion of the +self-fertilised than of the crossed flowers of the Tropaeolum produced +fruit.) + +Limnanthes douglasii--crossed and self-fertilised flowers yielded seeds +as (about): 100. + +Sarothamnus scoparius--crossed and self-fertilised flowers yielded seeds +as: 41. + +Ononis minutissima--crossed and self-fertilised flowers yielded seeds +as: 65. + +Cuphea purpurea--crossed and self-fertilised flowers yielded seeds as: +113. + +Passiflora gracilis--crossed and self-fertilised flowers yielded seeds +as: 85. + +Specularia speculum--crossed and self-fertilised flowers yielded seeds +as: 72. + +Lobelia fulgens--crossed and self-fertilised flowers yielded seeds as +(about): 100. + +Nemophila insignis--crossed and self-fertilised flowers yielded seeds as +(by weight): 69. + +Borago officinalis--crossed and self-fertilised flowers yielded seeds +as: 60. + +Nolana prostrata--crossed and self-fertilised flowers yielded seeds as: +100. + +Petunia violacea--crossed and self-fertilised flowers yielded seeds as +(by weight): 67. + +Nicotiana tabacum--crossed and self-fertilised flowers yielded seeds as +(by weight): 150. + +Cyclamen persicum--crossed and self-fertilised flowers yielded seeds as: +38. + +Anagallis collina--crossed and self-fertilised flowers yielded seeds as: +96. + +Canna warscewiczi--crossed and self-fertilised flowers (on three +generations of crossed and self-fertilised plants taken all together) +yielded seeds as: 85. + +Table 9/G gives the relative fertility of flowers on crossed plants +again cross-fertilised, and of flowers on self-fertilised plants again +self-fertilised, either in the first or in a later generation. Here two +causes combine to diminish the fertility of the self-fertilised flowers; +namely, the lesser efficacy of pollen from the same flower, and the +innate lessened fertility of plants derived from self-fertilised seeds, +which as we have seen in the previous Table 9/D is strongly marked. The +fertility was determined in the same manner as in Table 9/F, that is, by +the average number of seeds per capsule; and the same remarks as before, +with respect to the different proportion of flowers which set capsules +when they are cross-fertilised and self-fertilised, are here likewise +applicable. + +TABLE 9/G.--RELATIVE FERTILITY OF FLOWERS ON CROSSED AND SELF-FERTILISED +PLANTS OF THE FIRST OR SOME SUCCEEDING GENERATION; THE FORMER BEING +AGAIN FERTILISED WITH POLLEN FROM A DISTINCT PLANT, AND THE LATTER AGAIN +WITH THEIR OWN POLLEN. FERTILITY JUDGED OF BY THE AVERAGE NUMBER OF +SEEDS PER CAPSULE. FERTILITY OF CROSSED FLOWERS TAKEN AS 100. + +Column 1: Name of plant and feature observed. + +Column 2: x, in the expression, 100 to x. + +Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and +self-fertilised plants of the first generation yielded seeds as: 93. + +Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and +self-fertilised plants of the 3rd generation yielded seeds as: 94. + +Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and +self-fertilised plants of the 4th generation yielded seeds as: 94. + +Ipomoea purpurea--crossed and self-fertilised flowers on the crossed and +self-fertilised plants of the 5th generation yielded seeds as: 107. + +Mimulus luteus--crossed and self-fertilised flowers on the crossed and +self-fertilised plants of the 3rd generation yielded seeds as (by +weight): 65. + +Mimulus luteus--same plants of the 3rd generation treated in the same +manner on the following year yielded seeds as (by weight): 34. + +Mimulus luteus--crossed and self-fertilised flowers on the crossed and +self-fertilised plants of the 4th generation yielded seeds as (by +weight): 40. + +Viola tricolor--crossed and self-fertilised flowers on the crossed and +self-fertilised plants of the 1st generation yielded seeds as: 69. + +Dianthus caryophyllus--crossed and self-fertilised flowers on the +crossed and self-fertilised plants of the 1st generation yielded seeds +as: 65. + +Dianthus caryophyllus--flowers on self-fertilised plants of the 3rd +generation crossed by intercrossed plants, and other flowers again +self-fertilised yielded seeds as: 97. + +Dianthus caryophyllus--flowers on self-fertilised plants of the 3rd +generation crossed by a fresh stock, and other flowers again +self-fertilised yielded seeds as: 127. + +Lathytus odoratus--crossed and self-fertilised flowers on the crossed +and self-fertilised plants of the 1st generation yielded seeds as: 65. + +Lobelia ramosa--crossed and self-fertilised flowers on the crossed and +self-fertilised plants of the 1st generation yielded seeds as (by +weight): 60. + +Petunia violacea--crossed and self-fertilised flowers on the crossed and +self-fertilised plants of the 1st generation yielded seeds as (by +weight): 68. + +Petunia violacea--crossed and self-fertilised flowers on the crossed and +self-fertilised plants of the 4th generation yielded seeds as (by +weight): 72. + +Petunia violacea--flowers on self-fertilised plants of the 4th +generation crossed by a fresh stock, and other flowers again +self-fertilised yielded seeds as (by weight): 48. + +Nicotiana tabacum--crossed and self-fertilised flowers on the crossed +and self-fertilised plants of the 1st generation yielded seeds as (by +weight): 97. + +Nicotiana tabacum--flowers on self-fertilised plants of the 2nd +generation crossed by intercrossed plants, and other flowers again +self-fertilised yielded seeds as (by estimation): 110. + +Nicotiana tabacum--flowers on self-fertilised plants of the 3rd +generation crossed by a fresh stock, and other flowers again +self-fertilised yielded seeds as (by estimation): 110. + +Anagallis collina--flowers on red variety crossed by a blue variety, and +other flowers on the red variety self-fertilised yielded seeds as: 48. + +Canna warscewiczi--crossed and self-fertilised flowers on the crossed +and self-fertilised plants of three generations taken together yielded +seeds as: 85. + +As both these tables relate to the fertility of flowers fertilised by +pollen from another plant and by their own pollen, they may be +considered together. The difference between them consists in the +self-fertilised flowers in Table 9/G, being produced by self-fertilised +parents, and the crossed flowers by crossed parents, which in the later +generations had become somewhat closely inter-related, and had been +subjected all the time to nearly the same conditions. These two tables +include fifty cases relating to thirty-two species. The flowers on many +other species were crossed and self-fertilised, but as only a few were +thus treated, the results cannot be trusted, as far as fertility is +concerned, and are not here given. Some other cases have been rejected, +as the plants were in an unhealthy condition. If we look to the figures +in the two tables expressing the ratios between the mean relative +fertility of the crossed and self-fertilised flowers, we see that in a +majority of cases (i.e., in thirty-five out of fifty) flowers fertilised +by pollen from a distinct plant yield more, sometimes many more, seeds +than flowers fertilised with their own pollen; and they commonly set a +larger proportion of capsules. The degree of infertility of the +self-fertilised flowers differs extremely in the different species, and +even, as we shall see in the section on self-sterile plants, in the +individuals of the same species, as well as under slightly changed +conditions of life. Their fertility ranges from zero to fertility +equalling that of the crossed flowers; and of this fact no explanation +can be offered. There are fifteen cases in the two tables in which the +number of seeds per capsule produced by the self-fertilised flowers +equals or even exceeds that yielded by the crossed flowers. Some few of +these cases are, I believe, accidental; that is, would not recur on a +second trial. This was apparently the case with the plants of the fifth +generation of Ipomoea, and in one of the experiments with Dianthus. +Nicotiana offers the most anomalous case of any, as the self-fertilised +flowers on the parent-plants, and on their descendants of the second and +third generations, produced more seeds than did the crossed flowers; but +we shall recur to this case when we treat of highly self-fertile +varieties. + +It might have been expected that the difference in fertility between the +crossed and self-fertilised flowers would have been more strongly marked +in Table 9/G, in which the plants of one set were derived from +self-fertilised parents, than in Table 9/F, in which flowers on the +parent-plants were self-fertilised for the first time. But this is not +the case, as far as my scanty materials allow of any judgment. There is +therefore no evidence at present, that the fertility of plants goes on +diminishing in successive self-fertilised generations, although there is +some rather weak evidence that this does occur with respect to their +height or growth. But we should bear in mind that in the later +generations the crossed plants had become more or less closely +inter-related, and had been subjected all the time to nearly uniform +conditions. + +It is remarkable that there is no close correspondence, either in the +parent-plants or in the successive generations, between the relative +number of seeds produced by the crossed and self-fertilised flowers, and +the relative powers of growth of the seedlings raised from such seeds. +Thus, the crossed and self-fertilised flowers on the parent-plants of +Ipomoea, Gesneria, Salvia, Limnanthes, Lobelia fulgens, and Nolana +produced a nearly equal number of seeds, yet the plants raised from the +crossed seeds exceeded considerably in height those raised from the +self-fertilised seeds. The crossed flowers of Linaria and Viscaria +yielded far more seeds than the self-fertilised flowers; and although +the plants raised from the former were taller than those from the +latter, they were not so in any corresponding degree. With Nicotiana the +flowers fertilised with their own pollen were more productive than those +crossed with pollen from a slightly different variety; yet the plants +raised from the latter seeds were much taller, heavier, and more hardy +than those raised from the self-fertilised seeds. On the other hand, the +crossed seedlings of Eschscholtzia were neither taller nor heavier than +the self-fertilised, although the crossed flowers were far more +productive than the self-fertilised. But the best evidence of a want of +correspondence between the number of seeds produced by crossed and +self-fertilised flowers, and the vigour of the offspring raised from +them, is afforded by the plants of the Brazilian and European stocks of +Eschscholtzia, and likewise by certain individual plants of Reseda +odorata; for it might have been expected that the seedlings from plants, +the flowers of which were excessively self-sterile, would have profited +in a greater degree by a cross, than the seedlings from plants which +were moderately or fully self-fertile, and therefore apparently had no +need to be crossed. But no such result followed in either case: for +instance, the crossed and self-fertilised offspring from a highly +self-fertile plant of Reseda odorata were in average height to each +other as 100 to 82; whereas the similar offspring from an excessively +self-sterile plant were as 100 to 92 in average height. + +With respect to the innate fertility of the plants of crossed and +self-fertilised parentage, given in the previous Table 9/D--that is, the +number of seeds produced by both lots when their flowers were fertilised +in the same manner,--nearly the same remarks are applicable, in +reference to the absence of any close correspondence between their +fertility and powers of growth, as in the case of the plants in the +Tables 9/F and 9/G, just considered. Thus the crossed and +self-fertilised plants of Ipomoea, Papaver, Reseda odorata, and +Limnanthes were almost equally fertile, yet the former exceeded +considerably in height the self-fertilised plants. On the other hand, +the crossed and self-fertilised plants of Mimulus and Primula differed +to an extreme degree in innate fertility, but by no means to a +corresponding degree in height or vigour. + +In all the cases of self-fertilised flowers included in Tables 9/E, 9/F, +and 9/G, these were fertilised with their own pollen; but there is +another form of self-fertilisation, namely, by pollen from other flowers +on the same plant; but this latter method made no difference in +comparison with the former in the number of seeds produced, or only a +slight difference. Neither with Digitalis nor Dianthus were more seeds +produced by the one method than by the other, to any trustworthy degree. +With Ipomoea rather more seeds, in the proportion of 100 to 91, were +produced from a crossed between flowers on the same plant than from +strictly self-fertilised flowers; but I have reason to suspect that the +result was accidental. With Origanum vulgare, however, a cross between +flowers on plants propagated by stolons from the same stock certainly +increased slightly their fertility. This likewise occurred, as we shall +see in the next section, with Eschscholtzia, perhaps with Corydalis cava +and Oncidium; but not so with Bignonia, Abutilon, Tabernaemontana, +Senecio, and apparently Reseda odorata. + +SELF-STERILE PLANTS. + +The cases here to be described might have been introduced in Table 9/F, +which gives the relative fertility of flowers fertilised with their own +pollen, and with that from a distinct plant, but it has been found more +convenient to keep them for separate discussion. The present cases must +not be confounded with those to be given in the next chapter relatively +to flowers which are sterile when insects are excluded; for such +sterility depends not merely on the flowers being incapable of +fertilisation with their own pollen, but on mechanical causes, by which +their pollen is prevented from reaching the stigma, or on the pollen and +stigma of the same flower being matured at different periods. + +In the seventeenth chapter of my 'Variation of Animals and Plants under +Domestication' I had occasion to enter fully on the present subject; and +I will therefore here give only a brief abstract of the cases there +described, but others must be added, as they have an important bearing +on the present work. Kolreuter long ago described plants of Verbascum +phoeniceum which during two years were sterile with their own pollen, +but were easily fertilised by that of four other species; these plants +however afterwards became more or less self-fertile in a strangely +fluctuating manner. Mr. Scott also found that this species, as well as +two of its varieties, were self-sterile, as did Gartner in the case of +Verbascum nigrum. So it was, according to this latter author, with two +plants of Lobelia fulgens, though the pollen and ovules of both were in +an efficient state in relation to other species. Five species of +Passiflora and certain individuals of a sixth species have been found +sterile with their own pollen; but slight changes in their conditions, +such as being grafted on another stock or a change of temperature, +rendered them self-fertile. Flowers on a completely self-impotent plant +of Passiflora alata fertilised with pollen from its own self-impotent +seedlings were quite fertile. Mr. Scott, and afterwards Mr. Munro, found +that some species of Oncidium and of Maxillaria cultivated in a hothouse +in Edinburgh were quite sterile with their own pollen; and Fritz Muller +found this to be the case with a large number of Orchidaceous genera +growing in their native home of South Brazil. (9/2. 'Botanische Zeitung' +1868 page 114.) He also discovered that the pollen-masses of some +orchids acted on their own stigmas like a poison; and it appears that +Gartner formerly observed indications of this extraordinary fact in the +case of some other plants. + +Fritz Muller also states that a species of Bignonia and Tabernaemontana +echinata are both sterile with their own pollen in their native country +of Brazil. (9/3. Ibid 1868 page 626 and 1870 page 274.) Several +Amaryllidaceous and Liliaceous plants are in the same predicament. +Hildebrand observed with care Corydalis cava, and found it completely +self-sterile (9/4. 'Report of the International Horticultural Congress' +1866.); but according to Caspary a few self-fertilised seeds are +occasionally produced: Corydalis halleri is only slightly self-sterile, +and C. intermedia not at all so. (9/5. 'Botanische Zeitung' June 27, +1873.) In another Fumariaceous genus, Hypecoum, Hildebrand observed that +H. grandiflorum was highly self-sterile, whilst H. procumbens was fairly +self-fertile. (9/6. 'Jahrb. fur wiss. Botanik' B. 7 page 464.) +Thunbergia alata kept by me in a warm greenhouse was self-sterile early +in the season, but at a later period produced many spontaneously +self-fertilised fruits. So it was with Papaver vagum: another species, +P. alpinum, was found by Professor H. Hoffmann to be quite self-sterile +excepting on one occasion (9/7. 'Zur Speciesfrage' 1875 page 47.); +whilst P. somniferum has been with me always completely self-sterile. + +Eschscholtzia californica. + +This species deserves a fuller consideration. A plant cultivated by +Fritz Muller in South Brazil happened to flower a month before any of +the others, and it did not produce a single capsule. This led him to +make further observations during the next six generations, and he found +that all his plants were completely sterile, unless they were crossed by +insects or were artificially fertilised with pollen from a distinct +plant, in which case they were completely fertile. (9/8. 'Botanische +Zeitung' 1868 page 115 and 1869 page 223.) I was much surprised at this +fact, as I had found that English plants, when covered by a net, set a +considerable number of capsules; and that these contained seeds by +weight, compared with those on plants intercrossed by the bees, as 71 to +100. Professor Hildebrand, however, found this species much more +self-sterile in Germany than it was with me in England, for the capsules +produced by self-fertilised flowers, compared with those from +intercrossed flowers, contained seeds in the ratio of only 11 to 100. At +my request Fritz Muller sent me from Brazil seeds of his self-sterile +plants, from which I raised seedlings. Two of these were covered with a +net, and one produced spontaneously only a single capsule containing no +good seeds, but yet, when artificially fertilised with its own pollen, +produced a few capsules. The other plant produced spontaneously under +the net eight capsules, one of which contained no less than thirty +seeds, and on an average about ten seeds per capsule. Eight flowers on +these two plants were artificially self-fertilised, and produced seven +capsules, containing on an average twelve seeds; eight other flowers +were fertilised with pollen from a distinct plant of the Brazilian +stock, and produced eight capsules, containing on an average about +eighty seeds: this gives a ratio of 15 seeds for the self-fertilised +capsules to 100 for the crossed capsules. Later in the season twelve +other flowers on these two plants were artificially self-fertilised; but +they yielded only two capsules, containing three and six seeds. It +appears therefore that a lower temperature than that of Brazil favours +the self-fertility of this plant, whilst a still lower temperature +lessens it. As soon as the two plants which had been covered by the net +were uncovered, they were visited by many bees,and it was interesting to +observe how quickly they became, even the more sterile plant of the two, +covered with young capsules. On the following year eight flowers on +plants of the Brazilian stock of self-fertilised parentage (i.e., +grandchildren of the plants which grew in Brazil) were again +self-fertilised, and produced five capsules, containing on an average +27.4 seeds, with a maximum in one of forty-two seeds; so that their +self-fertility had evidently increased greatly by being reared for two +generations in England. On the whole we may conclude that plants of the +Brazilian stock are much more self-fertile in this country than in +Brazil, and less so than plants of the English stock in England; so that +the plants of Brazilian parentage retained by inheritance some of their +former sexual constitution. Conversely, seeds from English plants sent +by me to Fritz Muller and grown in Brazil, were much more self-fertile +than his plants which had been cultivated there for several generations; +but he informs me that one of the plants of English parentage which did +not flower the first year, and was thus exposed for two seasons to the +climate of Brazil, proved quite self-sterile, like a Brazilian plant, +showing how quickly the climate had acted on its sexual constitution. + +Abutilon darwinii. + +Seeds of this plant were sent me by Fritz Muller, who found it, as well +as some other species of the same genus, quite sterile in its native +home of South Brazil, unless fertilised with pollen from a distinct +plant, either artificially or naturally by humming-birds. (9/9. +'Jenaische Zeitschr. fur Naturwiss' B. 7 1872 page 22 and 1873 page +441.) Several plants were raised from these seeds and kept in the +hothouse. They produced flowers very early in the spring, and twenty of +them were fertilised, some with pollen from the same flower, and some +with pollen from other flowers on the same plants; but not a single +capsule was thus produced, yet the stigmas twenty-seven hours after the +application of the pollen were penetrated by the pollen-tubes. At the +same time nineteen flowers were crossed with pollen from a distinct +plant, and these produced thirteen capsules, all abounding with fine +seeds. A greater number of capsules would have been produced by the +cross, had not some of the nineteen flowers been on a plant which was +afterwards proved to be from some unknown cause completely sterile with +pollen of any kind. Thus far these plants behaved exactly like those in +Brazil; but later in the season, in the latter part of May and in June, +they began to produce under a net a few spontaneously self-fertilised +capsules. As soon as this occurred, sixteen flowers were fertilised with +their own pollen, and these produced five capsules, containing on an +average 3.4 seeds. At the same time I selected by chance four capsules +from the uncovered plants growing close by, the flowers of which I had +seen visited by humble-bees, and these contained on an average 21.5 +seeds; so that the seeds in the naturally intercrossed capsules to those +in the self-fertilised capsules were as 100 to 16. The interesting point +in this case is that these plants, which were unnaturally treated by +being grown in pots in a hothouse, under another hemisphere, with a +complete reversal of the seasons, were thus rendered slightly +self-fertile, whereas they seem always to be completely self-sterile in +their native home. + +Senecio cruentus (greenhouse varieties, commonly called Cinerarias, +probably derived from several fruticose or herbaceous species much +intercrossed (9/10. I am much obliged to Mr. Moore and to Mr. Thiselton +Dyer for giving me information with respect to the varieties on which I +experimented. Mr. Moore believes that Senecio cruentas, tussilaginis, +and perhaps heritieri, maderensis and populifolius have all been more or +less blended together in our Cinerarias.)) + +Two purple-flowered varieties were placed under a net in the greenhouse, +and four corymbs on each were repeatedly brushed with flowers from the +other plant, so that their stigmas were well covered with each other's +pollen. Two of the eight corymbs thus treated produced very few seeds, +but the other six produced on an average 41.3 seeds per corymb, and +these germinated well. The stigmas on four other corymbs on both plants +were well smeared with pollen from the flowers on their own corymbs; +these eight corymbs produced altogether ten extremely poor seeds, which +proved incapable of germinating. I examined many flowers on both plants, +and found the stigmas spontaneously covered with pollen; but they +produced not a single seed. These plants were afterwards left uncovered +in the same house where many other Cinerarias were in flower; and the +flowers were frequently visited by bees. They then produced plenty of +seed, but one of the two plants less than the other, as this species +shows some tendency to be dioecious. + +The trial was repeated on another variety with white petals tipped with +red. Many stigmas on two corymbs were covered with pollen from the +foregoing purple variety, and these produced eleven and twenty-two +seeds, which germinated well. A large number of the stigmas on several +of the other corymbs were repeatedly smeared with pollen from their own +corymb; but they yielded only five very poor seeds, which were incapable +of germination. Therefore the above three plants belonging to two +varieties, though growing vigorously and fertile with pollen from either +of the other two plants, were utterly sterile with pollen from other +flowers on the same plant. + +Reseda odorata. + +Having observed that certain individuals were self-sterile, I covered +during the summer of 1868 seven plants under separate nets, and will +call these plants A, B, C, D, E, F, G. They all appeared to be quite +sterile with their own pollen, but fertile with that of any other plant. + +Fourteen flowers on A were crossed with pollen from B or C, and produced +thirteen fine capsules. Sixteen flowers were fertilised with pollen from +other flowers on the same plant, but yielded not a single capsule. + +Fourteen flowers on B were crossed with pollen from A, C or D, and all +produced capsules; some of these were not very fine, yet they contained +plenty of seeds. Eighteen flowers were fertilised with pollen from other +flowers on the same plant, and produced not one capsule. + +Ten flowers on C were crossed with pollen from A, B, D or E, and +produced nine fine capsules. Nineteen flowers were fertilised with +pollen from other flowers on the same plant, and produced no capsules. + +Ten flowers on D were crossed with pollen from A, B, C or E, and +produced nine fine capsules. Eighteen flowers were fertilised with +pollen from other flowers on the same plant, and produced no capsules. + +Seven flowers on E were crossed with pollen from A, C, or D, and all +produced fine capsules. Eight flowers were fertilised with pollen from +other flowers on the same plant, and produced no capsules. + +On the plants F and G no flowers were crossed, but very many (number not +recorded) were fertilised with pollen from other flowers on the same +plants, and these did not produce a single capsule. + +We thus see that fifty-five flowers on five of the above plants were +reciprocally crossed in various ways; several flowers on each of these +plants being fertilised with pollen from several of the other plants. +These fifty-five flowers produced fifty-two capsules, almost all of +which were of full size and contained an abundance of seeds. On the +other hand, seventy-nine flowers (besides many others not recorded) were +fertilised with pollen from other flowers on the same plants, and these +did not produce a single capsule. In one case in which I examined the +stigmas of the flowers fertilised with their own pollen, these were +penetrated by the pollen-tubes, although such penetration produced no +effect. Pollen falls generally, and I believe always, from the anthers +on the stigmas of the same flower; yet only three out of the above seven +protected plants produced spontaneously any capsules, and these it might +have been thought must have been self-fertilised. There were altogether +seven such capsules; but as they were all seated close to the +artificially crossed flowers, I can hardly doubt that a few grains of +foreign pollen had accidentally fallen on their stigmas. Besides the +above seven plants, four others were kept covered under the SAME large +net; and some of these produced here and there in the most capricious +manner little groups of capsules; and this makes me believe that a bee, +many of which settled on the outside of the net, being attracted by the +odour, had on some one occasion found an entrance, and had intercrossed +a few of the flowers. + +In the spring of 1869 four plants raised from fresh seeds were carefully +protected under separate nets; and now the result was widely different +to what it was before. Three of these protected plants became actually +loaded with capsules, especially during the early part of the summer; +and this fact indicates that temperature produces some effect, but the +experiment given in the following paragraph shows that the innate +constitution of the plant is a far more important element. The fourth +plant produced only a few capsules, many of them of small size; yet it +was far more self-fertile than any of the seven plants tried during the +previous year. The flowers on four small branches of this +semi-self-sterile plant were smeared with pollen from one of the other +plants, and they all produced fine capsules. + +As I was much surprised at the difference in the results of the trials +made during the two previous years, six fresh plants were protected by +separate nets in the year 1870. Two of these proved almost completely +self-sterile, for on carefully searching them I found only three small +capsules, each containing either one or two seeds of small size, which, +however, germinated. A few flowers on both these plants were +reciprocally fertilised with each other's pollen, and a few with pollen +from one of the following self-fertile plants, and all these flowers +produced fine capsules. The four other plants whilst still remaining +protected beneath the nets presented a wonderful contrast (though one of +them in a somewhat less degree than the others), for they became +actually covered with spontaneously self-fertilised capsules, as +numerous as, or very nearly so, and as fine as those on the unprotected +plants growing near. + +The above three spontaneously self-fertilised capsules produced by the +two almost completely self-sterile plants, contained altogether five +seeds; and from these I raised in the following year (1871) five plants, +which were kept under separate nets. They grew to an extraordinarily +large size, and on August 29th were examined. At first sight they +appeared entirely destitute of capsules; but on carefully searching +their many branches, two or three capsules were found on three of the +plants, half-a-dozen on the fourth, and about eighteen on the fifth +plant. But all these capsules were small, some being empty; the greater +number contained only a single seed, and very rarely more than one. +After this examination the nets were taken off, and the bees immediately +carried pollen from one of these almost self-sterile plants to the +other, for no other plants grew near. After a few weeks the ends of the +branches on all five plants became covered with capsules, presenting a +curious contrast with the lower and naked parts of the same long +branches. These five plants therefore inherited almost exactly the same +sexual constitution as their parents; and without doubt a self-sterile +race of Mignonette could have been easily established. + +Reseda lutea. + +Plants of this species were raised from seeds gathered from a group of +wild plants growing at no great distance from my garden. After casually +observing that some of these plants were self-sterile, two plants taken +by hazard were protected under separate nets. One of these soon became +covered with spontaneously self-fertilised capsules, as numerous as +those on the surrounding unprotected plants; so that it was evidently +quite self-fertile. The other plant was partially self-sterile, +producing very few capsules, many of which were of small size. When, +however, this plant had grown tall, the uppermost branches became +pressed against the net and grew crooked, and in this position the bees +were able to suck the flowers through the meshes, and brought pollen to +them from the neighbouring plants. These branches then became loaded +with capsules; the other and lower branches remaining almost bare. The +sexual constitution of this species is therefore similar to that of +Reseda odorata. + +CONCLUDING REMARKS ON SELF-STERILE PLANTS. + +In order to favour as far as possible the self-fertilisation of some of +the foregoing plants, all the flowers on Reseda odorata and some of +those on the Abutilon were fertilised with pollen from other flowers on +the same plant, instead of with their own pollen, and in the case of the +Senecio with pollen from other flowers on the same corymb; but this made +no difference in the result. Fritz Muller tried both kinds of +self-fertilisation in the case of Bignonia, Tabernaemontana and +Abutilon, likewise with no difference in the result. With Eschscholtzia, +however, he found that pollen from other flowers on the same plant was a +little more effective than pollen from the same flower. So did +Hildebrand in Germany; as thirteen out of fourteen flowers of +Eschscholtzia thus fertilised set capsules, these containing on an +average 9.5 seeds; whereas only fourteen flowers out of twenty-one +fertilised with their own pollen set capsules, these containing on an +average 9.0 seeds. (9/11. 'Pringsheim's Jahrbuch fur wiss. Botanik' 7 +page 467.) Hildebrand found a trace of a similar difference with +Corydalis cava, as did Fritz Muller with an Oncidium. (9/12. 'Variation +under Domestication' chapter 17 2nd edition volume 2 pages 113-115.) + +In considering the several cases above given of complete or almost +complete self-sterility, we are first struck with their wide +distribution throughout the vegetable kingdom. Their number is not at +present large, for they can be discovered only by protecting plants from +insects and then fertilising them with pollen from another plant of the +same species and with their own pollen; and the latter must be proved to +be in an efficient state by other trials. Unless all this be done, it is +impossible to know whether their self-sterility may not be due to the +male or female reproductive organs, or to both, having been affected by +changed conditions of life. As in the course of my experiments I have +found three new cases, and as Fritz Muller has observed indications of +several others, it is probable that they will hereafter be proved to be +far from rare. (9/13. Mr. Wilder, the editor of a horticultural journal +in the United States quoted in 'Gardeners' Chronicle' 1868 page 1286, +states that Lilium auratum, Impatiens pallida and fulva, and Forsythia +viridissima, cannot be fertilised with their own pollen.) + +As with plants of the same species and parentage, some individuals are +self-sterile and others self-fertile, of which fact Reseda odorata +offers the most striking instances, it is not at all surprising that +species of the same genus differ in this same manner. Thus Verbascum +phoeniceum and nigrum are self-sterile, whilst V. thapsus and lychnitis +are quite self-fertile, as I know by trial. There is the same difference +between some of the species of Papaver, Corydalis, and of other genera. +Nevertheless, the tendency to self-sterility certainly runs to a certain +extent in groups, as we see in the genus Passiflora, and with the +Vandeae amongst Orchids. + +Self-sterility differs much in degree in different plants. In those +extraordinary cases in which pollen from the same flower acts on the +stigma like a poison, it is almost certain that the plants would never +yield a single self-fertilised seed. Other plants, like Corydalis cava, +occasionally, though very rarely, produce a few self-fertilised seeds. A +large number of species, as may be seen in Table 9/F, are less fertile +with their own pollen than with that from another plant; and lastly, +some species are perfectly self-fertile. Even with the individuals of +the same species, as just remarked, some are utterly self-sterile, +others moderately so, and some perfectly self-fertile. The cause, +whatever it may be, which renders many plants more or less sterile with +their own pollen, that is, when they are self-fertilised, must be +different, at least to a certain extent, from that which determines the +difference in height, vigour, and fertility of the seedlings raised from +self-fertilised and crossed seeds; for we have already seen that the two +classes of cases do not by any means run parallel. This want of +parallelism would be intelligible, if it could be shown that +self-sterility depended solely on the incapacity of the pollen-tubes to +penetrate the stigma of the same flower deeply enough to reach the +ovules; whilst the greater or less vigorous growth of the seedlings no +doubt depends on the nature of the contents of the pollen-grains and +ovules. Now it is certain that with some plants the stigmatic secretion +does not properly excite the pollen-grains, so that the tubes are not +properly developed, if the pollen is taken from the same flower. This is +the case according to Fritz Muller with Eschscholtzia, for he found that +the pollen-tubes did not penetrate the stigma deeply; and with the +Orchidaceous genus Notylia they failed altogether to penetrate it. +(9/14. 'Botanische Zeitung' 1868 pages 114, 115.) + +With dimorphic and trimorphic species, an illegitimate union between +plants of the same form presents the closest analogy with +self-fertilisation, whilst a legitimate union closely resembles +cross-fertilisation; and here again the lessened fertility or complete +sterility of an illegitimate union depends, at least in part, on the +incapacity for interaction between the pollen-grains and stigma. Thus +with Linum grandiflorum, as I have elsewhere shown, not more than two or +three out of hundreds of pollen-grains, either of the long-styled or +short-styled form, when placed on the stigma of their own form, emit +their tubes, and these do not penetrate deeply; nor does the stigma +itself change colour, as occurs when it is legitimately fertilised. +(9/15. 'Journal of the Linnean Society Botany' volume 7 1863 pages +73-75.) + +On the other hand the difference in innate fertility, as well as in +growth between plants raised from crossed and self-fertilised seeds, and +the difference in fertility and growth between the legitimate and +illegitimate offspring of dimorphic and trimorphic plants, must depend +on some incompatibility between the sexual elements contained within the +pollen-grains and ovules, as it is through their union that new +organisms are developed. + +If we now turn to the more immediate cause of self-sterility, we clearly +see that in most cases it is determined by the conditions to which the +plants have been subjected. Thus Eschscholtzia is completely +self-sterile in the hot climate of Brazil, but is perfectly fertile +there with the pollen of any other individual. The offspring of +Brazilian plants became in England in a single generation partially +self-fertile, and still more so in the second generation. Conversely, +the offspring of English plants, after growing for two seasons in +Brazil, became in the first generation quite self-sterile. Again, +Abutilon darwinii, which is self-sterile in its native home of Brazil, +became moderately self-fertile in a single generation in an English +hothouse. Some other plants are self-sterile during the early part of +the year, and later in the season become self-fertile. Passiflora alata +lost its self-sterility when grafted on another species. With Reseda, +however, in which some individuals of the same parentage are +self-sterile and others are self-fertile, we are forced in our ignorance +to speak of the cause as due to spontaneous variability; but we should +remember that the progenitors of these plants, either on the male or +female side, may have been exposed to somewhat different conditions. The +power of the environment thus to affect so readily and in so peculiar a +manner the reproductive organs, is a fact which has many important +bearings; and I have therefore thought the foregoing details worth +giving. For instance, the sterility of many animals and plants under +changed conditions of life, such as confinement, evidently comes within +the same general principle of the sexual system being easily affected by +the environment. It has already been proved, that a cross between plants +which have been self-fertilised or intercrossed during several +generations, having been kept all the time under closely similar +conditions, does not benefit the offspring; and on the other hand, that +a cross between plants that have been subjected to different conditions +benefits the offspring to an extraordinary degree. We may therefore +conclude that some degree of differentiation in the sexual system is +necessary for the full fertility of the parent-plants and for the full +vigour of their offspring. It seems also probable that with those plants +which are capable of complete self-fertilisation, the male and female +elements and organs already differ to an extent sufficient to excite +their mutual interaction; but that when such plants are taken to another +country, and become in consequence self-sterile, their sexual elements +and organs are so acted on as to be rendered too uniform for such +interaction, like those of a self-fertilised plant long cultivated under +the same conditions. Conversely, we may further infer that plants which +are self-sterile in their native country, but become self-fertile under +changed conditions, have their sexual elements so acted on, that they +become sufficiently differentiated for mutual interaction. + +We know that self-fertilised seedlings are inferior in many respects to +those from a cross; and as with plants in a state of nature pollen from +the same flower can hardly fail to be often left by insects or by the +wind on the stigma, it seems at first sight highly probable that +self-sterility has been gradually acquired through natural selection in +order to prevent self-fertilisation. It is no valid objection to this +belief that the structure of some flowers, and the dichogamous condition +of many others, suffice to prevent the pollen reaching the stigma of the +same flower; for we should remember that with most species many flowers +expand at the same time, and that pollen from the same plant is equally +injurious or nearly so as that from the same flower. Nevertheless, the +belief that self-sterility is a quality which has been gradually +acquired for the special purpose of preventing self-fertilisation must, +I believe, be rejected. In the first place, there is no close +correspondence in degree between the sterility of the parent-plants when +self-fertilised, and the extent to which their offspring suffer in +vigour by this process; and some such correspondence might have been +expected if self-sterility had been acquired on account of the injury +caused by self-fertilisation. The fact of individuals of the same +parentage differing greatly in their degree of self-sterility is +likewise opposed to such a belief; unless, indeed, we suppose that +certain individuals have been rendered self-sterile to favour +intercrossing, whilst other individuals have been rendered self-fertile +to ensure the propagation of the species. The fact of self-sterile +individuals appearing only occasionally, as in the case of Lobelia, does +not countenance this latter view. But the strongest argument against the +belief that self-sterility has been acquired to prevent +self-fertilisation, is the immediate and powerful effect of changed +conditions in either causing or in removing self-sterility. We are not +therefore justified in admitting that this peculiar state of the +reproductive system has been gradually acquired through natural +selection; but we must look at it as an incidental result, dependent on +the conditions to which the plants have been subjected, like the +ordinary sterility caused in the case of animals by confinement, and in +the case of plants by too much manure, heat, etc. I do not, however, +wish to maintain that self-sterility may not sometimes be of service to +a plant in preventing self-fertilisation; but there are so many other +means by which this result might be prevented or rendered difficult, +including as we shall see in the next chapter the prepotency of pollen +from a distinct individual over a plant's own pollen, that +self-sterility seems an almost superfluous acquirement for this purpose. + +Finally, the most interesting point in regard to self-sterile plants is +the evidence which they afford of the advantage, or rather of the +necessity, of some degree or kind of differentiation in the sexual +elements, in order that they should unite and give birth to a new being. +It was ascertained that the five plants of Reseda odorata which were +selected by chance, could be perfectly fertilised by pollen taken from +any one of them, but not by their own pollen; and a few additional +trials were made with some other individuals, which I have not thought +worth recording. So again, Hildebrand and Fritz Muller frequently speak +of self-sterile plants being fertile with the pollen of any other +individual; and if there had been any exceptions to the rule, these +could hardly have escaped their observation and my own. We may therefore +confidently assert that a self-sterile plant can be fertilised by the +pollen of any one out of a thousand or ten thousand individuals of the +same species, but not by its own. Now it is obviously impossible that +the sexual organs and elements of every individual can have been +specialised with respect to every other individual. But there is no +difficulty in believing that the sexual elements of each differ slightly +in the same diversified manner as do their external characters; and it +has often been remarked that no two individuals are absolutely alike. +Therefore we can hardly avoid the conclusion, that differences of an +analogous and indefinite nature in the reproductive system are +sufficient to excite the mutual action of the sexual elements, and that +unless there be such differentiation fertility fails. + +THE APPEARANCE OF HIGHLY SELF-FERTILE VARIETIES. + +We have just seen that the degree to which flowers are capable of being +fertilised with their own pollen differs much, both with the species of +the same genus, and sometimes with the individuals of the same species. +Some allied cases of the appearance of varieties which, when +self-fertilised, yield more seed and produce offspring growing taller +than their self-fertilised parents, or than the intercrossed plants of +the corresponding generation, will now be considered. + +Firstly, in the third and fourth generations of Mimulus luteus, a tall +variety, often alluded to, having large white flowers blotched with +crimson, appeared amongst both the intercrossed and self-fertilised +plants. It prevailed in all the later self-fertilised generations to the +exclusion of every other variety, and transmitted its characters +faithfully, but disappeared from the intercrossed plants, owing no doubt +to their characters being repeatedly blended by crossing. The +self-fertilised plants belonging to this variety were not only taller, +but more fertile than the intercrossed plants; though these latter in +the earlier generations were much taller and more fertile than the +self-fertilised plants. Thus in the fifth generation the self-fertilised +plants were to the intercrossed in height as 126 to 100. In the sixth +generation they were likewise much taller and finer plants, but were not +actually measured; they produced capsules compared with those on the +intercrossed plants, in number, as 147 to 100; and the self-fertilised +capsules contained a greater number of seeds. In the seventh generation +the self-fertilised plants were to the crossed in height as 137 to 100; +and twenty flowers on these self-fertilised plants fertilised with their +own pollen yielded nineteen very fine capsules,--a degree of +self-sterility which I have not seen equalled in any other case. This +variety seems to have become specially adapted to profit in every way by +self-fertilisation, although this process was so injurious to the +parent-plants during the first four generations. It should however be +remembered that seedlings raised from this variety, when crossed by a +fresh stock, were wonderfully superior in height and fertility to the +self-fertilised plants of the corresponding generation. + +Secondly, in the sixth self-fertilised generation of Ipomoea a single +plant named the Hero appeared, which exceeded by a little in height its +intercrossed opponent,--a case which had not occurred in any previous +generation. Hero transmitted the peculiar colour of its flowers, as well +as its increased tallness and a high degree of self-fertility, to its +children, grandchildren, and great-grandchildren. The self-fertilised +children of Hero were in height to other self-fertilised plants of the +same stock as 100 to 85. Ten self-fertilised capsules produced by the +grandchildren contained on an average 5.2 seeds; and this is a higher +average than was yielded in any other generation by the capsules of +self-fertilised flowers. The great-grandchildren of Hero derived from a +cross with a fresh stock were so unhealthy, from having been grown at an +unfavourable season, that their average height in comparison with that +of the self-fertilised plants cannot be judged of with any safety; but +it did not appear that they had profited even by a cross of this kind. + +Thirdly, the plants of Nicotiana on which I experimented appear to come +under the present class of cases; for they varied in their sexual +constitution and were more or less highly self-fertile. They were +probably the offspring of plants which had been spontaneously +self-fertilised under glass for several generations in this country. The +flowers on the parent-plants which were first fertilised by me with +their own pollen yielded half again as many seeds as did those which +were crossed; and the seedlings raised from these self-fertilised seeds +exceeded in height those raised from the crossed seeds to an +extraordinary degree. In the second and third generations, although the +self-fertilised plants did not exceed the crossed in height, yet their +self-fertilised flowers yielded on two occasions considerably more seeds +than the crossed flowers, even than those which were crossed with pollen +from a distinct stock or variety. + +Lastly, as certain individual plants of Reseda odorata and lutea are +incomparably more self-fertile than other individuals, the former might +be included under the present heading of the appearance of new and +highly self-fertile varieties. But in this case we should have to look +at these two species as normally self-sterile; and this, judging by my +experience, appears to be the correct view. + +We may therefore conclude from the facts now given, that varieties +sometimes arise which when self-fertilised possess an increased power of +producing seeds and of growing to a greater height, than the +intercrossed or self-fertilised plants of the corresponding +generation--all the plants being of course subjected to the same +conditions. The appearance of such varieties is interesting, as it bears +on the existence under nature of plants which regularly fertilise +themselves, such as Ophrys apifera and a few other orchids, or as +Leersia oryzoides, which produces an abundance of cleistogene flowers, +but most rarely flowers capable of cross-fertilisation. + +Some observations made on other plants lead me to suspect that +self-fertilisation is in some respects beneficial; although the benefit +thus derived is as a rule very small compared with that from a cross +with a distinct plant. Thus we have seen in the last chapter that +seedlings of Ipomoea and Mimulus raised from flowers fertilised with +their own pollen, which is the strictest possible form of +self-fertilisation, were superior in height, weight, and in early +flowering to the seedlings raised from flowers crossed with pollen from +other flowers on the same plant; and this superiority apparently was too +strongly marked to be accidental. Again, the cultivated varieties of the +common pea are highly self-fertile, although they have been +self-fertilised for many generations; and they exceeded in height +seedlings from a cross between two plants belonging to the same variety +in the ratio of 115 to 100; but then only four pairs of plants were +measured and compared. The self-fertility of Primula veris increased +after several generations of illegitimate fertilisation, which is a +process closely analogous to self-fertilisation, but only as long as the +plants were cultivated under the same favourable conditions. I have also +elsewhere shown that with Primula veris and sinensis, equal-styled +varieties occasionally appear which possess the sexual organs of the two +forms combined in the same flower. (9/16. 'Journal of the Linnean +Society Botany' volume 10 1867 pages 417, 419.) Consequently they +fertilise themselves in a legitimate manner and are highly self-fertile; +but the remarkable fact is that they are rather more fertile than +ordinary plants of the same species legitimately fertilised by pollen +from a distinct individual. Formerly it appeared to me probable, that +the increased fertility of these dimorphic plants might be accounted for +by the stigma lying so close to the anthers that it was impregnated at +the most favourable age and time of the day; but this explanation is not +applicable to the above given cases, in which the flowers were +artificially fertilised with their own pollen. + +Considering the facts now adduced, including the appearance of those +varieties which are more fertile and taller than their parents and than +the intercrossed plants of the corresponding generation, it is difficult +to avoid the suspicion that self-fertilisation is in some respects +advantageous; though if this be really the case, any such advantage is +as a rule quite insignificant compared with that from a cross with a +distinct plant, and especially with one of a fresh stock. Should this +suspicion be hereafter verified, it would throw light, as we shall see +in the next chapter, on the existence of plants bearing small and +inconspicuous flowers which are rarely visited by insects, and therefore +are rarely intercrossed. + +RELATIVE WEIGHT AND PERIOD OF GERMINATION OF SEEDS FROM CROSSED AND +SELF-FERTILISED FLOWERS. + +An equal number of seeds from flowers fertilised with pollen from +another plant, and from flowers fertilised with their own pollen, were +weighed, but only in sixteen cases. Their relative weights are given in +the following list; that of the seeds from the crossed flowers being +taken as 100. + +Column 1: Name of Plant. + +Column 2: x, in the expression, 100 to x. + +Ipomoea purpurea (parent plants): 127. +Ipomoea purpurea (third generation): 87. +Salvia coccinea: 100. +Brassica oleracea: 103. +Iberis umbellata (second generation): 136. +Delphinium consolida: 45. +Hibiscus africanus: 105. +Tropaeolum minus: 115. +Lathyrus odoratus (about): 100. +Sarothamnus scoparius: 88. +Specularia speculum: 86. +Nemophila insignis: 105. +Borago officinalis: 111. +Cyclamen persicum (about): 50. +Fagopyrum esculentum: 82. +Canna warscewiczi (3 generations): 102. + +It is remarkable that in ten out of these sixteen cases the +self-fertilised seeds were either superior or equal to the crossed in +weight; nevertheless, in six out of the ten cases (namely, with Ipomoea, +Salvia, Brassica, Tropaeolum, Lathyrus, and Nemophila) the plants raised +from these self-fertilised seeds were very inferior in height and in +other respects to those raised from the crossed seeds. The superiority +in weight of the self-fertilised seeds in at least six out of the ten +cases, namely, with Brassica, Hibiscus, Tropaeolum, Nemophila, Borago, +and Canna, may be accounted for in part by the self-fertilised capsules +containing fewer seeds; for when a capsule contains only a few seeds, +these will be apt to be better nourished, so as to be heavier, than when +many are contained in the same capsule. It should, however, be observed +that in some of the above cases, in which the crossed seeds were the +heaviest, as with Sarothamnus and Cyclamen, the crossed capsules +contained a larger number of seeds. Whatever may be the explanation of +the self-fertilised seeds being often the heaviest, it is remarkable in +the case of Brassica, Tropaeolum, Nemophila, and of the first generation +of Ipomoea, that the seedlings raised from them were inferior in height +and in other respects to the seedlings raised from the crossed seeds. +This fact shows how superior in constitutional vigour the crossed +seedlings must have been, for it cannot be doubted that heavy and fine +seeds tend to yield the finest plants. Mr. Galton has shown that this +holds good with Lathyrus odoratus; as has Mr. A.J. Wilson with the +Swedish turnip, Brassica campestris ruta baga. Mr. Wilson separated the +largest and smallest seeds of this latter plant, the ratio between the +weights of the two lots being as 100 to 59, and he found that the +seedlings "from the larger seeds took the lead and maintained their +superiority to the last, both in height and thickness of stem." (9/17. +'Gardeners' Chronicle' 1867 page 107. Loiseleur-Deslongchamp 'Les +Cereales' 1842 pages 208-219, was led by his observations to the +extraordinary conclusion that the smaller grains of cereals produce as +fine plants as the large. This conclusion is, however, contradicted by +Major Hallet's great success in improving wheat by the selection of the +finest grains. It is possible, however, that man, by long-continued +selection, may have given to the grains of the cereals a greater amount +of starch or other matter, than the seedlings can utilise for their +growth. There can be little doubt, as Humboldt long ago remarked, that +the grains of cereals have been rendered attractive to birds in a degree +which is highly injurious to the species.) Nor can this difference in +the growth of the seedling turnips be attributed to the heavier seeds +having been of crossed, and the lighter of self-fertilised origin, for +it is known that plants belonging to this genus are habitually +intercrossed by insects. + +With respect to the relative period of germination of crossed and +self-fertilised seeds, a record was kept in only twenty-one cases; and +the results are very perplexing. Neglecting one case in which the two +lots germinated simultaneously, in ten cases or exactly one-half many of +the self-fertilised seeds germinated before the crossed, and in the +other half many of the crossed before the self-fertilised. In four out +of these twenty cases, seeds derived from a cross with a fresh stock +were compared with self-fertilised seeds from one of the later +self-fertilised generations; and here again in half the cases the +crossed seeds, and in the other half the self-fertilised seeds, +germinated first. Yet the seedlings of Mimulus raised from such +self-fertilised seeds were inferior in all respects to the crossed +seedlings, and in the case of Eschscholtzia they were inferior in +fertility. Unfortunately the relative weight of the two lots of seeds +was ascertained in only a few instances in which their germination was +observed; but with Ipomoea and I believe with some of the other species, +the relative lightness of the self-fertilised seeds apparently +determined their early germination, probably owing to the smaller mass +being favourable to the more rapid completion of the chemical and +morphological changes necessary for germination. On the other hand, Mr. +Galton gave me seeds (no doubt all self-fertilised) of Lathyrus +odoratus, which were divided into two lots of heavier and lighter seeds; +and several of the former germinated first. It is evident that many more +observations are necessary before anything can be decided with respect +to the relative period of germination of crossed and self-fertilised +seeds. + + + +CHAPTER X. + +MEANS OF FERTILISATION. + +Sterility and fertility of plants when insects are excluded. +The means by which flowers are cross-fertilised. +Structures favourable to self-fertilisation. +Relation between the structure and conspicuousness of flowers, the +visits of insects, and the advantages of cross-fertilisation. +The means by which flowers are fertilised with pollen from a distinct +plant. +Greater fertilising power of such pollen. +Anemophilous species. +Conversion of anemophilous species into entomophilous. +Origin of nectar. +Anemophilous plants generally have their sexes separated. +Conversion of diclinous into hermaphrodite flowers. +Trees often have their sexes separated. + +In the introductory chapter I briefly specified the various means by +which cross-fertilisation is favoured or ensured, namely, the separation +of the sexes,--the maturity of the male and female sexual elements at +different periods,--the heterostyled or dimorphic and trimorphic +condition of certain plants,--many mechanical contrivances,--the more or +less complete inefficiency of a flower's own pollen on the stigma,--and +the prepotency of pollen from any other individual over that from the +same plant. Some of these points require further consideration; but for +full details I must refer the reader to the several excellent works +mentioned in the introduction. I will in the first place give two lists: +the first, of plants which are either quite sterile or produce less than +about half the full complement of seeds, when insects are excluded; and +a second list of plants which, when thus treated, are fully fertile or +produce at least half the full complement of seeds. These lists have +been compiled from the several previous tables, with some additional +cases from my own observations and those of others. The species are +arranged nearly in the order followed by Lindley in his 'Vegetable +Kingdom.' The reader should observe that the sterility or fertility of +the plants in these two lists depends on two wholly distinct causes; +namely, the absence or presence of the proper means by which pollen is +applied to the stigma, and its less or greater efficiency when thus +applied. As it is obvious that with plants in which the sexes are +separate, pollen must be carried by some means from flower to flower, +such species are excluded from the lists; as are likewise dimorphic and +trimorphic plants, in which the same necessity occurs to a limited +extent. Experience has proved to me that, independently of the exclusion +of insects, the seed-bearing power of a plant is not lessened by +covering it while in flower under a thin net supported on a frame; and +this might indeed have been inferred from the consideration of the two +following lists, as they include a considerable number of species +belonging to the same genera, some of which are quite sterile and others +quite fertile when protected by a net from the access of insects. + +[LIST OF PLANTS WHICH, WHEN INSECTS ARE EXCLUDED, ARE EITHER QUITE +STERILE, OR PRODUCE, AS FAR AS I COULD JUDGE, LESS THAN HALF THE NUMBER +OF SEEDS PRODUCED BY UNPROTECTED PLANTS. + +Passiflora alata, racemosa, coerulea, edulis, laurifolia, and some +individuals of P. quadrangularis (Passifloraceae), are quite sterile +under these conditions: see 'Variation of Animals and Plants under +Domestication' chapter 17 2nd edition volume 2 page 118. + +Viola canina (Violaceae).--Perfect flowers quite sterile unless +fertilised by bees, or artificially fertilised. + +Viola tricolor.--Sets very few and poor capsules. + +Reseda odorata (Resedaceae).--Some individuals quite sterile. + +Reseda lutea.--Some individuals produce very few and poor capsules. + +Abutilon darwinii (Malvaceae).--Quite sterile in Brazil: see previous +discussion on self-sterile plants. + +Nymphaea (Nymphaeaceae).--Professor Caspary informs me that some of the +species are quite sterile if insects are excluded. + +Euryale amazonica (Nymphaeaceae).--Mr. J. Smith, of Kew, informs me that +capsules from flowers left to themselves, and probably not visited by +insects, contained from eight to fifteen seeds; those from flowers +artificially fertilised with pollen from other flowers on the same plant +contained from fifteen to thirty seeds; and that two flowers fertilised +with pollen brought from another plant at Chatsworth contained +respectively sixty and seventy-five seeds. I have given these statements +because Professor Caspary advances this plant as a case opposed to the +doctrine of the necessity or advantage of cross-fertilisation: see +Sitzungsberichte der Phys.-okon. Gesell.zu Konigsberg, B.6 page 20.) + +Delphinium consolida (Ranunculaceae).--Produces many capsules, but these +contain only about half the number of seeds compared with capsules from +flowers naturally fertilised by bees. + +Eschscholtzia californica (Papaveraceae).--Brazilian plants quite +sterile: English plants produce a few capsules. + +Papaver vagum (Papaveraceae).--In the early part of the summer produced +very few capsules, and these contained very few seeds. + +Papaver alpinum.--H. Hoffmann ('Speciesfrage' 1875 page 47) states that +this species produced seeds capable of germination only on one occasion. + +Corydalis cava (Fumariaceae).--Sterile: see the previous discussion on +self-sterile plants. + +Corydalis solida.--I had a single plant in my garden (1863), and saw +many hive-bees sucking the flowers, but not a single seed was produced. +I was much surprised at this fact, as Professor Hildebrand's discovery +that C. cava is sterile with its own pollen had not then been made. He +likewise concludes from the few experiments which he made on the present +species that it is self-sterile. The two foregoing cases are +interesting, because botanists formerly thought (see, for instance, +Lecoq, 'De la Fecondation et de l'Hybridation' 1845 page 61 and Lindley +'Vegetable Kingdom' 1853 page 436) that all the species of the +Fumariaceae were specially adapted for self-fertilisation. + +Corydalis lutea.--A covered-up plant produced (1861) exactly half as +many capsules as an exposed plant of the same size growing close +alongside. When humble-bees visit the flowers (and I repeatedly saw them +thus acting) the lower petals suddenly spring downwards and the pistil +upwards; this is due to the elasticity of the parts, which takes effect, +as soon as the coherent edges of the hood are separated by the entrance +of an insect. Unless insects visit the flowers the parts do not move. +Nevertheless, many of the flowers on the plants which I had protected +produced capsules, notwithstanding that their petals and pistils still +retained their original position; and I found to my surprise that these +capsules contained more seeds than those from flowers, the petals of +which had been artificially separated and allowed to spring apart. Thus, +nine capsules produced by undisturbed flowers contained fifty-three +seeds; whilst nine capsules from flowers, the petals of which had been +artificially separated, contained only thirty-two seeds. But we should +remember that if bees had been permitted to visit these flowers, they +would have visited them at the best time for fertilisation. The flowers, +the petals of which had been artificially separated, set their capsules +before those which were left undisturbed under the net. To show with +what certainty the flowers are visited by bees, I may add that on one +occasion all the flowers on some unprotected plants were examined, and +every single one had its petals separated; and, on a second occasion, +forty-one out of forty-three flowers were in this state. Hildebrand +states (Pring. Jahr. f. wiss. Botanik, B. 7 page 450) that the mechanism +of the parts in this species is nearly the same as in C. ochroleuca, +which he has fully described. + +Hypecoum grandiflorum (Fumariaceae).--Highly self-sterile (Hildebrand, +ibid.). + +Kalmia latifolia (Ericaceae).--Mr. W.J. Beal says ('American Naturalist' +1867) that flowers protected from insects wither and drop off, with +"most of the anthers still remaining in the pockets." + +Pelargonium zonale (Geraniaceae).--Almost sterile; one plant produced +two fruits. It is probable that different varieties would differ in this +respect, as some are only feebly dichogamous. + +Dianthus caryophyllus (Caryophyllaceae).--Produces very few capsules +which contain any good seeds. + +Phaseolus multiflorus (Leguminosae).--Plants protected from insects +produced on two occasions about one-third and one-eighth of the full +number of seeds: see my article in 'Gardeners' Chronicle' 1857 page 225 +and 1858 page 828; also 'Annals and Magazine of Natural History' 3rd +series volume 2 1858 page 462. Dr. Ogle ('Popular Science Review' 1870 +page 168) found that a plant was quite sterile when covered up. The +flowers are not visited by insects in Nicaragua, and, according to Mr. +Belt, the species is there quite sterile: 'The Naturalist in Nicaragua' +page 70. + +Vicia faba (Leguminosae).--Seventeen covered-up plants yielded 40 beans, +whilst seventeen plants left unprotected and growing close alongside +produced 135 beans; these latter plants were, therefore, between three +and four times more fertile than the protected plants: see 'Gardeners' +Chronicle' for fuller details, 1858 page 828. + +Erythrina (sp.?) (Leguminosae).--Sir W. MacArthur informed me that in +New South Wales the flowers do not set, unless the petals are moved in +the same manner as is done by insects. + +Lathyrus grandiflorus (Leguminosae).--Is in this country more or less +sterile. It never sets pods unless the flowers are visited by +humble-bees (and this happens only rarely), or unless they are +artificially fertilised: see my article in 'Gardeners' Chronicle' 1858 +page 828. + +Sarothamnus scoparius (Leguminosae).--Extremely sterile when the flowers +are neither visited by bees, nor disturbed by being beaten by the wind +against the surrounding net. + +Melilotus officinalis (Leguminosae).--An unprotected plant visited by +bees produced at least thirty times more seeds than a protected one. On +this latter plant many scores of racemes did not produce a single pod; +several racemes produced each one or two pods; five produced three; six +produced four; and one produced six pods. On the unprotected plant each +of several racemes produced fifteen pods; nine produced between sixteen +and twenty-two pods, and one produced thirty pods. + +Lotus corniculatus (Leguminosae).--Several covered-up plants produced +only two empty pods, and not a single good seed. + +Trifolium repens (Leguminosae).--Several plants were protected from +insects, and the seeds from ten flowers-heads on these plants, and from +ten heads on other plants growing outside the net (which I saw visited +by bees), were counted; and the seeds from the latter plants were very +nearly ten times as numerous as those from the protected plants. The +experiment was repeated on the following year; and twenty protected +heads now yielded only a single aborted seed, whilst twenty heads on the +plants outside the net (which I saw visited by bees) yielded 2290 seeds, +as calculated by weighing all the seed, and counting the number in a +weight of two grains. + +Trifolium pratense.--One hundred flower-heads on plants protected by a +net did not produce a single seed, whilst 100 heads on plants growing +outside, which were visited by bees, yielded 68 grains weight of seeds; +and as eighty seeds weighed two grains, the 100 heads must have yielded +2720 seeds. I have often watched this plant, and have never seen +hive-bees sucking the flowers, except from the outside through holes +bitten by humble-bees, or deep down between the flowers, as if in search +of some secretion from the calyx, almost in the same manner as described +by Mr. Farrer, in the case of Coronilla ('Nature' 1874 July 2 page 169). +I must, however, except one occasion, when an adjoining field of +sainfoin (Hedysarum onobrychis) had just been cut down, and when the +bees seemed driven to desperation. On this occasion most of the flowers +of the clover were somewhat withered, and contained an extraordinary +quantity of nectar, which the bees were able to suck. An experienced +apiarian, Mr. Miner, says that in the United States hive-bees never suck +the red clover; and Mr. R. Colgate informs me that he has observed the +same fact in New Zealand after the introduction of the hive-bee into +that island. On the other hand, H. Muller ('Befruchtung' page 224) has +often seen hive-bees visiting this plant in Germany, for the sake both +of pollen and nectar, which latter they obtained by breaking apart the +petals. It is at least certain that humble-bees are the chief +fertilisers of the common red clover. + +Trifolium incarnatum.--The flower-heads containing ripe seeds, on some +covered and uncovered plants, appeared equally fine, but this was a +false appearance; 60 heads on the latter yielded 349 grains weight of +seeds, whereas 60 on the covered-up plants yielded only 63 grains, and +many of the seeds in the latter lot were poor and aborted. Therefore the +flowers which were visited by bees produced between five and six times +as many seeds as those which were protected. The covered-up plants not +having been much exhausted by seed-bearing, bore a second considerable +crop of flower-stems, whilst the exposed plants did not do so. + +Cytisus laburnum (Leguminosae).--Seven flower-racemes ready to expand +were enclosed in a large bag made of net, and they did not seem in the +least injured by this treatment. Only three of them produced any pods, +each a single one; and these three pods contained one, four, and five +seeds. So that only a single pod from the seven racemes included a fair +complement of seeds. + +Cuphea purpurea (Lythraceae).--Produced no seeds. Other flowers on the +same plant artificially fertilised under the net yielded seeds. + +Vinca major (Apocynaceae).--Is generally quite sterile, but sometimes +sets seeds when artificially cross-fertilised: see my notice 'Gardeners' +Chronicle' 1861 page 552. + +Vinca rosea.--Behaves in the same manner as the last species: +'Gardeners' Chronicle' 1861 page 699, 736, 831. + +Tabernaemontana echinata (Apocynaceae).--Quite sterile. + +Petunia violacea (Solanaceae).--Quite sterile, as far as I have +observed. + +Solanum tuberosum (Solanaceae).--Tinzmann says ('Gardeners' Chronicle' +1846 page 183) that some varieties are quite sterile unless fertilised +by pollen from another variety. + +Primula scotica (Primulaceae).--A non-dimorphic species, which is +fertile with its own pollen, but is extremely sterile if insects are +excluded. J. Scott in 'Journal of the Linnean Society Botany' volume 8 +1864 page 119. + +Cortusa matthioli (Primulaceae).--Protected plants completely sterile; +artificially self-fertilised flowers perfectly fertile. J. Scott ibid. +page 84. + +Cyclamen persicum (Primulaceae).--During one season several covered-up +plants did not produce a single seed. + +Borago officinalis (Boraginaceae).--Protected plants produced about half +as many seeds as the unprotected. + +Salvia tenori (Labiatae).--Quite sterile; but two or three flowers on +the summits of three of the spikes, which touched the net when the wind +blew, produced a few seeds. This sterility was not due to the injurious +effects of the net, for I fertilised five flowers with pollen from an +adjoining plant, and these all yielded fine seeds. I removed the net, +whilst one little branch still bore a few not completely faded flowers, +and these were visited by bees and yielded seeds. + +Salvia coccinea.--Some covered-up plants produced a good many fruits, +but not, I think, half as many as did the uncovered plants; twenty-eight +of the fruits spontaneously produced by the protected plant contained on +an average only 1.45 seeds, whilst some artificially self-fertilised +fruits on the same plant contained more than twice as many, namely 3.3 +seeds. + +Bignonia (unnamed species) (Bignoniaceae).--Quite sterile: see my +account of self-sterile plants. + +Digitalis purpurea (Scrophulariaceae).--Extremely sterile, only a few +poor capsules being produced. + +Linaria vulgaris (Scrophulariaceae).--Extremely sterile. + +Antirrhinum majus, red var. (Scrophulariaceae).--Fifty pods gathered +from a large plant under a net contained 9.8 grains weight of seeds; but +many (unfortunately not counted) of the fifty pods contained no seeds. +Fifty pods on a plant fully exposed to the visits of humble-bees +contained 23.1 grains weight of seed, that is, more than twice the +weight; but in this case again, several of the fifty pods contained no +seeds. + +Antirrhinum majus (white var., with a pink mouth to the corolla).--Fifty +pods, of which only a very few were empty, on a covered-up plant +contained 20 grains weight of seed; so that this variety seems to be +much more self-fertile than the previous one. With Dr. W. Ogle ('Popular +Science Review' January 1870 page 52) a plant of this species was much +more sterile when protected from insects than with me, for it produced +only two small capsules. As showing the efficiency of bees, I may add +that Mr. Crocker castrated some young flowers and left them uncovered; +and these produced as many seeds as the unmutilated flowers. + +Antirrhinum majus (peloric var.).--This variety is quite fertile when +artificially fertilised with its own pollen, but is utterly sterile when +left to itself and uncovered, as humble-bees cannot crawl into the +narrow tubular flowers. + +Verbascum phoeniceum (Scrophulariaceae).--Quite sterile. See my account +of self-sterile plants. + +Verbascum nigrum.--Quite sterile. See my account of self-sterile plants. + +Campanula carpathica (Lobeliaceae).--Quite sterile. + +Lobelia ramosa (Lobeliaceae).--Quite sterile. + +Lobelia fulgens.--This plant is never visited in my garden by bees, and +is quite sterile; but in a nursery-garden at a few miles' distance I saw +humble-bees visiting the flowers, and they produced some capsules. + +Isotoma (a white-flowered var.) (Lobeliaceae).--Five plants left +unprotected in my greenhouse produced twenty-four fine capsules, +containing altogether 12.2 grains weight of seed, and thirteen other +very poor capsules, which were rejected. Five plants protected from +insects, but otherwise exposed to the same conditions as the above +plants, produced sixteen fine capsules, and twenty other very poor and +rejected ones. The sixteen fine capsules contained seeds by weight in +such proportion that twenty-four would have yielded 4.66 grains. So that +the unprotected plants produced nearly thrice as many seeds by weight as +the protected plants. + +Leschenaultia formosa (Goodeniaceae).--Quite sterile. My experiments on +this plant, showing the necessity of insect aid, are given in the +'Gardeners' Chronicle' 1871 page 1166. + +Senecio cruentus (Compositae).--Quite sterile: see my account of +self-sterile plants. + +Heterocentron mexicanum (Malastomaceae).--Quite sterile; but this +species and the following members of the group produce plenty of seed +when artificially self-fertilised. + +Rhexia glandulosa (Melastomaceae).--Set spontaneously only two or three +capsules. + +Centradenia floribunda (Melastomaceae).--During some years produced +spontaneously two or three capsules, sometimes none. + +Pleroma (unnamed species from Kew) (Melastomaceae).--During some years +produced spontaneously two or three capsules, sometimes none. + +Monochaetum ensiferum (Melastomaceae).--During some years produced +spontaneously two or three capsules, sometimes none. + +Hedychium (unnamed species) (Marantaceae).--Almost self-sterile without +aid. + +Orchideae.--An immense proportion of the species sterile, if insects are +excluded. + +LIST OF PLANTS, WHICH WHEN PROTECTED FROM INSECTS ARE EITHER QUITE +FERTILE, OR YIELD MORE THAN HALF THE NUMBER OF SEEDS PRODUCED BY +UNPROTECTED PLANTS. + +Passiflora gracilis (Passifloraceae).--Produces many fruits, but these +contain fewer seeds than fruits from intercrossed flowers. + +Brassica oleracea (Cruciferae).--Produces many capsules, but these +generally not so rich in seed as those on uncovered plants. + +Raphanus sativus (Cruciferae).--Half of a large branching plant was +covered by a net, and was as thickly covered with capsules as the other +and unprotected half; but twenty of the capsules on the latter contained +on an average 3.5 seeds, whilst twenty of the protected capsules +contained only 1.85 seeds, that is, only a little more than half the +number. This plant might perhaps have been more properly included in the +former list. + +Iberis umbellata (Cruciferae).--Highly fertile. + +Iberis amara.--Highly fertile. + +Reseda odorata and lutea (Resedaceae).--Certain individuals completely +self-fertile. + +Euryale ferox (Nymphaeaceae).--Professor Caspary informs me that this +plant is highly self-fertile when insects are excluded. He remarks in +the paper before referred to, that his plants (as well as those of the +Victoria regia) produce only one flower at a time; and that as this +species is an annual, and was introduced in 1809, it must have been +self-fertilised for the last fifty-six generations; but Dr. Hooker +assures me that to his knowledge it has been repeatedly introduced, and +that at Kew the same plant both of the Euryale and of the Victoria +produce several flowers at the same time. + +Nymphaea (Nymphaeaceae).--Some species, as I am informed by Professor +Caspary, are quite self-fertile when insects are excluded. + +Adonis aestivalis (Ranunculaceae).--Produces, according to Professor H. +Hoffmann ('Speciesfrage' page 11), plenty of seeds when protected from +insects. + +Ranunculus acris (Ranunculaceae).--Produces plenty of seeds under a net. + +Papaver somniferum (Papaveraceae).--Thirty capsules from uncovered +plants yielded 15.6 grains weight of seed, and thirty capsules from +covered-up plants, growing in the same bed, yielded 16.5 grains weight; +so that the latter plants were more productive than the uncovered. +Professor H. Hoffmann ('Speciesfrage' 1875 page 53) also found this +species self-fertile when protected from insects. + +Papaver vagum.--Produced late in the summer plenty of seeds, which +germinated well. + +Papaver argemonoides.--According to Hildebrand ('Jahrbuch fur w. Bot.' +B.7 page 466), spontaneously self-fertilised flowers are by no means +sterile. + +Glaucium luteum (Papaveraceae).--According to Hildebrand ('Jahrbuch fur +w. Bot.' B.7 page 466), spontaneously self-fertilised flowers are by no +means sterile. + +Argemone ochroleuca (Papaveraceae).--According to Hildebrand ('Jahrbuch +fur w. Bot.' B.7 page 466), spontaneously self-fertilised flowers are by +no means sterile. + +Adlumia cirrhosa (Fumariaceae).--Sets an abundance of capsules. + +Hypecoum procumbens (Fumariaceae).--Hildebrand says (idem), with respect +to protected flowers, that "eine gute Fruchtbildung eintrete." + +Fumaria officinalis (Fumariaceae).--Covered-up and unprotected plants +apparently produced an equal number of capsules, and the seeds of the +former seemed to the eye equally good. I have often watched this plant, +and so has Hildebrand, and we have never seen an insect visit the +flowers. Hermann Muller has likewise been struck with the rarity of the +visits of insects to it, though he has sometimes seen hive-bees at work. +The flowers may perhaps be visited by small moths, as is probably the +case with the following species. + +Fumaria capreolata.--Several large beds of this plant growing wild were +watched by me during many days, but the flowers were never visited by +any insects, though a humble-bee was once seen closely to inspect them. +Nevertheless, as the nectary contains much nectar, especially in the +evening, I felt convinced that they were visited, probably by moths. The +petals do not naturally separate or open in the least; but they had been +opened by some means in a certain proportion of the flowers, in the same +manner as follows when a thick bristle is pushed into the nectary; so +that in this respect they resemble the flowers of Corydalis lutea. +Thirty-four heads, each including many flowers, were examined, and +twenty of them had from one to four flowers, whilst fourteen had not a +single flower thus opened. It is therefore clear that some of the +flowers had been visited by insects, while the majority had not; yet +almost all produced capsules. + +Linum usitatissimum (Linaceae).--Appears to be quite fertile. H. +Hoffmann 'Botanische Zeitung' 1876 page 566. + +Impatiens barbigerum (Balsaminaceae).--The flowers, though excellently +adapted for cross-fertilisation by the bees which freely visit them, set +abundantly under a net. + +Impatiens noli-me-tangere (Balsaminaceae).--This species produces +cleistogene and perfect flowers. A plant was covered with a net, and +some perfect flowers, marked with threads, produced eleven spontaneously +self-fertilised capsules, which contained on an average 3.45 seeds. I +neglected to ascertain the number of seeds produced by perfect flowers +exposed to the visits of insects, but I believe it is not greatly in +excess of the above average. Mr. A.W. Bennett has carefully described +the structure of the flowers of I. fulva in 'Journal of the Linnean +Society' volume 13 Bot. 1872 page 147. This latter species is said to be +sterile with its own pollen ('Gardeners' Chronicle' 1868 page 1286), and +if so, it presents a remarkable contrast with I. barbigerum and +noli-me-tangere. + +Limnanthes douglasii (Geraniaceae).--Highly fertile. + +Viscaria oculata (Caryophyllaceae).--Produces plenty of capsules with +good seeds. + +Stellaria media (Caryophyllaceae).--Covered-up and uncovered plants +produced an equal number of capsules, and the seeds in both appeared +equally numerous and good. + +Beta vulgaris (Chenopodiaceae).--Highly self-fertile. + +Vicia sativa (Leguminosae).--Protected and unprotected plants produced +an equal number of pods and equally fine seeds. If there was any +difference between the two lots, the covered-up plants were the most +productive. + +Vicia hirsuta.--This species bears the smallest flowers of any British +leguminous plant. The result of covering up plants was exactly the same +as in the last species. + +Pisum sativum (Leguminosae).--Fully fertile. + +Lathyrus odoratus (Leguminosae).--Fully fertile. + +Lathyrus nissolia.--Fully fertile. + +Lupinus luteus (Leguminosae).--Fairly productive. + +Lupinus pilosus.--Produced plenty of pods. + +Ononis minutissima (Leguminosae).--Twelve perfect flowers on a plant +under a net were marked by threads, and produced eight pods, containing +on an average 2.38 seeds. Pods produced by flowers visited by insects +would probably have contained on an average 3.66 seeds, judging from the +effects of artificial cross-fertilisation. + +Phaseolus vulgaris (Leguminosae).--Quite fertile. + +Trifolium arvense (Leguminosae).--The excessively small flowers are +incessantly visited by hive and humble-bees. When insects were excluded +the flower-heads seemed to produce as many and as fine seeds as the +exposed heads. + +Trifolium procumbens.--On one occasion covered-up plants seemed to yield +as many seeds as the uncovered. On a second occasion sixty uncovered +flower-heads yielded 9.1 grains weight of seeds, whilst sixty heads on +protected plants yielded no less than 17.7 grains; so that these latter +plants were much more productive; but this result I suppose was +accidental. I have often watched this plant, and have never seen the +flowers visited by insects; but I suspect that the flowers of this +species, and more especially of Trifolium minus, are frequented by small +nocturnal moths which, as I hear from Mr. Bond, haunt the smaller +clovers. + +Medicago lupulina (Leguminosae).--On account of the danger of losing the +seeds, I was forced to gather the pods before they were quite ripe; 150 +flower-heads on plants visited by bees yielded pods weighing 101 grains; +whilst 150 heads on protected plants yielded pods weighing 77 grains. +The inequality would probably have been greater if the mature seeds +could have been all safely collected and compared. Ig. Urban (Keimung, +Bluthen, etc., bei Medicago 1873) has described the means of +fertilisation in this genus, as has the Reverend G. Henslow in the +'Journal of the Linnean Society Botany' volume 9 1866 pages 327 and 355. + +Nicotiana tabacum (Solanaceae).--Fully self-fertile. + +Ipomoea purpurea (Convolvulaceae).--Highly self-fertile. + +Leptosiphon androsaceus (Polemoniacae).--Plants under a net produced a +good many capsules. + +Primula mollis (Primulaceae).--A non-dimorphic species, self-fertile: J. +Scott, in 'Journal of the Linnean Society Botany' volume 8 1864 page +120. + +Nolana prostrata (Nolanaceae).--Plants covered up in the greenhouse, +yielded seeds by weight compared with uncovered plants, the flowers of +which were visited by many bees, in the ratio of 100 to 61. + +Ajuga reptans (Labiatae).--Set a good many seeds; but none of the stems +under a net produced so many as several uncovered stems growing closely +by. + +Euphrasia officinalis (Scrophulariaceae).--Covered-up plants produced +plenty of seed; whether less than the exposed plants I cannot say. I saw +two small Dipterous insects (Dolichopos nigripennis and Empis chioptera) +repeatedly sucking the flowers; as they crawled into them, they rubbed +against the bristles which project from the anthers, and became dusted +with pollen. + +Veronica agrestis (Scrophulariaceae).--Covered-up plants produced an +abundance of seeds. I do not know whether any insects visit the flowers; +but I have observed Syrphidae repeatedly covered with pollen visiting +the flowers of V. hederaefolia and chamoedrys. + +Mimulus luteus (Scrophulariaceae).--Highly self-fertile. + +Calceolaria (greenhouse variety) (Scrophulariaceae).--Highly +self-fertile. + +Verbascum thapsus (Scrophulariaceae).--Highly self-fertile. + +Verbascum lychnitis.--Highly self-fertile. + +Vandellia nummularifolia (Scrophulariaceae).--Perfect flowers produce a +good many capsules. + +Bartsia odontites (Scrophulariaceae).--Covered-up plants produced a good +many seeds; but several of these were shrivelled, nor were they so +numerous as those produced by unprotected plants, which were incessantly +visited by hive and humble-bees. + +Specularia speculum (Lobeliaceae).--Covered plants produced almost as +many capsules as the uncovered. + +Lactuca sativa (Compositae).--Covered plants produced some seeds, but +the summer was wet and unfavourable. + +Galium aparine (Rubiaceae).--Covered plants produced quite as many seeds +as the uncovered. + +Apium petroselinum (Umbelliferae).--Covered plants apparently were as +productive as the uncovered. + +Zea mays (Gramineae).--A single plant in the greenhouse produced a good +many grains. + +Canna warscewiczi (Marantaceae).--Highly self-fertile. + +Orchidaceae.--In Europe Ophrys apifera is as regularly self-fertilised +as is any cleistogene flower. In the United States, South Africa, and +Australia there are a few species which are perfectly self-fertile. +These several cases are given in the second edition of my work on the +Fertilisation of Orchids. + +Allium cepa (blood red var.) (Liliaceae).--Four flower-heads were +covered with a net, and they produced somewhat fewer and smaller +capsules than those on the uncovered heads. The capsules were counted on +one uncovered head, and were 289 in number; whilst those on a fine head +from under the net were only 199.] + +Each of these lists contains by a mere accident the same number of +genera, namely, forty-nine. The genera in the first list include +sixty-five species, and those in the second sixty species; the Orchideae +in both being excluded. If the genera in this latter order, as well as +in the Asclepiadae and Apocynaceae, had been included, the number of +species which are sterile if insects are excluded would have been +greatly increased; but the lists are confined to species which were +actually experimented on. The results can be considered as only +approximately accurate, for fertility is so variable a character, that +each species ought to have been tried many times. The above number of +species, namely, 125, is as nothing to the host of living plants; but +the mere fact of more than half of them being sterile within the +specified degree, when insects are excluded, is a striking one; for +whenever pollen has to be carried from the anthers to the stigma in +order to ensure full fertility, there is at least a good chance of +cross-fertilisation. I do not, however, believe that if all known plants +were tried in the same manner, half would be found to be sterile within +the specified limits; for many flowers were selected for experiment +which presented some remarkable structure; and such flowers often +require insect-aid. Thus out of the forty-nine genera in the first list, +about thirty-two have flowers which are asymmetrical or present some +remarkable peculiarity; whilst in the second list, including species +which are fully or moderately fertile when insects were excluded, only +about twenty-one out of the forty-nine are asymmetrical or present any +remarkable peculiarity. + +MEANS OF CROSS-FERTILISATION. + +The most important of all the means by which pollen is carried from the +anthers to the stigma of the same flower, or from flower to flower, are +insects, belonging to the orders of Hymenoptera, Lepidoptera, and +Diptera; and in some parts of the world, birds. (10/1. I will here give +all the cases known to me of birds fertilising flowers. In South Brazil, +humming-birds certainly fertilise the various species of Abutilon, which +are sterile without their aid (Fritz Muller 'Jenaische Zeitschrift f. +Naturwiss.' B. 7 1872 page 24.) Long-beaked humming-birds visit the +flowers of Brugmansia, whilst some of the short-beaked species often +penetrate its large corolla in order to obtain the nectar in an +illegitimate manner, in the same manner as do bees in all parts of the +world. It appears, indeed, that the beaks of humming-birds are specially +adapted to the various kinds of flowers which they visit: on the +Cordillera they suck the Salviae, and lacerate the flowers of the +Tacsoniae; in Nicaragua, Mr. Belt saw them sucking the flowers of +Marcgravia and Erythina, and thus they carried pollen from flower to +flower. In North America they are said to frequent the flowers of +Impatiens: (Gould 'Introduction to the Trochilidae' 1861 pages 15, 120; +'Gardeners' Chronicle' 1869 page 389; 'The Naturalist in Nicaragua' page +129; 'Journal of the Linnean Society Botany' volume 13 1872 page 151.) I +may add that I often saw in Chile a Mimus with its head yellow with +pollen from, as I believe, a Cassia. I have been assured that at the +Cape of Good Hope, Strelitzia is fertilised by the Nectarinidae. There +can hardly be a doubt that many Australian flowers are fertilised by the +many honey-sucking birds of that country. Mr. Wallace remarks (address +to the Biological Section, British Association 1876) that he has "often +observed the beaks and faces of the brush-tongued lories of the Moluccas +covered with pollen." In New Zealand, many specimens of the Anthornis +melanura had their heads coloured with pollen from the flowers of an +endemic species of Fuchsia (Potts 'Transactions of the New Zealand +Institute' volume 3 1870 page 72.) Next in importance, but in a quite +subordinate degree, is the wind; and with some aquatic plants, according +to Delpino, currents of water. The simple fact of the necessity in many +cases of extraneous aid for the transport of the pollen, and the many +contrivances for this purpose, render it highly probable that some great +benefit is thus gained; and this conclusion has now been firmly +established by the proved superiority in growth, vigour, and fertility +of plants of crossed parentage over those of self-fertilised parentage. +But we should always keep in mind that two somewhat opposed ends have to +be gained; the first and more important one being the production of +seeds by any means, and the second, cross-fertilisation. + +The advantages derived from cross-fertilisation throw a flood of light +on most of the chief characters of flowers. We can thus understand their +large size and bright colours, and in some cases the bright tints of the +adjoining parts, such as the peduncles, bracteae, etc. By this means +they are rendered conspicuous to insects, on the same principle that +almost every fruit which is devoured by birds presents a strong contrast +in colour with the green foliage, in order that it may be seen, and its +seeds freely disseminated. With some flowers conspicuousness is gained +at the expense even of the reproductive organs, as with the ray-florets +of many Compositae, the exterior flowers of Hydrangea, and the terminal +flowers of the Feather-hyacinth or Muscari. There is also reason to +believe, and this was the opinion of Sprengel, that flowers differ in +colour in accordance with the kinds of insects which frequent them. + +Not only do the bright colours of flowers serve to attract insects, but +dark-coloured streaks and marks are often present, which Sprengel long +ago maintained served as guides to the nectary. These marks follow the +veins in the petals, or lie between them. They may occur on only one, or +on all excepting one or more of the upper or lower petals; or they may +form a dark ring round the tubular part of the corolla, or be confined +to the lips of an irregular flower. In the white varieties of many +flowers, such as of Digitalis purpurea, Antirrhinum majus, several +species of Dianthus, Phlox, Myosotis, Rhododendron, Pelargonium, Primula +and Petunia, the marks generally persist, whilst the rest of the corolla +has become of a pure white; but this may be due merely to their colour +being more intense and thus less readily obliterated. Sprengel's notion +of the use of these marks as guides appeared to me for a long time +fanciful; for insects, without such aid, readily discover and bite holes +through the nectary from the outside. They also discover the minute +nectar-secreting glands on the stipules and leaves of certain plants. +Moreover, some few plants, such as certain poppies, which are not +nectariferous, have guiding marks; but we might perhaps expect that some +few plants would retain traces of a former nectariferous condition. On +the other hand, these marks are much more common on asymmetrical +flowers, the entrance into which would be apt to puzzle insects, than on +regular flowers. Sir J. Lubbock has also proved that bees readily +distinguish colours, and that they lose much time if the position of +honey which they have once visited be in the least changed. (10/2. +'British Wild Flowers in relation to Insects' 1875 page 44.) The +following case affords, I think, the best evidence that these marks have +really been developed in correlation with the nectary. The two upper +petals of the common Pelargonium are thus marked near their bases; and I +have repeatedly observed that when the flowers vary so as to become +peloric or regular, they lose their nectaries and at the same time the +dark marks. When the nectary is only partially aborted, only one of the +upper petals loses its mark. Therefore the nectary and these marks +clearly stand in some sort of close relation to one another; and the +simplest view is that they were developed together for a special +purpose; the only conceivable one being that the marks serve as a guide +to the nectary. It is, however, evident from what has been already said, +that insects could discover the nectar without the aid of guiding marks. +They are of service to the plant, only by aiding insects to visit and +suck a greater number of flowers within a given time than would +otherwise be possible; and thus there will be a better chance of +fertilisation by pollen brought from a distinct plant, and this we know +is of paramount importance. + +The odours emitted by flowers attract insects, as I have observed in the +case of plants covered by a muslin net. Nageli affixed artificial +flowers to branches, scenting some with essential oils and leaving +others unscented; and insects were attracted to the former in an +unmistakable manner. (10/3. 'Enstehung etc. der Naturhist. Art.' 1865 +page 23.) Not a few flowers are both conspicuous and odoriferous. Of all +colours, white is the prevailing one; and of white flowers a +considerably larger proportion smell sweetly than of any other colour, +namely, 14.6 per cent; of red, only 8.2 per cent are odoriferous. (10/4. +The colours and odours of the flowers of 4200 species have been +tabulated by Landgrabe and by Schubler and Kohler. I have not seen their +original works, but a very full abstract is given in Loudon's +'Gardeners' Magazine' volume 13 1837 page 367.) The fact of a larger +proportion of white flowers smelling sweetly may depend in part on those +which are fertilised by moths requiring the double aid of +conspicuousness in the dusk and of odour. So great is the economy of +nature, that most flowers which are fertilised by crepuscular or +nocturnal insects emit their odour chiefly or exclusively in the +evening. Some flowers, however, which are highly odoriferous depend +solely on this quality for their fertilisation, such as the +night-flowering stock (Hesperis) and some species of Daphne; and these +present the rare case of flowers which are fertilised by insects being +obscurely coloured. + +The storage of a supply of nectar in a protected place is manifestly +connected with the visits of insects. So is the position which the +stamens and pistils occupy, either permanently or at the proper period +through their own movements; for when mature they invariably stand in +the pathway leading to the nectary. The shape of the nectary and of the +adjoining parts are likewise related to the particular kinds of insects +which habitually visit the flowers; this has been well shown by Hermann +Muller by his comparison of lowland species which are chiefly visited by +bees, with alpine species belonging to the same genera which are visited +by butterflies. (10/5. 'Nature' 1874 page 110, 1875 page 190, 1876 pages +210, 289.) Flowers may also be adapted to certain kinds of insects, by +secreting nectar particularly attractive to them, and unattractive to +other kinds; of which fact Epipactis latifolia offers the most striking +instance known to me, as it is visited exclusively by wasps. Structures +also exist, such as the hairs within the corolla of the fox glove +(Digitalis), which apparently serve to exclude insects that are not well +fitted to bring pollen from one flower to another. (10/6. Belt 'The +Naturalist in Nicaragua' 1874 page 132.) I need say nothing here of the +endless contrivances, such as the viscid glands attached to the +pollen-masses of the Orchideae and Asclepiadae, or the viscid or +roughened state of the pollen-grains of many plants, or the irritability +of their stamens which move when touched by insects etc.--as all these +contrivances evidently favour or ensure cross-fertilisation. + +All ordinary flowers are so far open that insects can force an entrance +into them, notwithstanding that some, like the Snapdragon (Antirrhinum), +various Papilionaceous and Fumariaceous flowers, are in appearance +closed. It cannot be maintained that their openness is necessary for +fertility, as cleistogene flowers which are permanently closed yield a +full complement of seeds. Pollen contains much nitrogen and +phosphorus--the two most precious of all the elements for the growth of +plants--but in the case of most open flowers, a large quantity of pollen +is consumed by pollen-devouring insects, and a large quantity is +destroyed during long-continued rain. With many plants this latter evil +is guarded against, as far as is possible, by the anthers opening only +during dry weather (10/7. Mr. Blackley observed that the ripe anthers of +rye did not dehisce whilst kept under a bell-glass in a damp atmosphere, +whilst other anthers exposed to the same temperature in the open air +dehisced freely. He also found much more pollen adhering to the sticky +slides, which were attached to kites and sent high up in the atmosphere, +during the first fine and dry days after wet weather, than at other +times: 'Experimental Researches on Hay Fever' 1873 page 127.)--by the +position and form of some or all of the petals,--by the presence of +hairs, etc., and as Kerner has shown in his interesting essay, by the +movements of the petals or of the whole flower during cold and wet +weather. (10/8. 'Die Schutzmittel des Pollens' 1873.) In order to +compensate the loss of pollen in so many ways, the anthers produce a far +larger amount than is necessary for the fertilisation of the same +flower. I know this from my own experiments on Ipomoea, given in the +Introduction; and it is still more plainly shown by the astonishingly +small quantity produced by cleistogene flowers, which lose none of their +pollen, in comparison with that produced by the open flowers borne by +the same plants; and yet this small quantity suffices for the +fertilisation of all their numerous seeds. Mr. Hassall took pains in +estimating the number of pollen-grains produced by a flower of the +Dandelion (Leontodon), and found the number to be 243,600, and in a +Paeony 3,654,000 grains. (10/9. 'Annals and Magazine of Natural History' +volume 8 1842 page 108.) The editor of the 'Botanical Register' counted +the ovules in the flowers of Wistaria sinensis, and carefully estimated +the number of pollen-grains, and he found that for each ovule there were +7000 grains. (10/10. Quoted in 'Gardeners' Chronicle' 1846 page 771.) +With Mirabilis, three or four of the very large pollen-grains are +sufficient to fertilise an ovule; but I do not know how many grains a +flower produces. With Hibiscus, Kolreuter found that sixty grains were +necessary to fertilise all the ovules of a flower, and he calculated +that 4863 grains were produced by a single flower, or eighty-one times +too many. With Geum urbanum, however, according to Gartner, the pollen +is only ten times too much. (10/11. Kolreuter 'Vorlaufige Nachricht' +1761 page 9. Gartner 'Beitrage zur Kenntniss' etc. page 346.) As we thus +see that the open state of all ordinary flowers, and the consequent loss +of much pollen, necessitate the development of so prodigious an excess +of this precious substance, why, it may be asked, are flowers always +left open? As many plants exist throughout the vegetable kingdom which +bear cleistogene flowers, there can hardly be a doubt that all open +flowers might easily have been converted into closed ones. The graduated +steps by which this process could have been effected may be seen at the +present time in Lathyrus nissolia, Biophytum sensitivum, and several +other plants. The answer to the above question obviously is, that with +permanently closed flowers there could be no cross-fertilisation. + +The frequency, almost regularity, with which pollen is transported by +insects from flower to flower, often from a considerable distance, well +deserves attention. (10/12. An experiment made by Kolreuter 'Forsetsung' +etc. 1763 page 69, affords good evidence on this head. Hibiscus +vesicarius is strongly dichogamous, its pollen being shed before the +stigmas are mature. Kolreuter marked 310 flowers, and put pollen from +other flowers on their stigmas every day, so that they were thoroughly +fertilised; and he left the same number of other flowers to the agency +of insects. Afterwards he counted the seeds of both lots: the flowers +which he had fertilised with such astonishing care produced 11,237 +seeds, whilst those left to the insects produced 10,886; that is, a less +number by only 351; and this small inferiority is fully accounted for by +the insects not having worked during some days, when the weather was +cold with continued rain.) This is best shown by the impossibility in +many cases of raising two varieties of the same species pure, if they +grow at all near together; but to this subject I shall presently return; +also by the many cases of hybrids which have appeared spontaneously both +in gardens and a state of nature. With respect to the distance from +which pollen is often brought, no one who has had any experience would +expect to obtain pure cabbage-seed, for instance, if a plant of another +variety grew within two or three hundred yards. An accurate observer, +the late Mr. Masters of Canterbury, assured me that he once had his +whole stock of seeds "seriously affected with purple bastards," by some +plants of purple kale which flowered in a cottager's garden at the +distance of half a mile; no other plant of this variety growing any +nearer. (10/13. Mr. W.C. Marshall caught no less than seven specimens of +a moth (Cucullia umbratica) with the pollinia of the butterfly-orchis +(Habenaria chlorantha) sticking to their eyes, and, therefore, in the +proper position for fertilising the flowers of this species, on an +island in Derwentwater, at the distance of half a mile from any place +where this plant grew: 'Nature' 1872 page 393.) But the most striking +case which has been recorded is that by M. Godron, who shows by the +nature of the hybrids produced that Primula grandiflora must have been +crossed with pollen brought by bees from P. officinalis, growing at the +distance of above two kilometres, or of about one English mile and a +quarter. (10/14. 'Revue des Sc. Nat.' 1875 page 331.) + +All those who have long attended to hybridisation, insist in the +strongest terms on the liability of castrated flowers to be fertilised +by pollen brought from distant plants of the same species. (10/15. See, +for instance, the remarks by Herbert 'Amaryllidaceae' 1837 page 349. +Also Gartner's strong expressions on this subject in his +'Bastarderzeugung' 1849 page 670 and 'Kenntniss der Befruchtung' 1844 +pages 510, 573. Also Lecoq 'De la Fecondation' etc. 1845 page 27. Some +statements have been published during late years of the extraordinary +tendency of hybrid plants to revert to their parent forms; but as it is +not said how the flowers were protected from insects, it may be +suspected that they were often fertilised with pollen brought from a +distance from the parent-species.) The following case shows this in the +clearest manner: Gartner, before he had gained much experience, +castrated and fertilised 520 flowers on various species with pollen of +other genera or other species, but left them unprotected; for, as he +says, he thought it a laughable idea that pollen should be brought from +flowers of the same species, none of which grew nearer than between 500 +and 600 yards. (10/16. 'Kenntniss der Befruchtung' pages 539, 550, 575, +576.) The result was that 289 of these 520 flowers yielded no seed, or +none that germinated; the seed of 29 flowers produced hybrids, such as +might have been expected from the nature of the pollen employed; and +lastly, the seed of the remaining 202 flowers produced perfectly pure +plants, so that these flowers must have been fertilised by pollen +brought by insects from a distance of between 500 and 600 yards. (10/17. +Henschel's experiments quoted by Gartner 'Kenntniss' etc. page 574, +which are worthless in all other respects, likewise show how largely +flowers are intercrossed by insects. He castrated many flowers on +thirty-seven species, belonging to twenty-two genera, and put on their +stigmas either no pollen, or pollen from distinct genera, yet they all +seeded, and all the seedlings raised from them were of course pure.) It +is of course possible that some of these 202 flowers might have been +fertilised by pollen left accidentally in them when they were castrated; +but to show how improbable this is, I may add that Gartner, during the +next eighteen years, castrated no less than 8042 flowers and hybridised +them in a closed room; and the seeds from only seventy of these, that is +considerably less than 1 per cent, produced pure or unhybridised +offspring. (10/18. 'Kenntniss' etc. pages 555, 576.) + +From the various facts now given, it is evident that most flowers are +adapted in an admirable manner for cross-fertilisation. Nevertheless, +the greater number likewise present structures which are manifestly +adapted, though not in so striking a manner, for self-fertilisation. The +chief of these is their hermaphrodite condition; that is, their +including within the same corolla both the male and female reproductive +organs. These often stand close together and are mature at the same +time; so that pollen from the same flower cannot fail to be deposited at +the proper period on the stigma. There are also various details of +structure adapted for self-fertilisation. (10/19. Hermann Muller 'Die +Befruchtung' etc. page 448.) Such structures are best shown in those +curious cases discovered by Hermann Muller, in which a species exists +under two forms,--one bearing conspicuous flowers fitted for +cross-fertilisation, and the other smaller flowers fitted for +self-fertilisation, with many parts in the latter slightly modified for +this special purpose. (10/20. 'Nature' 1873 pages 44, 433.) + +As two objects in most respects opposed, namely, cross-fertilisation and +self-fertilisation, have in many cases to be gained, we can understand +the co-existence in so many flowers of structures which appear at first +sight unnecessarily complex and of an opposed nature. We can thus +understand the great contrast in structure between cleistogene flowers, +which are adapted exclusively for self-fertilisation, and ordinary +flowers on the same plant, which are adapted so as to allow of at least +occasional cross-fertilisation. (10/21. Fritz Muller has discovered in +the animal kingdom 'Jenaische Zeitschr.' B. 4 page 451, a case curiously +analogous to that of the plants which bear cleistogene and perfect +flowers. He finds in the nests of termites in Brazil, males and females +with imperfect wings, which do not leave the nests and propagate the +species in a cleistogene manner, but only if a fully-developed queen +after swarming does not enter the old nest. The fully-developed males +and females are winged, and individuals from distinct nests can hardly +fail often to intercross. In the act of swarming they are destroyed in +almost infinite numbers by a host of enemies, so that a queen may often +fail to enter an old nest; and then the imperfectly developed males and +females propagate and keep up the stock.) The former are always minute, +completely closed, with their petals more or less rudimentary and never +brightly coloured; they never secrete nectar, never are odoriferous, +have very small anthers which produce only a few grains of pollen, and +their stigmas are but little developed. Bearing in mind that some +flowers are cross-fertilised by the wind (called anemophilous by +Delpino), and others by insects (called entomophilous), we can further +understand, as was pointed out by me several years ago, the great +contrast in appearance between these two classes of flowers. (10/22. +'Journal of the Linnean Society' volume 7 Botany 1863 page 77.) +Anemophilous flowers resemble in many respects cleistogene flowers, but +differ widely in not being closed, in producing an extraordinary amount +of pollen which is always incoherent, and in the stigma often being +largely developed or plumose. We certainly owe the beauty and odour of +our flowers and the storage of a large supply of honey to the existence +of insects. + +ON THE RELATION BETWEEN THE STRUCTURE AND CONSPICUOUSNESS OF FLOWERS, +THE VISITS OF INSECTS, AND THE ADVANTAGES OF CROSS-FERTILISATION. + +It has already been shown that there is no close relation between the +number of seeds produced by flowers when crossed and self-fertilised, +and the degree to which their offspring are aaffected by the two +processes. I have also given reasons for believing that the inefficiency +of a plant's own pollen is in most cases an incidental result, or has +not been specially acquired for the sake of preventing +self-fertilisation. On the other hand, there can hardly be a doubt that +dichogamy, which prevails according to Hildebrand in the greater number +of species (10/23. 'Die Geschlecter Vertheiling' etc. page 32.),--that +the heterostyled condition of certain plants,--and that many mechanical +structures--have all been acquired so as both to check +self-fertilisation and to favour cross-fertilisation. The means for +favouring cross-fertilisation must have been acquired before those which +prevent self-fertilisation; as it would manifestly be injurious to a +plant that its stigma should fail to receive its own pollen, unless it +had already become well adapted for receiving pollen from another +individual. It should be observed that many plants still possess a high +power of self-fertilisation, although their flowers are excellently +constructed for cross-fertilisation--for instance, those of many +papilionaceous species. + +It may be admitted as almost certain that some structures, such as a +narrow elongated nectary, or a long tubular corolla, have been developed +in order that certain kinds of insects alone should obtain the nectar. +These insects would thus find a store of nectar preserved from the +attacks of other insects; and they would thus be led to visit frequently +such flowers and to carry pollen from one to the other. (10/24. See the +interesting discussion on this subject by Hermann Muller, 'Die +Befruchtung' etc. page 431.) It might perhaps have been expected that +plants having their flowers thus peculiarly constructed would profit in +a greater degree by being crossed, than ordinary or simple flowers; but +this does not seem to hold good. Thus Tropaeolum minus has a long +nectary and an irregular corolla, whilst Limnanthes douglasii has a +regular flower and no proper nectary, yet the crossed seedlings of both +species are to the self-fertilised in height as 100 to 79. Salvia +coccinea has an irregular corolla, with a curious apparatus by which +insects depress the stamens, while the flowers of Ipomoea are regular; +and the crossed seedlings of the former are in height to the +self-fertilised as 100 to 76, whilst those of the Ipomoea are as 100 to +77. Fagopyrum is dimorphic, and Anagallis collina is non-dimorphic, and +the crossed seedlings of both are in height to the self-fertilised as +100 to 69. + +With all European plants, excepting the comparatively rare anemophilous +kinds, the possibility of distinct individuals intercrossing depends on +the visits of insects; and Hermann Muller has proved by his valuable +observations, that large conspicuous flowers are visited much more +frequently and by many more kinds of insects, than are small +inconspicuous flowers. He further remarks that the flowers which are +rarely visited must be capable of self-fertilisation, otherwise they +would quickly become extinct. (10/25. 'Die Befruchtung' etc. page 426. +'Nature' 1873 page 433.) There is, however, some liability to error in +forming a judgment on this head, from the extreme difficulty of +ascertaining whether flowers which are rarely or never visited during +the day (as in the above given case of Fumaria capreolata) are not +visited by small nocturnal Lepidoptera, which are known to be strongly +attracted by sugar. (10/26. In answer to a question by me, the editor of +an entomological journal writes--"The Depressariae, as is notorious to +every collector of Noctuae, come very freely to sugar, and no doubt +naturally visit flowers:" the 'Entomologists' Weekly Intelligencer' 1860 +page 103.) The two lists given in the early part of this chapter support +Muller's conclusion that small and inconspicuous flowers are completely +self-fertile: for only eight or nine out of the 125 species in the two +lists come under this head, and all of these were proved to be highly +fertile when insects were excluded. The singularly inconspicuous flowers +of the Fly Ophrys (O. muscifera), as I have elsewhere shown, are rarely +visited by insects; and it is a strange instance of imperfection, in +contradiction to the above rule, that these flowers are not +self-fertile, so that a large proportion of them do not produce seeds. +The converse of the rule that plants bearing small and inconspicuous +flowers are self-fertile, namely, that plants with large and conspicuous +flowers are self-sterile, is far from true, as may be seen in our second +list of spontaneously self-fertile species; for this list includes such +species as Ipomoea purpurea, Adonis aestivalis, Verbascum thapsus, Pisum +sativum, Lathyrus odoratus, some species of Papaver and of Nymphaea, and +others. + +The rarity of the visits of insects to small flowers, does not depend +altogether on their inconspicuousness, but likewise on the absence of +some sufficient attraction; for the flowers of Trifolium arvense are +extremely small, yet are incessantly visited by hive and humble-bees, as +are the small and dingy flowers of the asparagus. The flowers of Linaria +cymbalaria are small and not very conspicuous, yet at the proper time +they are freely visited by hive-bees. I may add that, according to Mr. +Bennett, there is another and quite distinct class of plants which +cannot be much frequented by insects, as they flower either exclusively +or often during the winter, and these seem adapted for +self-fertilisation, as they shed their pollen before the flowers expand. +(10/27. 'Nature' 1869 page 11.) + +That many flowers have been rendered conspicuous for the sake of guiding +insects to them is highly probable or almost certain; but it may be +asked, have other flowers been rendered inconspicuous so that they may +not be frequently visited, or have they merely retained a former and +primitive condition? If a plant were much reduced in size, so probably +would be the flowers through correlated growth, and this may possibly +account for some cases; but the size and colour of the corolla are both +extremely variable characters, and it can hardly be doubted that if +large and brightly-coloured flowers were advantageous to any species, +these could be acquired through natural selection within a moderate +lapse of time, as indeed we see with most alpine plants. Papilionaceous +flowers are manifestly constructed in relation to the visits of insects, +and it seems improbable, from the usual character of the group, that the +progenitors of the genera Vicia and Trifolium produced such minute and +unattractive flowers as those of V. hirsuta and T. procumbens. We are +thus led to infer that some plants either have not had their flowers +increased in size, or have actually had them reduced and purposely +rendered inconspicuous, so that they are now but little visited by +insects. In either case they must also have acquired or retained a high +degree of self-fertility. + +If it became from any cause advantageous to a species to have its +capacity for self-fertilisation increased, there is little difficulty in +believing that this could readily be effected; for three cases of plants +varying in such a manner as to be more fertile with their own pollen +than they originally were, occurred in the course of my few experiments, +namely, with Mimulus, Ipomoea, and Nicotiana. Nor is there any reason to +doubt that many kinds of plants are capable under favourable +circumstances of propagating themselves for very many generations by +self-fertilisation. This is the case with the varieties of Pisum sativum +and of Lathyrus odoratus which are cultivated in England, and with +Ophrys apifera and some other plants in a state of nature. Nevertheless, +most or all of these plants retain structures in an efficient state +which cannot be of the least use excepting for cross-fertilisation. We +have also seen reason to suspect that self-fertilisation is in some +peculiar manner beneficial to certain plants; but if this be really the +case, the benefit thus derived is far more than counter-balanced by a +cross with a fresh stock or with a slightly different variety. + +Notwithstanding the several considerations just advanced, it seems to me +highly improbable that plants bearing small and inconspicuous flowers +have been or should continue to be subjected to self-fertilisation for a +long series of generations. I think so, not from the evil which +manifestly follows from self-fertilisation, in many cases even in the +first generation, as with Viola tricolor, Sarothamnus, Nemophila, +Cyclamen, etc.; nor from the probability of the evil increasing after +several generations, for on this latter head I have not sufficient +evidence, owing to the manner in which my experiments were conducted. +But if plants bearing small and inconspicuous flowers were not +occasionally intercrossed, and did not profit by the process, all their +flowers would probably have been rendered cleistogene, as they would +thus have largely benefited by having to produce only a small quantity +of safely-protected pollen. In coming to this conclusion, I have been +guided by the frequency with which plants belonging to distinct orders +have been rendered cleistogene. But I can hear of no instance of a +species with all its flowers rendered permanently cleistogene. Leersia +makes the nearest approach to this state; but as already stated, it has +been known to produce perfect flowers in one part of Germany. Some other +plants of the cleistogene class, for instance Aspicarpa, have failed to +produce perfect flowers during several years in a hothouse; but it does +not follow that they would fail to do so in their native country, any +more than with Vandellia, which with me produced only cleistogene +flowers during certain years. Plants belonging to this class commonly +bear both kinds of flowers every season, and the perfect flowers of +Viola canina yield fine capsules, but only when visited by bees. We have +also seen that the seedlings of Ononis minutissima, raised from the +perfect flowers fertilised with pollen from another plant, were finer +than those from self-fertilised flowers; and this was likewise the case +to a certain extent with Vandellia. As therefore no species which at one +time bore small and inconspicuous flowers has had all its flowers +rendered cleistogene, I must believe that plants now bearing small and +inconspicuous flowers profit by their still remaining open, so as to be +occasionally intercrossed by insects. It has been one of the greatest +oversights in my work that I did not experimentise on such flowers, +owing to the difficulty of fertilising them, and to my not having seen +the importance of the subject. (10/28. Some of the species of Solanum +would be good ones for such experiments, for they are said by Hermann +Muller 'Befruchtung' page 434, to be unattractive to insects from not +secreting nectar, not producing much pollen, and not being very +conspicuous. Hence probably it is that, according to Verlot 'Production +des Varieties' 1865 page 72, the varieties of "les aubergines et les +tomates" (species of Solanum) do not intercross when they are cultivated +near together; but it should be remembered that these are not endemic +species. On the other hand, the flowers of the common potato (S. +tuberosum), though they do not secrete nectar Kurr 'Bedeutung der +Nektarien' 1833 page 40, yet cannot be considered as inconspicuous, and +they are sometimes visited by diptera (Muller), and, as I have seen, by +humble-bees. Tinzmann (as quoted in 'Gardeners' Chronicle' 1846 page +183, found that some of the varieties did not bear seed when fertilised +with pollen from the same variety, but were fertile with that from +another variety.) + +It should be remembered that in two of the cases in which highly +self-fertile varieties appeared amongst my experimental plants, namely, +with Mimulus and Nicotiana, such varieties were greatly benefited by a +cross with a fresh stock or with a slightly different variety; and this +likewise was the case with the cultivated varieties of Pisum sativum and +Lathyrus odoratus, which have been long propagated by +self-fertilisation. Therefore until the contrary is distinctly proved, I +must believe that as a general rule small and inconspicuous flowers are +occasionally intercrossed by insects; and that after long-continued +self-fertilisation, if they are crossed with pollen brought from a plant +growing under somewhat different conditions, or descended from one thus +growing, their offspring would profit greatly. It cannot be admitted, +under our present state of knowledge, that self-fertilisation continued +during many successive generations is ever the most beneficial method of +reproduction. + +THE MEANS WHICH FAVOUR OR ENSURE FLOWERS BEING FERTILISED WITH POLLEN +FROM A DISTINCT PLANT. + +We have seen in four cases that seedlings raised from a cross between +flowers on the same plant, even on plants appearing distinct from having +been propagated by stolons or cuttings, were not superior to seedlings +from self-fertilised flowers; and in a fifth case (Digitalis) superior +only in a slight degree. Therefore we might expect that with plants +growing in a state of nature a cross between the flowers on distinct +individuals, and not merely between the flowers on the same plant, would +generally or often be effected by some means. The fact of bees and of +some Diptera visiting the flowers of the same species as long as they +can, instead of promiscuously visiting various species, favours the +intercrossing of distinct plants. On the other hand, insects usually +search a large number of flowers on the same plant before they fly to +another, and this is opposed to cross-fertilisation. The extraordinary +number of flowers which bees are able to search within a very short +space of time, as will be shown in a future chapter, increases the +chance of cross-fertilisation; as does the fact that they are not able +to perceive without entering a flower whether other bees have exhausted +the nectar. For instance, Hermann Muller found that four-fifths of the +flowers of Lamium album which a humble-bee visited had been already +exhausted of their nectar. (10/29. 'Die Befruchtung' etc. page 311.) In +order that distinct plants should be intercrossed, it is of course +indispensable that two or more individuals should grow near one another; +and this is generally the case. Thus A. de Candolle remarks that in +ascending a mountain the individuals of the same species do not commonly +disappear near its upper limit quite gradually, but rather abruptly. +This fact can hardly be explained by the nature of the conditions, as +these graduate away in an insensible manner, and it probably depends in +large part on vigorous seedlings being produced only as high up the +mountain as many individuals can subsist together. + +With respect to dioecious plants, distinct individuals must always +fertilise each other. With monoecious plants, as pollen has to be +carried from flower to flower, there will always be a good chance of its +being carried from plant to plant. Delpino has also observed the curious +fact that certain individuals of the monoecious walnut (Juglans regia) +are proterandrous, and others proterogynous, and these will reciprocally +fertilise each other. (10/30. 'Ult. Osservazioni' etc. part 2 fasc 2 +page 337.) So it is with the common nut (Corylus avellana) (10/31. +'Nature' 1875 page 26.), and, what is more surprising, with some few +hermaphrodite plants, as observed by Hermann Muller. (10/32. 'Die +Befruchtung' etc. pages 285, 339.) These latter plants cannot fail to +act on each other like dimorphic or trimorphic species, in which the +union of two individuals is necessary for full and normal fertility. +With ordinary hermaphrodite species, the expansion of only a few flowers +at the same time is one of the simplest means for favouring the +intercrossing of distinct individuals; but this would render the plants +less conspicuous to insects, unless the flowers were of large size, as +in the case of several bulbous plants. Kerner thinks that it is for this +object that the Australian Villarsia parnassifolia produces daily only a +single flower. (10/33. 'Die Schutzmittel' etc page 23.) Mr. Cheeseman +also remarks, that as certain Orchids in New Zealand which require +insect-aid for their fertilisation bear only a single flower, distinct +plants cannot fail to intercross. (10/34. 'Transactions of the New +Zealand Institute' volume 5 1873 page 356.) + +Dichogamy, which prevails so extensively throughout the vegetable +kingdom, much increases the chance of distinct individuals +intercrossing. With proterandrous species, which are far more ccommon +than proterogynous, the young flowers are exclusively male in function, +and the older ones exclusively female; and as bees habitually alight low +down on the spikes of flowers in order to crawl upwards, they get dusted +with pollen from the uppermost flowers, which they carry to the stigmas +of the lower and older flowers on the next spike which they visit. The +degree to which distinct plants will thus be intercrossed depends on the +number of spikes in full flower at the same time on the same plant. With +proterogynous flowers and with depending racemes, the manner in which +insects visit the flowers ought to be reversed in order that distinct +plants should be intercrossed. But this whole subject requires further +investigation, as the great importance of crosses between distinct +individuals, instead of merely between distinct flowers, has hitherto +been hardly recognised. + +In some few cases the special movements of certain organs almost ensure +pollen being carried from plant to plant. Thus with many orchids, the +pollen-masses after becoming attached to the head or proboscis of an +insect do not move into the proper position for striking the stigma, +until ample time has elapsed for the insect to fly to another plant. +With Spiranthes autumnalis, the pollen-masses cannot be applied to the +stigma until the labellum and rostellum have moved apart, and this +movement is very slow. (10/35. 'The Various Contrivances by which +British and Foreign Orchids are fertilised' first edition page 128.) +With Posoqueria fragrans (one of the Rubiaceae) the same end is gained +by the movement of a specially constructed stamen, as described by Fritz +Muller. + +We now come to a far more general and therefore more important means by +which the mutual fertilisation of distinct plants is effected, namely, +the fertilising power of pollen from another variety or individual being +greater than that of a plant's own pollen. The simplest and best known +case of prepotent action in pollen, though it does not bear directly on +our present subject, is that of a plant's own pollen over that from a +distinct species. If pollen from a distinct species be placed on the +stigma of a castrated flower, and then after the interval of several +hours, pollen from the same species be placed on the stigma, the effects +of the former are wholly obliterated, excepting in some rare cases. If +two varieties are treated in the same manner, the result is analogous, +though of directly opposite nature; for pollen from any other variety is +often or generally prepotent over that from the same flower. I will give +some instances: the pollen of Mimulus luteus regularly falls on the +stigma of its own flower, for the plant is highly fertile when insects +are excluded. Now several flowers on a remarkably constant whitish +variety were fertilised without being castrated with pollen from a +yellowish variety; and of the twenty-eight seedlings thus raised, every +one bore yellowish flowers, so that the pollen of the yellow variety +completely overwhelmed that of the mother-plant. Again, Iberis umbellata +is spontaneously self-fertile, and I saw an abundance of pollen from +their own flowers on the stigmas; nevertheless, of thirty seedlings +raised from non-castrated fflowers of a crimson variety crossed with +pollen from a pink variety, twenty-four bore pink flowers, like those of +the male or pollen-bearing parent. + +In these two cases flowers were fertilised with pollen from a distinct +variety, and this was shown to be prepotent by the character of the +offspring. Nearly similar results often follow when two or more +self-fertile varieties are allowed to grow near one another and are +visited by insects. The common cabbage produces a large number of +flowers on the same stalk, and when insects are excluded these set many +capsules, moderately rich in seeds. I planted a white Kohl-rabi, a +purple Kohl-rabi, a Portsmouth broccoli, a Brussels sprout, and a +Sugar-loaf cabbage near together and left them uncovered. Seeds +collected from each kind were sown in separate beds; and the majority of +the seedlings in all five beds were mongrelised in the most complicated +manner, some taking more after one variety, and some after another. The +effects of the Kohl-rabi were particularly plain in the enlarged stems +of many of the seedlings. Altogether 233 plants were raised, of which +155 were mongrelised in the plainest manner, and of the remaining 78 not +half were absolutely pure. I repeated the experiment by planting near +together two varieties of cabbage with purple-green and white-green +lacinated leaves; and of the 325 seedlings raised from the purple-green +variety, 165 had white-green and 160 purple-green leaves. Of the 466 +seedlings raised from the white-green variety, 220 had purple-green and +246 white-green leaves. These cases show how largely pollen from a +neighbouring variety of the cabbage effaces the action of the plant's +own pollen. We should bear in mind that pollen must be carried by the +bees from flower to flower on the same large branching stem much more +abundantly than from plant to plant; and in the case of plants the +flowers of which are in some degree dichogamous, those on the same stem +would be of different ages, and would thus be as ready for mutual +fertilisation as the flowers on distinct plants, were it not for the +prepotency of pollen from another variety. (10/36. A writer in the +'Gardeners' Chronicle' 1855 page 730, says that he planted a bed of +turnips (Brassica rapa) and of rape (B. napus) close together, and sowed +the seeds of the former. The result was that scarcely one seedling was +true to its kind, and several closely resembled rape.) + +Several varieties of the radish (Raphanus sativus), which is moderately +self-fertile when insects are excluded, were in flower at the same time +in my garden. Seed was collected from one of them, and out of twenty-two +seedlings thus raised only twelve were true to their kind. (10/37. +Duhamel as quoted by Godron 'De l'Espece' tome 2 page 50, makes an +analogous statement with respect to this plant.) + +The onion produces a large number of flowers, all crowded together into +a large globular head, each flower having six stamens; so that the +stigmas receive plenty of pollen from their own and the adjoining +anthers. Consequently the plant is fairly self-fertile when protected +from insects. A blood-red, silver, globe and Spanish onion were planted +near together; and seedlings were raised from each kind in four separate +beds. In all the beds mongrels of various kinds were numerous, except +amongst the ten seedlings from the blood-red onion, which included only +two. Altogether forty-six seedlings were raised, of which thirty-one had +been plainly crossed. + +A similar result is known to follow with the varieties of many other +plants, if allowed to flower near together: I refer here only to species +which are capable of fertilising themselves, for if this be not the +case, they would of course be liable to be crossed by any other variety +growing near. Horticulturists do not commonly distinguish between the +effects of variability and intercrossing; but I have collected evidence +on the natural crossing of varieties of the tulip, hyacinth, anemone, +ranunculus, strawberry, Leptosiphon androsaceus, orange, rhododendron +and rhubarb, all of which plants I believe to be self-fertile. (10/38. +With respect to tulips and some other flowers, see Godron 'De l'Espece' +tome 1 page 252. For anemones 'Gardeners' Chronicle' 1859 page 98. For +strawberries see Herbert in 'Transactions of the Horticultural Society' +volume 4 page 17. The same observer elsewhere speaks of the spontaneous +crossing of rhododendrons. Gallesio makes the same statement with +respect to oranges. I have myself known extensive crossing to occur with +the common rhubarb. For Leptosiphon, Verlot 'Des Varieties' 1865 page +20. I have not included in my list the Carnation, Nemophila, or +Antirrhinum, the varieties of which are known to cross freely, because +these plants are not always self-fertile. I know nothing about the +self-fertility of Trollius Lecoq 'De la Fecondation' 1862 page 93, +Mahonia, and Crinum, in which genera the species intercross largely. +With respect to Mahonia it is now scarcely possible to procure in this +country pure specimens of M. aquifolium or repens; and the various +species of Crinum sent by Herbert 'Amaryllidaceae' page 32, to Calcutta, +crossed there so freely that pure seed could not be saved.) Much other +indirect evidence could be given with respect to the extent to which +varieties of the same species spontaneously intercross. + +Gardeners who raise seed for sale are compelled by dearly bought +experience to take extraordinary precautions against intercrossing. Thus +Messrs. Sharp "have land engaged in the growth of seed in no less than +eight parishes." The mere fact of a vast number of plants belonging to +the same variety growing together is a considerable protection, as the +chances are strong in favour of plants of the same variety +intercrossing; and it is in chief part owing to this circumstance, that +certain villages have become famous for pure seed of particular +varieties. (10/39. With respect to Messrs. Sharp see 'Gardeners' +Chronicle' 1856 page 823. Lindley's 'Theory of Horticulture' page 319.) +Only two trials were made by me to ascertain after how long an interval +of time, pollen from a distinct variety would obliterate more or less +completely the action of a plant's own pollen. The stigmas in two lately +expanded flowers on a variety of cabbage, called Ragged Jack, were well +covered with pollen from the same plant. After an interval of +twenty-three hours, pollen from the Early Barnes Cabbage growing at a +distance was placed on both stigmas; and as the plant was left +uncovered, pollen from other flowers on the Ragged Jack would certainly +have been left by the bees during the next two or three days on the same +two stigmas. Under these circumstances it seemed very unlikely that the +pollen of the Barnes cabbage would produce any effect; but three out of +the fifteen plants raised from the two capsules thus produced were +plainly mongrelised: and I have no doubt that the twelve other plants +were affected, for they grew much more vigorously than the +self-fertilised seedlings from the Ragged Jack planted at the same time +and under the same conditions. Secondly, I placed on several stigmas of +a long-styled cowslip (Primula veris) plenty of pollen from the same +plant, and after twenty-four hours added some from a short-styled +dark-red Polyanthus, which is a variety of the cowslip. From the flowers +thus treated thirty seedlings were raised, and all these without +exception bore reddish flowers; so that the effect of the plant's own +pollen, though placed on the stigmas twenty-four hours previously, was +quite destroyed by that of the red variety. It should, however, be +observed that these plants are dimorphic, and that the second union was +a legitimate one, whilst the first was illegitimate; but flowers +illegitimately fertilised with their own pollen yield a moderately fair +supply of seeds. + +We have hitherto considered only the prepotent fertilising power of +pollen from a distinct variety over a plants' own pollen,--both kinds of +pollen being placed on the same stigma. It is a much more remarkable +fact that pollen from another individual of the same variety is +prepotent over a plant's own pollen, as shown by the superiority of the +seedlings raised from a cross of this kind over seedlings from +self-fertilised flowers. Thus in Tables 7/A, B, and C, there are at +least fifteen species which are self-fertile when insects are excluded; +and this implies that their stigmas must receive their own pollen; +nevertheless, most of the seedlings which were raised by fertilising the +non-castrated flowers of these fifteen species with pollen from another +plant were greatly superior, in height, weight, and fertility, to the +self-fertilised offspring. (10/40. These fifteen species consist of +Brassica oleracea, Reseda odorata and lutea, Limnanthes douglasii, +Papaver vagum, Viscaria oculata, Beta vulgaris, Lupinus luteus, Ipomoea +purpurea, Mimulus luteus, Calceolaria, Verbascum thapsus, Vandellia +nummularifolia, Lactuca sativa, and Zea mays.) For instance, with +Ipomoea purpurea every single intercrossed plant exceeded in height its +self-fertilised opponent until the sixth generation; and so it was with +Mimulus luteus until the fourth generation. Out of six pairs of crossed +and self-fertilised cabbages, every one of the former was much heavier +than the latter. With Papaver vagum, out of fifteen pairs, all but two +of the crossed plants were taller than their self-fertilised opponents. +Of eight pairs of Lupinus luteus, all but two of the crossed were +taller; of eight pairs of Beta vulgaris all but one; and of fifteen +pairs of Zea mays all but two were taller. Of fifteen pairs of +Limnanthes douglasii, and of seven pairs of Lactuca sativa, every single +crossed plant was taller than its self-fertilised opponent. It should +also be observed that in these experiments no particular care was taken +to cross-fertilise the flowers immediately after their expansion; it is +therefore almost certain that in many of these cases some pollen from +the same flower will have already fallen on and acted on the stigma. + +There can hardly be a doubt that several other species of which the +crossed seedlings are more vigorous than the self-fertilised, as shown +in Tables 7/A, 7/B and 7/C, besides the above fifteen, must have +received their own pollen and that from another plant at nearly the same +time; and if so, the same remarks as those just given are applicable to +them. Scarcely any result from my experiments has surprised me so much +as this of the prepotency of pollen from a distinct individual over each +plant's own pollen, as proved by the greater constitutional vigour of +the crossed seedlings. The evidence of prepotency is here deduced from +the comparative growth of the two lots of seedlings; but we have similar +evidence in many cases from the much greater fertility of the +non-castrated flowers on the mother-plant, when these received at the +same time their own pollen and that from a distinct plant, in comparison +with the flowers which received only their own pollen. + +From the various facts now given on the spontaneous intercrossing of +varieties growing near together, and on the effects of cross-fertilising +flowers which are self-fertile and have not been castrated, we may +conclude that pollen brought by insects or by the wind from a distinct +plant will generally prevent the action of pollen from the same flower, +even though it may have been applied some time before; and thus the +intercrossing of plants in a state of nature will be greatly favoured or +ensured. + +The case of a great tree covered with innumerable hermaphrodite flowers +seems at first sight strongly opposed to the belief in the frequency of +intercrosses between distinct individuals. The flowers which grow on the +opposite sides of such a tree will have been exposed to somewhat +different conditions, and a cross between them may perhaps be in some +degree beneficial; but it is not probable that it would be nearly so +beneficial as a cross between flowers on distinct trees, as we may infer +from the inefficiency of pollen taken from plants which have been +propagated from the same stock, though growing on separate roots. The +number of bees which frequent certain kinds of trees when in full flower +is very great, and they may be seen flying from tree to tree more +frequently than might have been expected. Nevertheless, if we consider +how numerous are the flowers, for instance, on a horse-chestnut or +lime-tree, an incomparably larger number of flowers must be fertilised +by pollen brought from other flowers on the same tree, than from flowers +on a distinct tree. But we should bear in mind that with the +horse-chestnut, for instance, only one or two of the several flowers on +the same peduncle produce a seed; and that this seed is the product of +only one out of several ovules within the same ovarium. Now we know from +the experiments of Herbert and others that if one flower is fertilised +with pollen which is more efficient than that applied to the other +flowers on the same peduncle, the latter often drop off (10/41. +'Variation under Domestication' chapter 17 2nd edition volume 2 page +120.); and it is probable that this would occur with many of the +self-fertilised flowers on a large tree, if other and adjoining flowers +were cross-fertilised. Of the flowers annually produced by a great tree, +it is almost certain that a large number would be self-fertilised; and +if we assume that the tree produced only 500 flowers, and that this +number of seeds were requisite to keep up the stock, so that at least +one seedling should hereafter struggle to maturity, then a large +proportion of the seedlings would necessarily be derived from +self-fertilised seeds. But if the tree annually produced 50,000 flowers, +of which the self-fertilised dropped off without yielding seeds, then +the cross-fertilised flowers might yield seeds in sufficient number to +keep up the stock, and most of the seedlings would be vigorous from +being the product of a cross between distinct individuals. In this +manner the production of a vast number of flowers, besides serving to +entice numerous insects and to compensate for the accidental destruction +of many flowers by spring-frosts or otherwise, would be a very great +advantage to the species; and when we behold our orchard-trees covered +with a white sheet of bloom in the spring, we should not falsely accuse +nature of wasteful expenditure, though comparatively little fruit is +produced in the autumn. + +ANEMOPHILOUS PLANTS. + +The nature and relations of plants which are fertilised by the wind have +been admirably discussed by Delpino and Hermann Muller; and I have +already made some remarks on the structure of their flowers in contrast +with those of entomophilous species. (10/42. Delpino 'Ult. Osservazioni +sulla Dicogamia' part 2 fasc. 1 1870 and 'Studi sopra un Lignaggio +anemofilo' etc. 1871. Hermann Muller 'Die Befruchtung' etc. pages 412, +442. Both these authors remark that plants must have been anemophilous +before they were entomophilous. Hermann Muller further discusses in a +very interesting manner the steps by which entomophilous flowers became +nectariferous and gradually acquired their present structure through +successive beneficial changes.) There is good reason to believe that the +first plants which appeared on this earth were cryptogamic; and judging +from what now occurs, the male fertilising element must either have +possessed the power of spontaneous movement through the water or over +damp surfaces, or have been carried by currents of water to the female +organs. That some of the most ancient plants, such as ferns, possessed +true sexual organs there can hardly be a doubt; and this shows, as +Hildebrand remarks, at how early a period the sexes were separated. +(10/43. 'Die Geschlechter-Vertheilung' 1867 pages 84-90.) As soon as +plants became phanerogamic and grew on the dry ground, if they were ever +to intercross, it would be indispensable that the male fertilising +element should be transported by some means through the air; and the +wind is the simplest means of transport. There must also have been a +period when winged insects did not exist, and plants would not then have +been rendered entomophilous. Even at a somewhat later period the more +specialised orders of the Hymenoptera, Lepidoptera, and Diptera, which +are now chiefly concerned with the transport of pollen, did not exist. +Therefore the earliest terrestrial plants known to us, namely, the +Coniferae and Cycadiae, no doubt were anemophilous, like the existing +species of these same groups. A vestige of this early state of things is +likewise shown by some other groups of plants which are anemophilous, as +these on the whole stand lower in the scale than entomophilous species. + +There is no great difficulty in understanding how an anemophilous plant +might have been rendered entomophilous. Pollen is a nutritious +substance, and would soon have been discovered and devoured by insects; +and if any adhered to their bodies it would have been carried from the +anthers to the stigma of the same flower, or from one flower to another. +One of the chief characteristics of the pollen of anemophilous plants is +its incoherence; but pollen in this state can adhere to the hairy bodies +of insects, as we see with some Leguminosae, Ericaceae, and +Melastomaceae. We have, however, better evidence of the possibility of a +transition of the above kind in certain plants being now fertilised +partly by the wind and partly by insects. The common rhubarb (Rheum +rhaponticum) is so far in an intermediate condition, that I have seen +many Diptera sucking the flowers, with much pollen adhering to their +bodies; and yet the pollen is so incoherent, that clouds of it are +emitted if the plant be gently shaken on a sunny day, some of which +could hardly fail to fall on the large stigmas of the neighbouring +flowers. According to Delpino and Hermann Muller, some species of +Plantago are in a similar intermediate condition. (10/44. 'Die +Befruchtung' etc. page 342.) + +Although it is probable that pollen was aboriginally the sole attraction +to insects, and although many plants now exist whose flowers are +frequented exclusively by pollen-devouring insects, yet the great +majority secrete nectar as the chief attraction. Many years ago I +suggested that primarily the saccharine matter in nectar was excreted as +a waste product of chemical changes in the sap; and that when the +excretion happened to occur within the envelopes of a flower, it was +utilised for the important object of cross-fertilisation, being +subsequently much increased in quantity and stored in various ways. +(10/45. Nectar was regarded by De Candolle and Dunal as an excretion, as +stated by Martinet in 'Annal des Sc. Nat.' 1872 tome 14 page 211.) This +view is rendered probable by the leaves of some trees excreting, under +certain climatic conditions, without the aid of special glands, a +saccharine fluid, often called honey-dew. This is the case with the +leaves of the lime; for although some authors have disputed the fact, a +most capable judge, Dr. Maxwell Masters, informs me that, after having +heard the discussions on this subject before the Horticultural Society, +he feels no doubt on this head. The leaves, as well as the cut stems, of +the manna ash (Fraxinus ornus) secrete in a like manner saccharine +matter. (10/46. 'Gardeners' Chronicle' 1876 page 242.) According to +Treviranus, so do the upper surfaces of the leaves of Carduus arctioides +during hot weather. Many analogous facts could be given. (10/47. Kurr +'Untersuchungen uber die Bedeutung der Nektarien' 1833 page 115.) There +are, however, a considerable number of plants which bear small glands on +their leaves, petioles, phyllodia, stipules, bracteae, or flower +peduncles, or on the outside of their calyx, and these glands secrete +minute drops of a sweet fluid, which is eagerly sought by sugar-loving +insects, such as ants, hive-bees, and wasps. (10/48. A large number of +cases are given by Delpino in the 'Bulletino Entomologico' Anno 6 1874. +To these may be added those given in my text, as well as the excretion +of saccharine matter from the calyx of two species of Iris, and from the +bracteae of certain Orchideae: see Kurr 'Bedeutung der Nektarien' 1833 +pages 25, 28. Belt 'Nicaragua' page 224, also refers to a similar +excretion by many epiphytal orchids and passion-flowers. Mr. Rodgers has +seen much nectar secreted from the bases of the flower-peduncles of +Vanilla. Link says that the only example of a hypopetalous nectary known +to him is externally at the base of the flowers of Chironia decussata: +see 'Reports on Botany, Ray Society' 1846 page 355. An important memoir +bearing on this subject has lately appeared by Reinke 'Gottingen +Nachrichten' 1873 page 825, who shows that in many plants the tips of +the serrations on the leaves in the bud bear glands which secrete only +at a very early age, and which have the same morphological structure as +true nectar-secreting glands. He further shows that the nectar-secreting +glands on the petioles of Prunus avium are not developed at a very early +age, yet wither away on the old leaves. They are homologous with those +on the serrations of the blades of the same leaves, as shown by their +structure and by transition-forms; for the lowest serrations on the +blades of most of the leaves secrete nectar instead of resin (harz).) In +the case of the glands on the stipules of Vicia sativa, the excretion +manifestly depends on changes in the sap, consequent on the sun shining +brightly; for I repeatedly observed that as soon as the sun was hidden +behind clouds the secretion ceased, and the hive-bees left the field; +but as soon as the sun broke out again, they returned to their feast. +(10/49. I published a brief notice of this case in the 'Gardeners' +Chronicle' 1855 July 21 page 487, and afterwards made further +observations. Besides the hive-bee, another species of bee, a moth, +ants, and two kinds of flies sucked the drops of fluid on the stipules. +The larger drops tasted sweet. The hive-bees never even looked at the +flowers which were open at the same time; whilst two species of +humble-bees neglected the stipules and visited only the flowers.) I have +observed an analogous fact with the secretion of true nectar in the +flowers of Lobelia erinus. + +Delpino, however, maintains that the power of secreting a sweet fluid by +any extra-floral organ has been in every case specially gained, for the +sake of attracting ants and wasps as defenders of the plant against +their enemies; but I have never seen any reason to believe that this is +so with the three species observed by me, namely, Prunus laurocerasus, +Vicia sativa, and V. faba. No plant is so little attacked by enemies of +any kind as the common bracken-fern (Pteris aquilina); and yet, as my +son Francis has discovered, the large glands at the bases of the fronds, +but only whilst young, excrete much sweetish fluid, which is eagerly +sought by innumerable ants, chiefly belonging to Myrmica; and these ants +certainly do not serve as a protection against any enemy. Delpino argues +that such glands ought not to be considered as excretory, because if +they were so, they would be present in every species; but I cannot see +much force in this argument, as the leaves of some plants excrete sugar +only during certain states of the weather. That in some cases the +secretion serves to attract insects as defenders of the plant, and may +have been developed to a high degree for this special purpose, I have +not the least doubt, from the observations of Delpino, and more +especially from those of Mr. Belt on Acacia sphaerocephala, and on +passion-flowers. This acacia likewise produces, as an additional +attraction to ants, small bodies containing much oil and protoplasm, and +analogous bodies are developed by a Cecropia for the same purpose, as +described by Fritz Muller. (10/50. Mr. Belt 'The Naturalist in +Nicaragua' 1874 page 218, has given a most interesting account of the +paramount importance of ants as defenders of the above Acacia. With +respect to the Cecropia see 'Nature' 1876 page 304. My son Francis has +described the microscopical structure and development of these wonderful +food-bodies in a paper read before the Linnean Society.) + +The excretion of a sweet fluid by glands seated outside of a flower is +rarely utilised as a means for cross-fertilisation by the aid of +insects; but this occurs with the bracteae of the Marcgraviaceae, as the +late Dr. Cruger informed me from actual observation in the West Indies, +and as Delpino infers with much acuteness from the relative position of +the several parts of their flowers. (10/51. 'Ult. Osservaz. Dicogamia' +1868-69 page 188.) Mr. Farrer has also shown that the flowers of +Coronilla are curiously modified, so that bees may fertilise them whilst +sucking the fluid secreted from the outside of the calyx. (10/52. +'Nature' 1874 page 169.) It further appears probable from the +observations of the Reverend W.A. Leighton, that the fluid so abundantly +secreted by glands on the phyllodia of the Australian Acacia magnifica, +which stand near the flowers, is connected with their fertilisation. +(10/53. 'Annals and Magazine of Natural History' volume 16 1865 page 14. +In my work on the 'Fertilisation of Orchids' and in a paper subsequently +published in the 'Annals and Magazine of Natural History' it has been +shown that although certain kinds of orchids possess a nectary, no +nectar is actually secreted by it; but that insects penetrate the inner +walls and suck the fluid contained in the intercellular spaces. I +further suggested, in the case of some other orchids which do not +secrete nectar, that insects gnawed the labellum; and this suggestion +has since been proved true. Hermann Muller and Delpino have now shown +that some other plants have thickened petals which are sucked or gnawed +by insects, their fertilisation being thus aided. All the known facts on +this head have been collected by Delpino in his 'Ult. Osserv.' part 2 +fasc. 2 1875 pages 59-63.) + +The amount of pollen produced by anemophilous plants, and the distance +to which it is often transported by the wind, are both surprisingly +great. Mr. Hassall found that the weight of pollen produced by a single +plant of the Bulrush (Typha) was 144 grains. Bucketfuls of pollen, +chiefly of Coniferae and Gramineae, have been swept off the decks of +vessels near the North American shore; and Mr. Riley has seen the ground +near St. Louis, in Missouri, covered with pollen, as if sprinkled with +sulphur; and there was good reason to believe that this had been +transported from the pine-forests at least 400 miles to the south. +Kerner has seen the snow-fields on the higher Alps similarly dusted; and +Mr. Blackley found numerous pollen-grains, in one instance 1200, +adhering to sticky slides, which were sent up to a height of from 500 to +1000 feet by means of a kite, and then uncovered by a special mechanism. +It is remarkable that in these experiments there were on an average +nineteen times as many pollen-grains in the atmosphere at the higher +than at the lower levels. (10/54. For Mr. Hassall's observations see +'Annals and Magazine of Natural History' volume 8 1842 page 108. In the +'North American Journal of Science' January 1842, there is an account of +the pollen swept off the decks of a vessel. Riley 'Fifth Report on the +Noxious Insects of Missouri' 1873 page 86. Kerner 'Die Schutzmittel des +Pollens' 1873 page 6. This author has also seen a lake in the Tyrol so +covered with pollen, that the water no longer appeared blue. Mr. +Blackley 'Experimental Researches on Hay-fever' 1873 pages 132, +141-152.) Considering these facts, it is not so surprising as it at +first appears that all, or nearly all, the stigmas of anemophilous +plants should receive pollen brought to them by mere chance by the wind. +During the early part of summer every object is thus dusted with pollen; +for instance, I examined for another purpose the labella of a large +number of flowers of the Fly Ophrys (which is rarely visited by +insects), and found on all very many pollen-grains of other plants, +which had been caught by their velvety surfaces. + +The extraordinary quantity and lightness of the pollen of anemophilous +plants are no doubt both necessary, as their pollen has generally to be +carried to the stigmas of other and often distant flowers; for, as we +shall soon see, most anemophilous plants have their sexes separated. The +fertilisation of these plants is generally aided by the stigmas being of +large size or plumose; and in the case of the Coniferae, by the naked +ovules secreting a drop of fluid, as shown by Delpino. Although the +number of anemophilous species is small, as the author just quoted +remarks, the number of individuals is large in comparison with that of +entomophilous species. This holds good especially in cold and temperate +regions, where insects are not so numerous as under a warmer climate, +and where consequently entomophilous plants are less favourably +situated. We see this in our forests of Coniferae and other trees, such +as oaks, beeches, birches, ashes, etc.; and in the Gramineae, +Cyperaceae, and Juncaceae, which clothe our meadows and swamps; all +these trees and plants being fertilised by the wind. As a large quantity +of pollen is wasted by anemophilous plants, it is surprising that so +many vigorous species of this kind abounding with individuals should +still exist in any part of the world; for if they had been rendered +entomophilous, their pollen would have been transported by the aid of +the senses and appetites of insects with incomparably greater safety +than by the wind. That such a conversion is possible can hardly be +doubted, from the remarks lately made on the existence of intermediate +forms; and apparently it has been effected in the group of willows, as +we may infer from the nature of their nearest allies. (10/55. Hermann +Muller 'Die Befruchtung' etc. page 149.) + +It seems at first sight a still more surprising fact that plants, after +having been once rendered entomophilous, should ever again have become +anemophilous; but this has occasionally though rarely occurred, for +instance, with the common Poterium sanguisorba, as may be inferred from +its belonging to the Rosaceae. Such cases are, however, intelligible, as +almost all plants require to be occasionally intercrossed; and if any +entomiphilous species ceased to be visited by insects, it would probably +perish unless it were rendered anemophilous. A plant would be neglected +by insects if nectar failed to be secreted, unless indeed a large supply +of attractive pollen was present; and from what we have seen of the +excretion of saccharine fluid from leaves and glands being largely +governed in several cases by climatic influences, and from some few +flowers which do not now secrete nectar still retaining coloured +guiding-marks, the failure of the secretion cannot be considered as a +very improbable event. The same result would follow to a certainty, if +winged insects ceased to exist in any district, or became very rare. Now +there is only a single plant in the great order of the Cruciferae, +namely, Pringlea, which is anemophilous, and this plant is an inhabitant +of Kerguelen Land, where there are hardly any winged insects, owing +probably, as was suggested by me in the case of Madeira, to the risk +which they run of being blown out to sea and destroyed. (10/56. The +Reverend A.E. Eaton in 'Proceedings of the Royal Society' volume 23 1875 +page 351.) + +A remarkable fact with respect to anemophilous plants is that they are +often diclinous, that is, they are either monoecious with their sexes +separated on the same plant, or dioecious with their sexes on distinct +plants. In the class Monoecia of Linnaeus, Delpino shows that the +species of twenty-eight genera are anemophilous, and of seventeen genera +entomophilous. (10/57. 'Studi sopra un Lignaggio anemofilo delle +Compositae' 1871.) The larger proportion of entomophilous genera in this +latter class is probably the indirect result of insects having the power +of carrying pollen to another and sometimes distant plant much more +securely than the wind. In the above two classes taken together there +are thirty-eight anemophilous and thirty-six entomophilous genera; +whereas in the great mass of hermaphrodite plants the proportion of +anemophilous to entomophilous genera is extremely small. The cause of +this remarkable difference may be attributed to anemophilous plants +having retained in a greater degree than the entomophilous a primordial +condition, in which the sexes were separated and their mutual +fertilisation effected by means of the wind. That the earliest and +lowest members of the vegetable kingdom had their sexes separated, as is +still the case to a large extent, is the opinion of a high authority, +Nageli. (10/58. 'Entstehung und Begriff der Naturhist. Art' 1865 page +22.) It is indeed difficult to avoid this conclusion, if we admit the +view, which seems highly probable, that the conjugation of the Algae and +of some of the simplest animals is the first step towards sexual +reproduction; and if we further bear in mind that a greater and greater +degree of differentiation between the cells which conjugate can be +traced, thus leading apparently to the development of the two sexual +forms. (10/59. See the interesting discussion on this whole subject by +O. Butschli in his 'Studien uber die ersten Entwickelungsvorgange der +Eizelle; etc. 1876 pages 207-219. Also Engelmann "Ueber Entwickelung von +Infusorien" 'Morphol. Jahrbuch' B. 1 page 573. Also Dr. A. Dodel "Die +Kraushaar-Algae" 'Pringsheims Jahrbuch f. Wiss. Bot.' B. 10.) We have +also seen that as plants became more highly developed and affixed to the +ground, they would be compelled to be anemophilous in order to +intercross. Therefore all plants which have not since been greatly +modified, would tend still to be both diclinous and anemophilous; and we +can thus understand the connection between these two states, although +they appear at first sight quite disconnected. If this view is correct, +plants must have been rendered hermaphrodites at a later though still +very early period, and entomophilous at a yet later period, namely, +after the development of winged insects. So that the relationship +between hermaphroditism and fertilisation by means of insects is +likewise to a certain extent intelligible. + +Why the descendants of plants which were originally dioecious, and which +therefore profited by always intercrossing with another individual, +should have been converted into hermaphrodites, may perhaps be explained +by the risk which they ran, especially as long as they were +anemophilous, of not being always fertilised, and consequently of not +leaving offspring. This latter evil, the greatest of all to any +organism, would have been much lessened by their becoming +hermaphrodites, though with the contingent disadvantage of frequent +self-fertilisation. By what graduated steps an hermaphrodite condition +was acquired we do not know. But we can see that if a lowly organised +form, in which the two sexes were represented by somewhat different +individuals, were to increase by budding either before or after +conjugation, the two incipient sexes would be capable of appearing by +buds on the same stock, as occasionally occurs with various characters +at the present day. The organism would then be in a monoecious +condition, and this is probably the first step towards hermaphroditism; +for if very simple male and female flowers on the same stock, each +consisting of a single stamen or pistil, were brought close together and +surrounded by a common envelope, in nearly the same manner as with the +florets of the Compositae, we should have an hermaphrodite flower. + +There seems to be no limit to the changes which organisms undergo under +changing conditions of life; and some hermaphrodite plants, descended as +we must believe from aboriginally diclinous plants, have had their sexes +again separated. That this has occurred, we may infer from the presence +of rudimentary stamens in the flowers of some individuals, and of +rudimentary pistils in the flowers of other individuals, for example in +Lychnis dioica. But a conversion of this kind will not have occurred +unless cross-fertilisation was already assured, generally by the agency +of insects; but why the production of male and female flowers on +distinct plants should have been advantageous to the species, +cross-fertilisation having been previously assured, is far from obvious. +A plant might indeed produce twice as many seeds as were necessary to +keep up its numbers under new or changed conditions of life; and if it +did not vary by bearing fewer flowers, and did vary in the state of its +reproductive organs (as often occurs under cultivation), a wasteful +expenditure of seeds and pollen would be saved by the flowers becoming +diclinous. + +A related point is worth notice. I remarked in my Origin of Species that +in Britain a much larger proportion of trees and bushes than of +herbaceous plants have their sexes separated; and so it is, according to +Asa Gray and Hooker, in North America and New Zealand. (10/60. I find in +the 'London Catalogue of British Plants' that there are thirty-two +indigenous trees and bushes in Great Britain, classed under nine +families; but to err on the safe side, I have counted only six species +of willows. Of the thirty-two trees and bushes, nineteen, or more than +half, have their sexes separated; and this is an enormous proportion +compared with other British plants. New Zealand abounds with diclinous +plants and trees; and Dr. Hooker calculates that out of about 756 +phanerogamic plants inhabiting the islands, no less than 108 are trees, +belonging to thirty-five families. Of these 108 trees, fifty-two, or +very nearly half, have their sexes more or less separated. Of bushes +there are 149, of which sixty-one have their sexes in the same state; +whilst of the remaining 500 herbaceous plants only 121, or less than a +fourth, have their sexes separated. Lastly, Professor Asa Gray informs +me that in the United States there are 132 native trees (belonging to +twenty-five families) of which ninety-five (belonging to seventeen +families) "have their sexes more or less separated, for the greater part +decidedly separated.") It is, however, doubtful how far this rule holds +good generally, and it certainly does not do so in Australia. But I have +been assured that the flowers of the prevailing Australian trees, +namely, the Myrtaceae, swarm with insects, and if they are dichogamous +they would be practically diclinous. (10/61. With respect to the +Proteaceae of Australia, Mr. Bentham 'Journal of the Linnean Society +Botany' volume 13 1871 pages 58, 64, remarks on the various contrivances +by which the stigma in the several genera is screened from the action of +the pollen from the same flower. For instance, in Synaphea "the stigma +is held by the eunuch (i.e., one of the stamens which is barren) safe +from all pollution from her brother anthers, and is preserved intact for +any pollen that may be inserted by insects and other agencies.") As far +as anemophilous plants are concerned, we know that they are apt to have +their sexes separated, and we can see that it would be an unfavourable +circumstance for them to bear their flowers very close to the ground, as +their pollen is liable to be blown high up in the air (10/62. Kerner +'Schutzmittel des Pollens' 1873 page 4.); but as the culms of grasses +give sufficient elevation, we cannot thus account for so many trees and +bushes being diclinous. We may infer from our previous discussion that a +tree bearing numerous hermaphrodite flowers would rarely intercross with +another tree, except by means of the pollen of a distinct individual +being prepotent over the plants' own pollen. Now the separation of the +sexes, whether the plant were anemophilous are entomophilous, would most +effectually bar self-fertilisation, and this may be the cause of so many +trees and bushes being diclinous. Or to put the case in another way, a +plant would be better fitted for development into a tree, if the sexes +were separated, than if it were hermaphrodite; for in the former case +its numerous flowers would be less liable to continued +self-fertilisation. But it should also be observed that the long life of +a tree or bush permits of the separation of the sexes, with much less +risk of evil from impregnation occasionally failing and seeds not being +produced, than in the case of short-lived plants. Hence it probably is, +as Lecoq has remarked, that annual plants are rarely dioecious. + +Finally, we have seen reason to believe that the higher plants are +descended from extremely low forms which conjugated, and that the +conjugating individuals differed somewhat from one another,--the one +representing the male and the other the female--so that plants were +aboriginally dioecious. At a very early period such lowly organised +dioecious plants probably gave rise by budding to monoecious plants with +the two sexes borne by the same individual; and by a still closer union +of the sexes to hermaphrodite plants, which are now much the commonest +form. (10/63. There is a considerable amount of evidence that all the +higher animals are the descendants of hermaphrodites; and it is a +curious problem whether such hermaphroditism may not have been the +result of the conjugation of two slightly different individuals, which +represented the two incipient sexes. On this view, the higher animals +may now owe their bilateral structure, with all their organs double at +an early embryonic period, to the fusion or conjugation of two +primordial individuals.) As soon as plants became affixed to the ground, +their pollen must have been carried by some means from flower to flower, +at first almost certainly by the wind, then by pollen-devouring, and +afterwards by nectar-seeking insects. During subsequent ages some few +entomophilous plants have been again rendered anemophilous, and some +hermaphrodite plants have had their sexes again separated; and we can +vaguely see the advantages of such recurrent changes under certain +conditions. + +Dioecious plants, however fertilised, have a great advantage over other +plants in their cross-fertilisation being assured. But this advantage is +gained in the case of anemophilous species at the expense of the +production of an enormous superfluity of pollen, with some risk to them +and to entomophilous species of their fertilisation occasionally +failing. Half the individuals, moreover, namely, the males, produce no +seed, and this might possibly be a disadvantage. Delpino remarks that +dioecious plants cannot spread so easily as monoecious and hermaphrodite +species, for a single individual which happened to reach some new site +could not propagate its kind; but it may be doubted whether this is a +serious evil. Monoecious plants can hardly fail to be to a large extent +dioecious in function, owing to the lightness of their pollen and to the +wind blowing laterally, with the great additional advantage of +occasionally or often producing some self-fertilised seeds. When they +are also dichogamous, they are necessarily dioecious in function. +Lastly, hermaphrodite plants can generally produce at least some +self-fertilised seeds, and they are at the same time capable, through +the various means specified in this chapter, of cross-fertilisation. +When their structure absolutely prevents self-fertilisation, they are in +the same relative position to one another as monoecious and dioecious +plants, with what may be an advantage, namely, that every flower is +capable of yielding seeds. + + + +CHAPTER XI. + +THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS. + +Insects visit the flowers of the same species as long as they can. +Cause of this habit. +Means by which bees recognise the flowers of the same species. +Sudden secretion of nectar. +Nectar of certain flowers unattractive to certain insects. +Industry of bees, and the number of flowers visited within a short time. +Perforation of the corolla by bees. +Skill shown in the operation. +Hive-bees profit by the holes made by humble-bees. +Effects of habit. +The motive for perforating flowers to save time. +Flowers growing in crowded masses chiefly perforated. + +Bees and various other insects must be directed by instinct to search +flowers for nectar and pollen, as they act in this manner without +instruction as soon as they emerge from the pupa state. Their instincts, +however, are not of a specialised nature, for they visit many exotic +flowers as readily as the endemic kinds, and they often search for +nectar in flowers which do not secrete any; and they may be seen +attempting to suck it out of nectaries of such length that it cannot be +reached by them. (11/1. See, on this subject Hermann Muller +'Befruchtung' etc. page 427; and Sir J. Lubbock's 'British Wild Flowers' +etc. page 20. Muller 'Bienen Zeitung' June 1876 page 119, assigns good +reasons for his belief that bees and many other Hymenoptera have +inherited from some early nectar-sucking progenitor greater skill in +robbing flowers than that which is displayed by insects belonging to the +other Orders.) All kinds of bees and certain other insects usually visit +the flowers of the same species as long as they can, before going to +another species. This fact was observed by Aristotle with respect to the +hive-bee more than 2000 years ago, and was noticed by Dobbs in a paper +published in 1736 in the Philosophical Transactions. It may be observed +by any one, both with hive and humble-bees, in every flower-garden; not +that the habit is invariably followed. Mr. Bennett watched for several +hours many plants of Lamium album, L. purpureum, and another Labiate +plant, Nepeta glechoma, all growing mingled together on a bank near some +hives; and he found that each bee confined its visits to the same +species. (11/2. 'Nature' 1874 June 4 page 92.) The pollen of these three +plants differs in colour, so that he was able to test his observations +by examining that which adhered to the bodies of the captured bees, and +he found one kind on each bee. + +Humble and hive-bees are good botanists, for they know that varieties +may differ widely in the colour of their flowers and yet belong to the +same species. I have repeatedly seen humble-bees flying straight from a +plant of the ordinary red Dictamnus fraxinella to a white variety; from +one to another very differently coloured variety of Delphinium consolida +and of Primula veris; from a dark purple to a bright yellow variety of +Viola tricolor; and with two species of Papaver, from one variety to +another which differed much in colour; but in this latter case some of +the bees flew indifferently to either species, although passing by other +genera, and thus acted as if the two species were merely varieties. +Hermann Muller also has seen hive-bees flying from flower to flower of +Ranunculus bulbosus and arvensis, and of Trifolium fragiferum and +repens; and even from blue hyacinths to blue violets. (11/3. 'Bienen +Zeitung' July 1876 page 183.) + +Some species of Diptera or flies keep to the flowers of the same species +with almost as much regularity as do bees; and when captured they are +found covered with pollen. I have seen Rhingia rostrata acting in this +manner with the flowers of Lychnis dioica, Ajuga reptans, and Vici +sepium. Volucella plumosa and Empis cheiroptera flew straight from +flower to flower of Myosotis sylvatica. Dolichopus nigripennis behaved +in the same manner with Potentilla tormentilla; and other Diptera with +Stellaria holostea, Helianthemum vulgare, Bellis perennis, Veronica +hederaefolia and chamoedrys; but some flies visited indifferently the +flowers of these two latter species. I have seen more than once a minute +Thrips, with pollen adhering to its body, fly from one flower to another +of the same kind; and one was observed by me crawling about within a +convolvulus with four grains of pollen adhering to its head, which were +deposited on the stigma. + +Fabricius and Sprengel state that when flies have once entered the +flowers of Aristolochia they never escape,--a statement which I could +not believe, as in this case the insects would not aid in the +cross-fertilisation of the plant; and this statement has now been shown +by Hildebrand to be erroneous. As the spathes of Arum maculatum are +furnished with filaments apparently adapted to prevent the exit of +insects, they resemble in this respect the flowers of Aristolochia; and +on examining several spathes, from thirty to sixty minute Diptera +belonging to three species were found in some of them; and many of these +insects were lying dead at the bottom, as if they had been permanently +entrapped. In order to discover whether the living ones could escape and +carry pollen to another plant, I tied in the spring of 1842 a fine +muslin bag tightly round a spathe; and on returning in an hour's time +several little flies were crawling about on the inner surface of the +bag. I then gathered a spathe and breathed hard into it; several flies +soon crawled out, and all without exception were dusted with arum +pollen. These flies quickly flew away, and I distinctly saw three of +them fly to another plant about a yard off; they alighted on the inner +or concave surface of the spathe, and suddenly flew down into the +flower. I then opened this flower, and although not a single anther had +burst, several grains of pollen were lying at the bottom, which must +have been brought from another plant by one of these flies or by some +other insect. In another flower little flies were crawling about, and I +saw them leave pollen on the stigmas. + +I do not know whether Lepidoptera generally keep to the flowers of the +same species; but I once observed many minute moths (I believe Lampronia +(Tinea) calthella) apparently eating the pollen of Mercurialis annua, +and they had the whole front of their bodies covered with pollen. I then +went to a female plant some yards off, and saw in the course of fifteen +minutes three of these moths alight on the stigmas. Lepidoptera are +probably often induced to frequent the flowers of the same species, +whenever these are provided with a long and narrow nectary, as in this +case other insects cannot suck the nectar, which will thus be preserved +for those having an elongated proboscis. No doubt the Yucca moth visits +only the flowers whence its name is derived, for a most wonderful +instinct guides this moth to place pollen on the stigma, so that the +ovules may be developed on which the larvae feed. (11/4. Described by +Mr. Riley in the 'American Naturalist' volume 7 October 1873.)With +respect to Coleoptera, I have seen Meligethes covered with pollen flying +from flower to flower of the same species; and this must often occur, +as, according to M. Brisout, 'many of the species affect only one kind +of plant." (11/5. As quoted in 'American Nat.' May 1873 page 270.) + +It must not be supposed from these several statements that insects +strictly confine their visits to the same species. They often visit +other species when only a few plants of the same kind grow near +together. In a flower-garden containing some plants of Oenothera, the +pollen of which can easily be recognised, I found not only single grains +but masses of it within many flowers of Mimulus, Digitalis, Antirrhinum, +and Linaria. Other kinds of pollen were likewise detected in these same +flowers. A large number of the stigmas of a plant of Thyme, in which the +anthers were completely aborted, were examined; and these stigmas, +though scarcely larger than a split needle, were covered not only with +pollen of Thyme brought from other plants by the bees, but with several +other kinds of pollen. + +That insects should visit the flowers of the same species as long as +they can, is of great importance to the plant, as it favours the +cross-fertilisation of distinct individuals of the same species; but no +one will suppose that insects act in this manner for the good of the +plant. The cause probably lies in insects being thus enabled to work +quicker; they have just learnt how to stand in the best position on the +flower, and how far and in what direction to insert their proboscides. +(11/6. Since these remarks were written, I find that Hermann Muller has +come to almost exactly the same conclusion with respect to the cause of +insects frequenting as long as they can the flowers of the same species: +'Bienen Zeitung' July 1876 page 182.) They act on the same principle as +does an artificer who has to make half-a-dozen engines, and who saves +time by making consecutively each wheel and part for all of them. +Insects, or at least bees, seem much influenced by habit in all their +manifold operations; and we shall presently see that this holds good in +their felonious practice of biting holes through the corolla. + +It is a curious question how bees recognise the flowers of the same +species. That the coloured corolla is the chief guide cannot be doubted. +On a fine day, when hive-bees were incessantly visiting the little blue +flowers of Lobelia erinus, I cut off all the petals of some, and only +the lower striped petals of others, and these flowers were not once +again sucked by the bees, although some actually crawled over them. The +removal of the two little upper petals alone made no difference in their +visits. Mr. J. Anderson likewise states that when he removed the +corollas of the Calceolaria, bees never visited the flowers. (11/7. +'Gardeners' Chronicle' 1853 page 534. Kurr cut off the nectaries from a +large number of flowers of several species, and found that the greater +number yielded seeds; but insects probably would not perceive the loss +of the nectary until they had inserted their proboscides into the holes +thus formed, and in doing so would fertilise the flowers. He also +removed the whole corolla from a considerable number of flowers, and +these likewise yielded seeds. Flowers which are self-fertile would +naturally produce seeds under these circumstances; but I am greatly +surprised that Delphinium consolida, as well as another species of +Delphinium, and Viola tricolor, should have produced a fair supply of +seeds when thus treated; but it does not appear that he compared the +number of the seeds thus produced with those yielded by unmutilated +flowers left to the free access of insects: 'Bedeutung der Nektarien' +1833 pages 123-135.) On the other hand, in some large masses of Geranium +phaeum which had escaped out of a garden, I observed the unusual fact of +the flowers continuing to secrete an abundance of nectar after all the +petals had fallen off; and the flowers in this state were still visited +by humble-bees. But the bees might have learnt that these flowers with +all their petals lost were still worth visiting, by finding nectar in +those with only one or two lost. The colour alone of the corolla serves +as an approximate guide: thus I watched for some time humble-bees which +were visiting exclusively plants of the white-flowered Spiranthes +autumnalis, growing on short turf at a considerable distance apart; and +these bees often flew within a few inches of several other plants with +white flowers, and then without further examination passed onwards in +search of the Spiranthes. Again, many hive-bees which confined their +visits to the common ling (Calluna vulgaris), repeatedly flew towards +Erica tetralix, evidently attracted by the nearly similar tint of their +flowers, and then instantly passed on in search of the Calluna. + +That the colour of the flower is not the sole guide, is clearly shown by +the six cases above given of bees which repeatedly passed in a direct +line from one variety to another of the same species, although they bore +very differently coloured flowers. I observed also bees flying in a +straight line from one clump of a yellow-flowered Oenothera to every +other clump of the same plant in the garden, without turning an inch +from their course to plants of Eschscholtzia and others with yellow +flowers which lay only a foot or two on either side. In these cases the +bees knew the position of each plant in the garden perfectly well, as we +may infer by the directness of their flight; so that they were guided by +experience and memory. But how did they discover at first that the above +varieties with differently coloured flowers belonged to the same +species? Improbable as it may appear, they seem, at least sometimes, to +recognise plants even from a distance by their general aspect, in the +same manner as we should do. On three occasions I observed humble-bees +flying in a perfectly straight line from a tall larkspur (Delphinium) +which was in full flower to another plant of the same species at the +distance of fifteen yards which had not as yet a single flower open, and +on which the buds showed only a faint tinge of blue. Here neither odour +nor the memory of former visits could have come into play, and the tinge +of blue was so faint that it could hardly have served as a guide. (11/8. +A fact mentioned by Hermann Muller 'Die Befruchtung' etc. page 347, +shows that bees possess acute powers of vision and discrimination; for +those engaged in collecting pollen from Primula elatior invariably +passed by the flowers of the long-styled form, in which the anthers are +seated low down in the tubular corolla. Yet the difference in aspect +between the long-styled and short-styled forms is extremely slight.) + +The conspicuousness of the corolla does not suffice to induce repeated +visits from insects, unless nectar is at the same time secreted, +together perhaps with some odour emitted. I watched for a fortnight many +times daily a wall covered with Linaria cymbalaria in full flower, and +never saw a bee even looking at one. There was then a very hot day, and +suddenly many bees were industriously at work on the flowers. It appears +that a certain degree of heat is necessary for the secretion of nectar; +for I observed with Lobelia erinus that if the sun ceased to shine for +only half an hour, the visits of the bees slackened and soon ceased. An +analogous fact with respect to the sweet excretion from the stipules of +Vicia sativa has been already given. As in the case of the Linaria, so +with Pedicularis sylvatica, Polygala vulgaris, Viola tricolor, and some +species of Trifolium, I have watched the flowers day after day without +seeing a bee at work, and then suddenly all the flowers were visited by +many bees. Now how did so many bees discover at once that the flowers +were secreting nectar? I presume that it must have been by their odour; +and that as soon as a few bees began to suck the flowers, others of the +same and of different kinds observed the fact and profited by it. We +shall presently see, when we treat of the perforation of the corolla, +that bees are fully capable of profiting by the labour of other species. +Memory also comes into play, for, as already remarked, bees know the +position of each clump of flowers in a garden. I have repeatedly seen +them passing round a corner, but otherwise in as straight a line as +possible, from one plant of Fraxinella and of Linaria to another and +distant one of the same species; although, owing to the intervention of +other plants, the two were not in sight of each other. + +It would appear that either the taste or the odour of the nectar of +certain flowers is unattractive to hive or to humble-bees, or to both; +for there seems no other reason why certain open flowers which secrete +nectar are not visited by them. The small quantity of nectar secreted by +some of these flowers can hardly be the cause of their neglect, as +hive-bees search eagerly for the minute drops on the glands on the +leaves of the Prunus laurocerasus. Even the bees from different hives +sometimes visit different kinds of flowers, as is said to be the case by +Mr. Grant with respect to the Polyanthus and Viola tricolor. (11/9. +'Gardeners' Chronicle' 1844 page 374.) I have known humble-bees to visit +the flowers of Lobelia fulgens in one garden and not in another at the +distance of only a few miles. The cupful of nectar in the labellum of +Epipactis latifolia is never touched by hive- or humble-bees, although I +have seen them flying close by; and yet the nectar has a pleasant taste +to us, and is habitually consumed by the common wasp. As far as I have +seen, wasps seek for nectar in this country only from the flowers of +this Epipactis, Scrophularia aquatica, Symphoricarpus racemosa (11/10. +The same fact apparently holds good in Italy, for Delpino says that the +flowers of these three plants are alone visited by wasps: 'Nettarii +Estranuziali, Bulletino Entomologico' anno 6.), and Tritoma; the two +former plants being endemic, and the two latter exotic. As wasps are so +fond of sugar and of any sweet fluid, and as they do not disdain the +minute drops on the glands of Prunus laurocerasus, it is a strange fact +that they do not suck the nectar of many open flowers, which they could +do without the aid of a proboscis. Hive-bees visit the flowers of the +Symphoricarpus and Tritoma, and this makes it all the stranger that they +do not visit the flowers of the Epipactis, or, as far as I have seen, +those of the Scrophularia aquatica; although they do visit the flowers +of Scrophularia nodosa, at least in North America. (11/11. 'Silliman's +American Journal of Science' August 1871.) + +The extraordinary industry of bees and the number of flowers which they +visit within a short time, so that each flower is visited repeatedly, +must greatly increase the chance of each receiving pollen from a +distinct plant. When the nectar is in any way hidden, bees cannot tell +without inserting their proboscides whether it has lately been exhausted +by other bees, and this, as remarked in a former chapter, forces them to +visit many more flowers than they otherwise would. But they endeavour to +lose as little time as they can; thus in flowers having several +nectaries, if they find one dry they do not try the others, but as I +have often observed, pass on to another flower. They work so +industriously and effectually, that even in the case of social plants, +of which hundreds of thousands grow together, as with the several kinds +of heath, every single flower is visited, of which evidence will +presently be given. They lose no time and fly very quickly from plant to +plant, but I do not know the rate at which hive-bees fly. Humble-bees +fly at the rate of ten miles an hour, as I was able to ascertain in the +case of the males from their curious habit of calling at certain fixed +points, which made it easy to measure the time taken in passing from one +place to another. + +With respect to the number of flowers which bees visit in a given time, +I observed that in exactly one minute a humble-bee visited twenty-four +of the closed flowers of the Linaria cymbalaria; another bee visited in +the same time twenty-two flowers of the Symphoricarpus racemosa; and +another seventeen flowers on two plants of a Delphinium. In the course +of fifteen minutes a single flower on the summit of a plant of Oenothera +was visited eight times by several humble-bees, and I followed the last +of these bees, whilst it visited in the course of a few additional +minutes every plant of the same species in a large flower-garden. In +nineteen minutes every flower on a small plant of Nemophila insignis was +visited twice. In one minute six flowers of a Campanula were entered by +a pollen-collecting hive-bee; and bees when thus employed work slower +than when sucking nectar. Lastly, seven flower-stalks on a plant of +Dictamnus fraxinella were observed on the 15th of June 1841 during ten +minutes; they were visited by thirteen humble-bees each of which entered +many flowers. On the 22nd the same flower-stalks were visited within the +same time by eleven humble-bees. This plant bore altogether 280 flowers, +and from the above data, taking into consideration how late in the +evening humble-bees work, each flower must have been visited at least +thirty times daily, and the same flower keeps open during several days. +The frequency of the visits of bees is also sometimes shown by the +manner in which the petals are scratched by their hooked tarsi; I have +seen large beds of Mimulus, Stachys, and Lathyrus with the beauty of +their flowers thus sadly defaced. + +PERFORATION OF THE COROLLA BY BEES. + +I have already alluded to bees biting holes in flowers for the sake of +obtaining the nectar. They often act in this manner, both with endemic +and exotic species, in many parts of Europe, in the United States, and +in the Himalaya; and therefore probably in all parts of the world. The +plants, the fertilisation of which actually depends on insects entering +the flowers, will fail to produce seed when their nectar is stolen from +the outside; and even with those species which are capable of +fertilising themselves without any aid, there can be no +cross-fertilisation, and this, as we know, is a serious evil in most +cases. The extent to which humble-bees carry on the practice of biting +holes is surprising: a remarkable case was observed by me near +Bournemouth, where there were formerly extensive heaths. I took a long +walk, and every now and then gathered a twig of Erica tetralix, and when +I had got a handful all the flowers were examined through a lens. This +process was repeated many times; but though many hundreds were examined, +I did not succeed in finding a single flower which had not been +perforated. Humble-bees were at the time sucking the flowers through +these perforations. On the following day a large number of flowers were +examined on another heath with the same result, but here hive-bees were +sucking through the holes. This case is all the more remarkable, as the +innumerable holes had been made within a fortnight, for before that time +I saw the bees everywhere sucking in the proper manner at the mouths of +the corolla. In an extensive flower-garden some large beds of Salvia +grahami, Stachys coccinea, and Pentstemon argutus (?) had every flower +perforated, and many scores were examined. I have seen whole fields of +red clover (Trifolium pratense) in the same state. Dr. Ogle found that +90 per cent of the flowers of Salvia glutinosa had been bitten. In the +United States Mr. Bailey says it is difficult to find a blossom of the +native Gerardia pedicularia without a hole in it; and Mr. Gentry, in +speaking of the introduced Wistaria sinensis, says "that nearly every +flower had been perforated." (11/12. Dr. Ogle 'Pop. Science Review' July +1869 page 267. Bailey 'American Naturalist' November 1873 page 690. +Gentry ibid May 1875 page 264.) + +As far as I have seen, it is always humble-bees which first bite the +holes, and they are well fitted for the work by possessing powerful +mandibles; but hive-bees afterwards profit by the holes thus made. Dr. +Hermann Muller, however, writes to me that hive-bees sometimes bite +holes through the flowers of Erica tetralix. No insects except bees, +with the single exception of wasps in the case of Tritoma, have sense +enough, as far as I have observed, to profit by the holes already made. +Even humble-bees do not always discover that it would be advantageous to +them to perforate certain flowers. There is an abundant supply of nectar +in the nectary of Tropaeolum tricolor, yet I have found this plant +untouched in more than one garden, while the flowers of other plants had +been extensively perforated; but a few years ago Sir J. Lubbock's +gardener assured me that he had seen humble-bees boring through the +nectary of this Tropaeolum. Muller has observed humble-bees trying to +suck at the mouths of the flowers of Primula elatior and of an +Aquilegia, and, failing in their attempts, they made holes through the +corolla; but they often bite holes, although they could with very little +more trouble obtain the nectar in a legitimate manner by the mouth of +the corolla. + +Dr. W. Ogle has communicated to me a curious case. He gathered in +Switzerland 100 flower-stems of the common blue variety of the monkshood +(Aconitum napellus), and not a single flower was perforated; he then +gathered 100 stems of a white variety growing close by, and every one of +the open flowers had been perforated. (11/13. Dr. Ogle 'Popular Science +Review' July 1869 page 267. Bailey 'American Naturalist' November 1873 +page 690. Gentry ibid May 1875 page 264.) This surprising difference in +the state of the flowers may be attributed with much probability to the +blue variety being distasteful to bees, from the presence of the acrid +matter which is so general in the Ranunculaceae, and to its absence in +the white variety in correlation with the loss of the blue tint. +According to Sprengel, this plant is strongly proterandrous (11/14. 'Das +Entdeckte' etc. page 278.); it would therefore be more or less sterile +unless bees carried pollen from the younger to the older flowers. +Consequently the white variety, the flowers of which were always bitten +instead of being properly entered by the bees, would fail to yield the +full number of seeds and would be a comparatively rare plant, as Dr. +Ogle informs me was the case. + +Bees show much skill in their manner of working, for they always make +their holes from the outside close to the spot where the nectar lies +hidden within the corolla. All the flowers in a large bed of Stachys +coccinea had either one or two slits made on the upper side of the +corolla near the base. The flowers of a Mirabilis and of Salvia coccinea +were perforated in the same manner; whilst those of Salvia grahami, in +which the calyx is much elongated, had both the calyx and the corolla +invariably perforated. The flowers of Pentstemon argutus are broader +than those of the plants just named, and two holes alongside each other +had here always been made just above the calyx. In these several cases +the perforations were on the upper side, but in Antirrhinum majus one or +two holes had been made on the lower side, close to the little +protuberance which represents the nectary, and therefore directly in +front of and close to the spot where the nectar is secreted. + +But the most remarkable case of skill and judgment known to me, is that +of the perforation of the flowers of Lathyrus sylvestris, as described +by my son Francis. (11/15. 'Nature' January 8, 1874 page 189.) The +nectar in this plant is enclosed within a tube, formed by the united +stamens, which surround the pistil so closely that a bee is forced to +insert its proboscis outside the tube; but two natural rounded passages +or orifices are left in the tube near the base, in order that the nectar +may be reached by the bees. Now my son found in sixteen out of +twenty-four flowers on this plant, and in eleven out of sixteen of those +on the cultivated everlasting pea, which is either a variety of the same +species or a closely allied one, that the left passage was larger than +the right one. And here comes the remarkable point,--the humble-bees +bite holes through the standard-petal, and they always operated on the +left side over the passage, which is generally the larger of the two. My +son remarks: "It is difficult to say how the bees could have acquired +this habit. Whether they discovered the inequality in the size of the +nectar-holes in sucking the flowers in the proper way, and then utilised +this knowledge in determining where to gnaw the hole; or whether they +found out the best situation by biting through the standard at various +points, and afterwards remembered its situation in visiting other +flowers. But in either case they show a remarkable power of making use +of what they have learnt by experience." It seems probable that bees owe +their skill in biting holes through flowers of all kinds to their having +long practised the instinct of moulding cells and pots of wax, or of +enlarging their old cocoons with tubes of wax; for they are thus +compelled to work on the inside and outside of the same object. + +In the early part of the summer of 1857 I was led to observe during some +weeks several rows of the scarlet kidney-bean (Phaseolus multiflorus), +whilst attending to the fertilisation of this plant, and daily saw +humble- and hive-bees sucking at the mouths of the flowers. But one day +I found several humble-bees employed in cutting holes in flower after +flower; and on the next day every single hive-bee, without exception, +instead of alighting on the left wing-petal and sucking the flower in +the proper manner, flew straight without the least hesitation to the +calyx, and sucked through the holes which had been made only the day +before by the humble-bees; and they continued this habit for many +following days. (11/16. 'Gardeners' Chronicle' 1857 page 725.) Mr. Belt +has communicated to me (July 28th, 1874) a similar case, with the sole +difference that less than half of the flowers had been perforated by the +humble-bees; nevertheless, all the hive-bees gave up sucking at the +mouths of the flowers and visited exclusively the bitten ones. Now how +did the hive-bees find out so quickly that holes had been made? Instinct +seems to be out of the question, as the plant is an exotic. The holes +cannot be seen by bees whilst standing on the wing-petals, where they +had always previously alighted. From the ease with which bees were +deceived when the petals of Lobelia erinus were cut off, it was clear +that in this case they were not guided to the nectar by its smell; and +it may be doubted whether they were attracted to the holes in the +flowers of the Phaseolus by the odour emitted from them. Did they +perceive the holes by the sense of touch in their proboscides, whilst +sucking the flowers in the proper manner, and then reason that it would +save them time to alight on the outside of the flowers and use the +holes? This seems almost too abstruse an act of reason for bees; and it +is more probable that they saw the humble-bees at work, and +understanding what they were about, imitated them and took advantage of +the shorter path to the nectar. Even with animals high in the scale, +such as monkeys, we should be surprised at hearing that all the +individuals of one species within the space of twenty-four hours +understood an act performed by a distinct species, and profited by it. + +I have repeatedly observed with various kinds of flowers that all the +hive and humble-bees which were sucking through the perforations, flew +to them, whether on the upper or under side of the corolla, without the +least hesitation; and this shows how quickly all the individuals within +the district had acquired the same knowledge. Yet habit comes into play +to a certain extent, as in so many of the other operations of bees. Dr. +Ogle, Messrs. Farrer and Belt have observed in the case of Phaseolus +multiflorus that certain individuals went exclusively to the +perforations, while others of the same species visited only the mouths +of the flowers. (11/17. Dr. Ogle 'Pop. Science Review' April 1870 page +167. Mr. Farrer 'Annals and Magazine of Natural History' 4th series +volume 2 1868 page 258. Mr. Belt in a letter to me.) I noticed in 1861 +exactly the same fact with Trifolium pratense. So persistent is the +force of habit, that when a bee which is visiting perforated flowers +comes to one which has not been bitten, it does not go to the mouth, but +instantly flies away in search of another bitten flower. Nevertheless, I +once saw a humble-bee visiting the hybrid Rhododendron azaloides, and it +entered the mouths of some flowers and cut holes into the others. Dr. +Hermann Muller informs me that in the same district he has seen some +individuals of Bombus mastrucatus boring through the calyx and corolla +of Rhinanthus alecterolophus, and others through the corolla alone. +Different species of bees may, however, sometimes be observed acting +differently at the same time on the same plant. I have seen hive-bees +sucking at the mouths of the flowers of the common bean; humble-bees of +one kind sucking through holes bitten in the calyx, and humble-bees of +another kind sucking the little drops of fluid excreted by the stipules. +Mr. Beal of Michigan informs me that the flowers of the Missouri currant +(Ribes aureum) abound with nectar, so that children often suck them; and +he saw hive-bees sucking through holes made by a bird, the oriole, and +at the same time humble-bees sucking in the proper manner at the mouths +of the flowers. (11/18. The flowers of the Ribes are however sometimes +perforated by humble-bees, and Mr. Bundy says that they were able to +bite through and rob seven flowers of their honey in a minute: 'American +Naturalist' 1876 page 238.) This statement about the oriole calls to +mind what I have before said of certain species of humming-birds boring +holes through the flowers of the Brugmansia, whilst other species +entered by the mouth. + +The motive which impels bees to gnaw holes through the corolla seems to +be the saving of time, for they lose much time in climbing into and out +of large flowers, and in forcing their heads into closed ones. They were +able to visit nearly twice as many flowers, as far as I could judge, of +a Stachys and Pentstemon by alighting on the upper surface of the +corolla and sucking through the cut holes, than by entering in the +proper way. Nevertheless each bee before it has had much practice, must +lose some time in making each new perforation, especially when the +perforation has to be made through both calyx and corolla. This action +therefore implies foresight, of which faculty we have abundant evidence +in their building operations; and may we not further believe that some +trace of their social instinct, that is, of working for the good of +other members of the community, may here likewise play a part? + +Many years ago I was struck with the fact that humble-bees as a general +rule perforate flowers only when these grow in large numbers near +together. In a garden where there were some very large beds of Stachys +coccinea and of Pentstemon argutus, every single flower was perforated, +but I found two plants of the former species growing quite separate with +their petals much scratched, showing that they had been frequently +visited by bees, and yet not a single flower was perforated. I found +also a separate plant of the Pentstemon, and saw bees entering the mouth +of the corolla, and not a single flower had been perforated. In the +following year (1842) I visited the same garden several times: on the +19th of July humble-bees were sucking the flowers of Stachys coccinea +and Salvia grahami in the proper manner, and none of the corollas were +perforated. On the 7th of August all the flowers were perforated, even +those on some few plants of the Salvia which grew at a little distance +from the great bed. On the 21st of August only a few flowers on the +summits of the spikes of both species remained fresh, and not one of +these was now bored. Again, in my own garden every plant in several rows +of the common bean had many flowers perforated; but I found three plants +in separate parts of the garden which had sprung up accidentally, and +these had not a single flower perforated. General Strachey formerly saw +many perforated flowers in a garden in the Himalaya, and he wrote to the +owner to inquire whether this relation between the plants growing +crowded and their perforation by the bees there held good, and was +answered in the affirmative. Hence it follows that the red clover +(Trifolium pratense) and the common bean when cultivated in great masses +in fields,--that Erica tetralix growing in large numbers on +heaths,--rows of the scarlet kidney-bean in the kitchen-garden,--and +masses of any species in the flower-garden,--are all eminently liable to +be perforated. + +The explanation of this fact is not difficult. Flowers growing in large +numbers afford a rich booty to the bees, and are conspicuous from a +distance. They are consequently visited by crowds of these insects, and +I once counted between twenty and thirty bees flying about a bed of +Pentstemon. They are thus stimulated to work quickly by rivalry, and, +what is much more important, they find a large proportion of the +flowers, as suggested by my son, with their nectaries sucked dry. +(11/19. 'Nature' January 8, 1874 page 189.) They thus waste much time in +searching many empty flowers, and are led to bite the holes, so as to +find out as quickly as possible whether there is any nectar present, and +if so, to obtain it. + +Flowers which are partially or wholly sterile unless visited by insects +in the proper manner, such as those of most species of Salvia, of +Trifolium pratense, Phaseolus multiflorus, etc., will fail more or less +completely to produce seeds if the bees confine their visits to the +perforations. The perforated flowers of those species, which are capable +of fertilising themselves, will yield only self-fertilised seeds, and +the seedlings will in consequence be less vigorous. Therefore all plants +must suffer in some degree when bees obtain their nectar in a felonious +manner by biting holes through the corolla; and many species, it might +be thought, would thus be exterminated. But here, as is so general +throughout nature, there is a tendency towards a restored equilibrium. +If a plant suffers from being perforated, fewer individuals will be +reared, and if its nectar is highly important to the bees, these in +their turn will suffer and decrease in number; but, what is much more +effective, as soon as the plant becomes somewhat rare so as not to grow +in crowded masses, the bees will no longer be stimulated to gnaw holes +in the flowers, but will enter them in a legitimate manner. More seed +will then be produced, and the seedlings being the product of +cross-fertilisation will be vigorous, so that the species will tend to +increase in number, to be again checked, as soon as the plant again +grows in crowded masses. + + + +CHAPTER XII. + +GENERAL RESULTS. + +Cross-fertilisation proved to be beneficial, and self-fertilisation +injurious. +Allied species differ greatly in the means by which cross-fertilisation +is favoured and self-fertilisation avoided. +The benefits and evils of the two processes depend on the degree of +differentiation in the sexual elements. +The evil effects not due to the combination of morbid tendencies in the +parents. +Nature of the conditions to which plants are subjected when growing near +together in a state of nature or under culture, and the effects of such +conditions. +Theoretical considerations with respect to the interaction of +differentiated sexual elements. +Practical lessons. +Genesis of the two sexes. +Close correspondence between the effects of cross-fertilisation and +self-fertilisation, and of the legitimate and illegitimate unions of +heterostyled plants, in comparison with hybrid unions. + +The first and most important of the conclusions which may be drawn from +the observations given in this volume, is that cross-fertilisation is +generally beneficial, and self-fertilisation injurious. This is shown by +the difference in height, weight, constitutional vigour, and fertility +of the offspring from crossed and self-fertilised flowers, and in the +number of seeds produced by the parent-plants. With respect to the +second of these two propositions, namely, that self-fertilisation is +generally injurious, we have abundant evidence. The structure of the +flowers in such plants as Lobelia ramosa, Digitalis purpurea, etc., +renders the aid of insects almost indispensable for their fertilisation; +and bearing in mind the prepotency of pollen from a distinct individual +over that from the same individual, such plants will almost certainly +have been crossed during many or all previous generations. So it must +be, owing merely to the prepotency of foreign pollen, with cabbages and +various other plants, the varieties of which almost invariably +intercross when grown together. The same inference may be drawn still +more surely with respect to those plants, such as Reseda and +Eschscholtzia, which are sterile with their own pollen, but fertile with +that from any other individual. These several plants must therefore have +been crossed during a long series of previous generations, and the +artificial crosses in my experiments cannot have increased the vigour of +the offspring beyond that of their progenitors. Therefore the difference +between the self-fertilised and crossed plants raised by me cannot be +attributed to the superiority of the crossed, but to the inferiority of +the self-fertilised seedlings, due to the injurious effects of +self-fertilisation. + +With respect to the first proposition, namely, that cross-fertilisation +is generally beneficial, we likewise have excellent evidence. Plants of +Ipomoea were intercrossed for nine successive generations; they were +then again intercrossed, and at the same time crossed with a plant of a +fresh stock, that is, one brought from another garden; and the offspring +of this latter cross were to the intercrossed plants in height as 100 to +78, and in fertility as 100 to 51. An analogous experiment with +Eschscholtzia gave a similar result, as far as fertility was concerned. +In neither of these cases were any of the plants the product of +self-fertilisation. Plants of Dianthus were self-fertilised for three +generations, and this no doubt was injurious; but when these plants were +fertilised by a fresh stock and by intercrossed plants of the same +stock, there was a great difference in fertility between the two sets of +seedlings, and some difference in their height. Petunia offers a nearly +parallel case. With various other plants, the wonderful effects of a +cross with a fresh stock may be seen in Table 7/C. Several accounts have +also been published of the extraordinary growth of seedlings from a +cross between two varieties of the same species, some of which are known +never to fertilise themselves; so that here neither self-fertilisation +nor relationship even in a remote degree can have come into play. (12/1. +See 'Variation under Domestication' chapter 19 2nd edition volume 2 page +159.) We may therefore conclude that the above two propositions are +true,--that cross-fertilisation is generally beneficial and +self-fertilisation injurious to the offspring. + +That certain plants, for instance, Viola tricolor, Digitalis purpurea, +Sarothamnus scoparius, Cyclamen persicum, etc., which have been +naturally cross-fertilised for many or all previous generations, should +suffer to an extreme degree from a single act of self-fertilisation is a +most surprising fact. Nothing of the kkind has been observed in our +domestic animals; but then we must remember that the closest possible +interbreeding with such animals, that is, between brothers and sisters, +cannot be considered as nearly so close a union as that between the +pollen and ovules of the same flower. Whether the evil from +self-fertilisation goes on increasing during successive generations is +not as yet known; but we may infer from my experiments that the increase +if any is far from rapid. After plants have been propagated by +self-fertilisation for several generations, a single cross with a fresh +stock restores their pristine vigour; and we have a strictly analogous +result with our domestic animals. (12/2. Ibid chapter 19 2nd edition +volume 2 page 159.) The good effects of cross-fertilisation are +transmitted by plants to the next generation; and judging from the +varieties of the common pea, to many succeeding generations. But this +may merely be that crossed plants of the first generation are extremely +vigorous, and transmit their vigour, like any other character, to their +successors. + +Notwithstanding the evil which many plants suffer from +self-fertilisation, they can be thus propagated under favourable +conditions for many generations, as shown by some of my experiments, and +more especially by the survival during at least half a century of the +same varieties of the common pea and sweet-pea. The same conclusion +probably holds good with several other exotic plants, which are never or +most rarely cross-fertilised in this country. But all these plants, as +far as they have been tried, profit greatly by a cross with a fresh +stock. Some few plants, for instance, Ophrys apifera, have almost +certainly been propagated in a state of nature for thousands of +generations without having been once intercrossed; and whether they +would profit by a cross with a fresh stock is not known. But such cases +ought not to make us doubt that as a general rule crossing is +beneficial, any more than the existence of plants which, in a state of +nature, are propagated exclusively by rhizomes, stolons, etc. (their +flowers never producing seeds), (12/3. I have given several cases in my +'Variation under Domestication' chapter 18 2nd edition volume 2 page +152.) (their flowers never producing seeds), should make us doubt that +seminal generation must have some great advantage, as it is the common +plan followed by nature. Whether any species has been reproduced +asexually from a very remote period cannot, of course, be ascertained. +Our sole means for forming any judgment on this head is the duration of +the varieties of our fruit trees which have been long propagated by +grafts or buds. Andrew Knight formerly maintained that under these +circumstances they always become weakly, but this conclusion has been +warmly disputed by others. A recent and competent judge, Professor Asa +Gray, leans to the side of Andrew Knight, which seems to me, from such +evidence as I have been able to collect, the more probable view, +notwithstanding many opposed facts. (12/4. 'Darwiniana: Essays and +Reviews pertaining to Darwinism' 1876 page 338.) + +The means for favouring cross-fertilisation and preventing +self-fertilisation, or conversely for favouring self-fertilisation and +preventing to a certain extent cross-fertilisation, are wonderfully +diversified; and it is remarkable that these differ widely in closely +allied plants,--in the species of the same genus, and sometimes in the +individuals of the same species. (12/5. Hildebrand has insisted strongly +to this effect in his valuable observations on the fertilisation of the +Gramineae: 'Monatsbericht K. Akad. Berlin' October 1872 page 763.) It is +not rare to find hermaphrodite plants and others with separated sexes +within the same genus; and it is common to find some of the species +dichogamous and others maturing their sexual elements simultaneously. +The dichogamous genus Saxifraga contains proterandrous and proterogynous +species. (12/6. Dr. Engler 'Botanische Zeitung' 1868 page 833.) Several +genera include both heterostyled (dimorphic or trimorphic forms) and +homostyled species. Ophrys offers a remarkable instance of one species +having its structure manifestly adapted for self-fertilisation, and +other species as manifestly adapted for cross-fertilisation. Some +con-generic species are quite sterile and others quite fertile with +their own pollen. From these several causes we often find within the +same genus species which do not produce seeds, while others produce an +abundance, when insects are excluded. Some species bear cleistogene +flowers which cannot be crossed, as well as perfect flowers, whilst +others in the same genus never produce cleistogene flowers. Some species +exist under two forms, the one bearing conspicuous flowers adapted for +cross-fertilisation, the other bearing inconspicuous flowers adapted for +self-fertilisation, whilst other species in the same genus present only +a single form. Even with the individuals of the same species, the degree +of self-sterility varies greatly, as in Reseda. With polygamous plants, +the distribution of the sexes differs in the individuals of the same +species. The relative period at which the sexual elements in the same +flower are mature, differs in the varieties of Pelargonium; and Carriere +gives several cases, showing that the period varies according to the +temperature to which the plants are exposed. (12/7. 'Des Varieties' 1865 +page 30.) + +This extraordinary diversity in the means for favouring or preventing +cross- and self-fertilisation in closely allied forms, probably depends +on the results of both processes being highly beneficial to the species, +but directly opposed in many ways to one another and dependent on +variable conditions. Self-fertilisation assures the production of a +large supply of seeds; and the necessity or advantage of this will be +determined by the average length of life of the plant, which largely +depends on the amount of destruction suffered by the seeds and +seedlings. This destruction follows from the most various and variable +causes, such as the presence of animals of several kinds, and the growth +of surrounding plants. The possibility of cross-fertilisation depends +mainly on the presence and number of certain insects, often of insects +belonging to special groups, and on the degree to which they are +attracted to the flowers of any particular species in preference to +other flowers,--all circumstances likely to change. Moreover, the +advantages which follow from cross-fertilisation differ much in +different plants, so that it is probable that allied plants would often +profit in different degrees by cross-fertilisation. Under these +extremely complex and fluctuating conditions, with two somewhat opposed +ends to be gained, namely, the safe propagation of the species and the +production of cross-fertilised, vigorous offspring, it is not surprising +that allied forms should exhibit an extreme diversity in the means which +favour either end. If, as there is reason to suspect, self-fertilisation +is in some respects beneficial, although more than counterbalanced by +the advantages derived from a cross with a fresh stock, the problem +becomes still more complicated. + +As I only twice experimented on more than a single species in a genus, I +cannot say whether the crossed offspring of the several species within +the same genus differ in their degree of superiority over their +self-fertilised brethren; but I should expect that this would often +prove to be the case from what was observed with the two species of +Lobelia and with the individuals of the same species of Nicotiana. The +species belonging to distinct genera in the same family certainly differ +in this respect. The effects of cross- and self-fertilisation may be +confined either to the growth or to the fertility of the offspring, but +generally extends to both qualities. There does not seem to exist any +close correspondence between the degree to which their offspring profit +by this process; but we may easily err on this head, as there are two +means for ensuring cross-fertilisation which are not externally +perceptible, namely, self-sterility and the prepotent fertilising +influence of pollen from another individual. Lastly, it has been shown +in a former chapter that the effect produced by cross and +self-fertilisation on the fertility of the parent-plants does not always +correspond with that produced on the height, vigour, and fertility of +their offspring. The same remark applies to crossed and self-fertilised +seedlings when these are used as the parent-plants. This want of +correspondence probably depends, at least in part, on the number of +seeds produced being chiefly determined by the number of the +pollen-tubes which reach the ovules, and this will be governed by the +reaction between the pollen and the stigmatic secretion or tissues; +whereas the growth and constitutional vigour of the offspring will be +chiefly determined, not only by the number of pollen-tubes reaching the +ovules, but by the nature of the reaction between the contents of the +pollen-grains and ovules. + +There are two other important conclusions which may be deduced from my +observations: firstly, that the advantages of cross-fertilisation do not +follow from some mysterious virtue in the mere union of two distinct +individuals, but from such individuals having been subjected during +previous generations to different conditions, or to their having varied +in a manner commonly called spontaneous, so that in either case their +sexual elements have been in some degree differentiated. And secondly, +that the injury from self-fertilisation follows from the want of such +differentiation in the sexual elements. These two propositions are fully +established by my experiments. Thus, when plants of the Ipomoea and of +the Mimulus, which had been self-fertilised for the seven previous +generations and had been kept all the time under the same conditions, +were intercrossed one with another, the offspring did not profit in the +least by the cross. Mimulus offers another instructive case, showing +that the benefit of a cross depends on the previous treatment of the +progenitors: plants which had been self-fertilised for the eight +previous generations were crossed with plants which had been +intercrossed for the same number of generations, all having been kept +under the same conditions as far as possible; seedlings from this cross +were grown in competition with others derived from the same +self-fertilised mother-plant crossed by a fresh stock; and the latter +seedlings were to the former in height as 100 to 52, and in fertility as +100 to 4. An exactly parallel experiment was tried on Dianthus, with +this difference, that the plants had been self-fertilised only for the +three previous generations, and the result was similar though not so +strongly marked. The foregoing two cases of the offspring of Ipomoea and +Eschscholtzia, derived from a cross with a fresh stock, being as much +superior to the intercrossed plants of the old stock, as these latter +were to the self-fertilised offspring, strongly supports the same +conclusion. A cross with a fresh stock or with another variety seems to +be always highly beneficial, whether or not the mother-plants have been +intercrossed or self-fertilised for several previous generations. The +fact that a cross between two flowers on the same plant does no good or +very little good, is likewise a strong corroboration of our conclusion; +for the sexual elements in the flowers on the same plant can rarely have +been differentiated, though this is possible, as flower-buds are in one +sense distinct individuals, sometimes varying and differing from one +another in structure or constitution. Thus the proposition that the +benefit from cross-fertilisation depends on the plants which are crossed +having been subjected during previous generations to somewhat different +conditions, or to their having varied from some unknown cause as if they +had been thus subjected, is securely fortified on all sides. + +Before proceeding any further, the view which has been maintained by +several physiologists must be noticed, namely, that all the evils from +breeding animals too closely, and no doubt, as they would say, from the +self-fertilisation of plants, is the result of the increase of some +morbid tendency or weakness of constitution common to the closely +related parents, or to the two sexes of hermaphrodite plants. +Undoubtedly injury has often thus resulted; but it is a vain attempt to +extend this view to the numerous cases given in my Tables. It should be +remembered that the same mother-plant was both self-fertilised and +crossed, so that if she had been unhealthy she would have transmitted +half her morbid tendencies to her crossed offspring. But plants +appearing perfectly healthy, some of them growing wild, or the immediate +offspring of wild plants, or vigorous common garden-plants, were +selected for experiment. Considering the number of species which were +tried, it is nothing less than absurd to suppose that in all these cases +the mother-plants, though not appearing in any way diseased, were weak +or unhealthy in so peculiar a manner that their self-fertilised +seedlings, many hundreds in number, were rendered inferior in height, +weight, constitutional vigour and fertility to their crossed offspring. +Moreover, this belief cannot be extended to the strongly marked +advantages which invariably follow, as far as my experience serves, from +intercrossing the individuals of the same variety or of distinct +varieties, if these have been subjected during some generations to +different conditions. + +It is obvious that the exposure of two sets of plants during several +generations to different conditions can lead to no beneficial results, +as far as crossing is concerned, unless their sexual elements are thus +affected. That every organism is acted on to a certain extent by a +change in its environment, will not, I presume, be disputed. It is +hardly necessary to advance evidence on this head; we can perceive the +difference between individual plants of the same species which have +grown in somewhat more shady or sunny, dry or damp places. Plants which +have been propagated for some generations under different climates or at +different seasons of the year transmit different constitutions to their +seedlings. Under such circumstances, the chemical constitution of their +fluids and the nature of their tissues are often modified. (12/8. +Numerous cases together with references are given in my 'Variation under +Domestication' chapter 23 2nd edition volume 2 page 264. With respect to +animals, Mr. Brackenridge 'A Contribution to the Theory of Diathesis' +Edinburgh 1869, has well shown that the different organs of animals are +excited into different degrees of activity by differences of temperature +and food, and become to a certain extent adapted to them.) Many other +such facts could be adduced. In short, every alteration in the function +of a part is probably connected with some corresponding, though often +quite imperceptible change in structure or composition. + +Whatever affects an organism in any way, likewise tends to act on its +sexual elements. We see this in the inheritance of newly acquired +modifications, such as those from the increased use or disuse of a part, +and even from mutilations if followed by disease. (12/9. 'Variation +under Domestication' chapter 12 2nd edition volume 1 page 466.) We have +abundant evidence how susceptible the reproductive system is to changed +conditions, in the many instances of animals rendered sterile by +confinement; so that they will not unite, or if they unite do not +produce offspring, though the confinement may be far from close; and of +plants rendered sterile by cultivation. But hardly any cases afford more +striking evidence how powerfully a change in the conditions of life acts +on the sexual elements, than those already given, of plants which are +completely self-sterile in one country, and when brought to another, +yield, even in the first generation, a fair supply of self-fertilised +seeds. + +But it may be said, granting that changed conditions act on the sexual +elements, how can two or more plants growing close together, either in +their native country or in a garden, be differently acted on, inasmuch +as they appear to be exposed to exactly the same conditions? Although +this question has been already considered, it deserves further +consideration under several points of view. In my experiments with +Digitalis purpurea, some flowers on a wild plant were self-fertilised, +and others were crossed with pollen from another plant growing within +two or three feet's distance. The crossed and self-fertilised plants +raised from the seeds thus obtained, produced flower-stems in number as +100 to 47, and in average height as 100 to 70. Therefore the cross +between these two plants was highly beneficial; but how could their +sexual elements have been differentiated by exposure to different +conditions? If the progenitors of the two plants had lived on the same +spot during the last score of generations, and had never been crossed +with any plant beyond the distance of a few feet, in all probability +their offspring would have been reduced to the same state as some of the +plants in my experiments,--such as the intercrossed plants of the ninth +generation of Ipomoea,--or the self-fertilised plants of the eighth +generation of Mimulus,--or the offspring from flowers on the same +plant,--and in this case a cross between the two plants of Digitalis +would have done no good. But seeds are often widely dispersed by natural +means, and one of the above two plants or one of their ancestors may +have come from a distance, from a more shady or sunny, dry or moist +place, or from a different kind of soil containing other organic or +inorganic matter. We know from the admirable researches of Messrs. Lawes +and Gilbert that different plants require and consume very different +amounts of inorganic matter. (12/10. 'Journal of the Royal Agricultural +Society of England' volume 24 part 1.) But the amount in the soil would +probably not make so great a difference to the several individuals of +any particular species as might at first be expected; for the +surrounding species with different requirements would tend, from +existing in greater or lesser numbers, to keep each species in a sort of +equilibrium, with respect to what it could obtain from the soil. So it +would be even with respect to moisture during dry seasons; and how +powerful is the influence of a little more or less moisture in the soil +on the presence and distribution of plants, is often well shown in old +pasture fields which still retain traces of former ridges and furrows. +Nevertheless, as the proportional numbers of the surrounding plants in +two neighbouring places is rarely exactly the same, the individuals of +the same species will be subjected to somewhat different conditions with +respect to what they can absorb from the soil. It is surprising how the +free growth of one set of plants affects others growing mingled with +them; I allowed the plants on rather more than a square yard of turf +which had been closely mown for several years, to grow up; and nine +species out of twenty were thus exterminated; but whether this was +altogether due to the kinds which grew up robbing the others of +nutriment, I do not know. + +Seeds often lie dormant for several years in the ground, and germinate +when brought near the surface by any means, as by burrowing animals. +They would probably be affected by the mere circumstance of having long +lain dormant; for gardeners believe that the production of double +flowers and of fruit is thus influenced. Seeds, moreover, which were +matured during different seasons, will have been subjected during the +whole course of their development to different degrees of heat and +moisture. + +It was shown in the last chapter that pollen is often carried by insects +to a considerable distance from plant to plant. Therefore one of the +parents or ancestors of our two plants of Digitalis may have been +crossed by a distant plant growing under somewhat different conditions. +Plants thus crossed often produce an unusually large number of seeds; a +striking instance of this fact is afforded by the Bignonia, previously +mentioned, which was fertilised by Fritz Muller with pollen from some +adjoining plants and set hardly any seed, but when fertilised with +pollen from a distant plant, was highly fertile. Seedlings from a cross +of this kind grow with great vigour, and transmit their vigour to their +descendants. These, therefore, in the struggle for life, will generally +beat and exterminate the seedlings from plants which have long grown +near together under the same conditions, and will thus tend to spread. + +When two varieties which present well-marked differences are crossed, +their descendants in the later generations differ greatly from one +another in external characters; and this is due to the augmentation or +obliteration of some of these characters, and to the reappearance of +former ones through reversion; and so it will be, as we may feel almost +sure, with any slight differences in the constitution of their sexual +elements. Anyhow, my experiments indicate that crossing plants which +have been long subjected to almost though not quite the same conditions, +is the most powerful of all the means for retaining some degree of +differentiation in the sexual elements, as shown by the superiority in +the later generations of the intercrossed over the self-fertilised +seedlings. Nevertheless, the continued intercrossing of plants thus +treated does tend to obliterate such differentiation, as may be inferred +from the lessened benefit derived from intercrossing such plants, in +comparison with that from a cross with a fresh stock. It seems probable, +as I may add, that seeds have acquired their endless curious adaptations +for wide dissemination, not only that the seedlings would thus be +enabled to find new and fitting homes, but that the individuals which +have been long subjected to the same conditions should occasionally +intercross with a fresh stock. (12/11. See Professor Hildebrand's +excellent treatise 'Verbreitungsmittel der Pflanzen' 1873.) + +From the foregoing several considerations we may, I think, conclude that +in the above case of the Digitalis, and even in that of plants which +have grown for thousands of generations in the same district, as must +often have occurred with species having a much restricted range, we are +apt to over-estimate the degree to which the individuals have been +subjected to absolutely the same conditions. There is at least no +difficulty in believing that such plants have been subjected to +sufficiently distinct conditions to differentiate their sexual elements; +for we know that a plant propagated for some generations in another +garden in the same district serves as a fresh stock and has high +fertilising powers. The curious cases of plants which can fertilise and +be fertilised by any other individual of the same species, but are +altogether sterile with their own pollen, become intelligible, if the +view here propounded is correct, namely, that the individuals of the +same species growing in a state of nature near together, have not really +been subjected during several previous generations to quite the same +conditions. + +Some naturalists assume that there is an innate tendency in all beings +to vary and to advance in organisation, independently of external +agencies; and they would, I presume, thus explain the slight differences +which distinguish all the individuals of the same species both in +external characters and in constitution, as well as the greater +differences in both respects between nearly allied varieties. No two +individuals can be found quite alike; thus if we sow a number of seeds +from the same capsule under as nearly as possible the same conditions, +they germinate at different rates and grow more or less vigorously. They +resist cold and other unfavourable conditions differently. They would in +all probability, as we know to be the case with animals of the same +species, be somewhat differently acted on by the same poison, or by the +same disease. They have different powers of transmitting their +characters to their offspring; and many analogous facts could be given. +(12/12. Vilmorin as quoted by Verlot 'Des Varieties' pages 32, 38, 39.) +Now, if it were true that plants growing near together in a state of +nature had been subjected during many previous generations to absolutely +the same conditions, such differences as those just specified would be +quite inexplicable; but they are to a certain extent intelligible in +accordance with the views just advanced. + +As most of the plants on which I experimented were grown in my garden or +in pots under glass, a few words must be added on the conditions to +which they were exposed, as well as on the effects of cultivation. When +a species is first brought under culture, it may or may not be subjected +to a change of climate, but it is always grown in ground broken up, and +more or less manured; it is also saved from competition with other +plants. The paramount importance of this latter circumstance is proved +by the multitude of species which flourish and multiply in a garden, but +cannot exist unless they are protected from other plants. When thus +saved from competition they are able to get whatever they require from +the soil, probably often in excess; and they are thus subjected to a +great change of conditions. It is probably in chief part owing to this +cause that all plants with rare exceptions vary after being cultivated +for some generations. The individuals which have already begun to vary +will intercross one with another by the aid of insects; and this +accounts for the extreme diversity of character which many of our long +cultivated plants exhibit. But it should be observed that the result +will be largely determined by the degree of their variability and by the +frequency of the intercrosses; for if a plant varies very little, like +most species in a state of nature, frequent intercrosses tend to give +uniformity of character to it. + +I have attempted to show that with plants growing naturally in the same +district, except in the unusual case of each individual being surrounded +by exactly the same proportional numbers of other species having certain +powers of absorption, each will be subjected to slightly different +conditions. This does not apply to the individuals of the same species +when cultivated in cleared ground in the same garden. But if their +flowers are visited by insects, they will intercross; and this will give +to their sexual elements during a considerable number of generations a +sufficient amount of differentiation for a cross to be beneficial. +Moreover, seeds are frequently exchanged or procured from other gardens +having a different kind of soil; and the individuals of the same +cultivated species will thus be subjected to a change of conditions. If +the flowers are not visited by our native insects, or very rarely so, as +in the case of the common and sweet pea, and apparently in that of the +tobacco when kept in a hothouse, any differentiation in the sexual +elements caused by intercrosses will tend to disappear. This appears to +have occurred with the plants just mentioned, for they were not +benefited by being crossed one with another, though they were greatly +benefited by a cross with a fresh stock. + +I have been led to the views just advanced with respect to the causes of +the differentiation of the sexual elements and of the variability of our +garden plants, by the results of my various experiments, and more +especially by the four cases in which extremely inconstant species, +after having been self-fertilised and grown under closely similar +conditions for several generations, produced flowers of a uniform and +constant tint. These conditions were nearly the same as those to which +plants, growing in a garden clear of weeds, are subjected, if they are +propagated by self-fertilised seeds on the same spot. The plants in pots +were, however, exposed to less severe fluctuations of climate than those +out of doors; but their conditions, though closely uniform for all the +individuals of the same generation, differed somewhat in the successive +generations. Now, under these circumstances, the sexual elements of the +plants which were intercrossed in each generation retained sufficient +differentiation during several years for their offspring to be superior +to the self-fertilised, but this superiority gradually and manifestly +decreased, as was shown by the difference in the result between a cross +with one of the intercrossed plants and with a fresh stock. These +intercrossed plants tended also in a few cases to become somewhat more +uniform in some of their external characters than they were at first. +With respect to the plants which were self-fertilised in each +generation, their sexual elements apparently lost, after some years, all +differentiation, for a cross between them did no more good than a cross +between the flowers on the same plant. But it is a still more remarkable +fact, that although the seedlings of Mimulus, Ipomoea, Dianthus, and +Petunia which were first raised, varied excessively in the colour of +their flowers, their offspring, after being self-fertilised and grown +under uniform conditions for some generations, bore flowers almost as +uniform in tint as those on a natural species. In one case also the +plants themselves became remarkably uniform in height. + +The conclusion that the advantages of a cross depend altogether on the +differentiation of the sexual elements, harmonises perfectly with the +fact that an occasional and slight change in the conditions of life is +beneficial to all plants and animals. (12/13. I have given sufficient +evidence on this head in my 'Variation under Domestication' chapter 18 +volume 2 2nd edition page 127.) But the offspring from a cross between +organisms which have been exposed to different conditions, profit in an +incomparably higher degree than do young or old beings from a mere +change in the conditions. In this latter case we never see anything like +the effect which generally follows from a cross with another individual, +especially from a cross with a fresh stock. This might, perhaps, have +been expected, for the blending together of the sexual elements of two +differentiated beings will affect the whole constitution at a very early +period of life, whilst the organisation is highly flexible. We have, +moreover, reason to believe that changed conditions generally act +differently on the several parts or organs of the same individual +(12/14. See, for instance, Brackenridge 'Theory of Diathesis' Edinburgh +1869.); and if we may further believe that these now slightly +differentiated parts react on one another, the harmony between the +beneficial effects on the individual due to changed conditions, and +those due to the interaction of differentiated sexual elements, becomes +still closer. + +That wonderfully accurate observer, Sprengel, who first showed how +important a part insects play in the fertilisation of flowers, called +his book 'The Secret of Nature Displayed;' yet he only occasionally saw +that the object for which so many curious and beautiful adaptations have +been acquired, was the cross-fertilisation of distinct plants; and he +knew nothing of the benefits which the offspring thus receive in growth, +vigour, and fertility. But the veil of secrecy is as yet far from +lifted; nor will it be, until we can say why it is beneficial that the +sexual elements should be differentiated to a certain extent, and why, +if the differentiation be carried still further, injury follows. It is +an extraordinary fact that with many species, flowers fertilised with +their own pollen are either absolutely or in some degree sterile; if +fertilised with pollen from another flower on the same plant, they are +sometimes, though rarely, a little more fertile; if fertilised with +pollen from another individual or variety of the same species, they are +fully fertile; but if with pollen from a distinct species, they are +sterile in all possible degrees, until utter sterility is reached. We +thus have a long series with absolute sterility at the two ends;--at one +end due to the sexual elements not having been sufficiently +differentiated, and at the other end to their having been differentiated +in too great a degree, or in some peculiar manner. + +The fertilisation of one of the higher plants depends, in the first +place, on the mutual action of the pollen-grains and the stigmatic +secretion or tissues, and afterwards on the mutual action of the +contents of the pollen-grains and ovules. Both actions, judging from the +increased fertility of the parent-plants and from the increased powers +of growth in the offspring, are favoured by some degree of +differentiation in the elements which interact and unite so as to form a +new being. Here we have some analogy with chemical affinity or +attraction, which comes into play only between atoms or molecules of a +different nature. As Professor Miller remarks: "Generally speaking, the +greater the difference in the properties of two bodies, the more intense +is their tendency to mutual chemical action...But between bodies of a +similar character the tendency to unite is feeble." (12/15. 'Elements of +Chemistry' 4th edition 1867 part 1 page 11. Dr. Frankland informs me +that similar views with respect to chemical affinity are generally +accepted by chemists.) This latter proposition accords well with the +feeble effects of a plant's own pollen on the fertility of the +mother-plant and on the growth of the offspring; and the former +proposition accords well with the powerful influence in both ways of +pollen from an individual which has been differentiated by exposure to +changed conditions, or by so-called spontaneous variation. But the +analogy fails when we turn to the negative or weak effects of pollen +from one species on a distinct species; for although some substances +which are extremely dissimilar, for instance, carbon and chlorine, have +a very feeble affinity for each other, yet it cannot be said that the +weakness of the affinity depends in such cases on the extent to which +the substances differ. It is not known why a certain amount of +differentiation is necessary or favourable for the chemical affinity or +union of two substances, any more than for the fertilisation or union of +two organisms. + +Mr. Herbert Spencer has discussed this whole subject at great length, +and after stating that all the forces throughout nature tend towards an +equilibrium, remarks, "that the need of this union of sperm-cell and +germ-ccell is the need for overthrowing this equilibrium and +re-establishing active molecular change in the detached germ--a result +which is probably effected by mixing the slightly-different +physiological units of slightly-different individuals." (12/16. +'Principles of Biology' volume 1 page 274 1864. In my 'Origin of +Species' published in 1859, I spoke of the good effects from slight +changes in the condition of life and from cross-fertilisation, and of +the evil effects from great changes in the conditions and from crossing +widely distinct forms (i.e., species), as a series of facts "connected +together by some common but unknown bond, which is essentially related +to the principle of life.") But we must not allow this highly +generalised view, or the analogy of chemical affinity, to conceal from +us our ignorance. We do not know what is the nature or degree of the +differentiation in the sexual elements which is favourable for union, +and what is injurious for union, as in the case of distinct species. We +cannot say why the individuals of certain species profit greatly, and +others very little by being crossed. There are some few species which +have been self-fertilised for a vast number of generations, and yet are +vigorous enough to compete successfully with a host of surrounding +plants. We can form no conception why the advantage from a cross is +sometimes directed exclusively to the vegetative system, and sometimes +to the reproductive system, but commonly to both. It is equally +inconceivable why some individuals of the same species should be +sterile, whilst others are fully fertile with their own pollen; why a +change of climate should either lessen or increase the sterility of +self-sterile species; and why the individuals of some species should be +even more fertile with pollen from a distinct species than with their +own pollen. And so it is with many other facts, which are so obscure +that we stand in awe before the mystery of life. + +Under a practical point of view, agriculturists and horticulturists may +learn something from the conclusions at which we have arrived. Firstly, +we see that the injury from the close breeding of animals and from the +self-fertilisation of plants, does not necessarily depend on any +tendency to disease or weakness of constitution common to the related +parents, and only indirectly on their relationship, in so far as they +are apt to resemble each other in all respects, including their sexual +nature. And, secondly, that the advantages of cross-fertilisation depend +on the sexual elements of the parents having become in some degree +differentiated by the exposure of their progenitors to different +conditions, or from their having intercrossed with individuals thus +exposed, or, lastly, from what we call in our ignorance spontaneous +variation. He therefore who wishes to pair closely related animals ought +to keep them under conditions as different as possible. Some few +breeders, guided by their keen powers of observation, have acted on this +principle, and have kept stocks of the same animals at two or more +distant and differently situated farms. They have then coupled the +individuals from these farms with excellent results. (12/17. 'Variation +of Animals and Plants under Domestication' chapter 17 2nd edition volume +2 pages 98, 105.) This same plan is also unconsciously followed whenever +the males, reared in one place, are let out for propagation to breeders +in other places. As some kinds of plants suffer much more from +self-fertilisation than do others, so it probably is with animals from +too close interbreeding. The effects of close interbreeding on animals, +judging again from plants, would be deterioration in general vigour, +including fertility, with no necessary loss of excellence of form; and +this seems to be the usual result. + +It is a common practice with horticulturists to obtain seeds from +another place having a very different soil, so as to avoid raising +plants for a long succession of generations under the same conditions; +but with all the species which freely intercross by aid of insects or +the wind, it would be an incomparably better plan to obtain seeds of the +required variety, which had been raised for some generations under as +different conditions as possible, and sow them in alternate rows with +seeds matured in the old garden. The two stocks would then intercross, +with a thorough blending of their whole organisations, and with no loss +of purity to the variety; and this would yield far more favourable +results than a mere exchange of seeds. We have seen in my experiments +how wonderfully the offspring profited in height, weight, hardiness, and +fertility, by crosses of this kind. For instance, plants of Ipomoea thus +crossed were to the intercrossed plants of the same stock, with which +they grew in competition, as 100 to 78 in height, and as 100 to 51 in +fertility; and plants of Eschscholtzia similarly compared were as 100 to +45 in fertility. In comparison with self-fertilised plants the results +are still more striking; thus cabbages derived from a cross with a fresh +stock were to the self-fertilised as 100 to 22 in weight. + +Florists may learn from the four cases which have been fully described, +that they have the power of fixing each fleeting variety of colour, if +they will fertilise the flowers of the desired kind with their own +pollen for half-a-dozen generations, and grow the seedlings under the +same conditions. But a cross with any other individual of the same +variety must be carefully prevented, as each has its own peculiar +constitution. After a dozen generations of self-fertilisation, it is +probable that the new variety would remain constant even if grown under +somewhat different conditions; and there would no longer be any +necessity to guard against intercrosses between the individuals of the +same variety. + +With respect to mankind, my son George has endeavoured to discover by a +statistical investigation whether the marriages of first cousins are at +all injurious, although this is a degree of relationship which would not +be objected to in our domestic animals; and he has come to the +conclusion from his own researches and those of Dr. Mitchell that the +evidence as to any evil thus caused is conflicting, but on the whole +points to its being very small. From the facts given in this volume we +may infer that with mankind the marriages of nearly related persons, +some of whose parents and ancestors had lived under very different +conditions, would be much less injurious than that of persons who had +always lived in the same place and followed the same habits of life. Nor +can I see reason to doubt that the widely different habits of life of +men and women in civilised nations, especially amongst the upper +classes, would tend to counterbalance any evil from marriages between +healthy and somewhat closely related persons. + +Under a theoretical point of view it is some gain to science to know +that numberless structures in hermaphrodite plants, and probably in +hermaphrodite animals, are special adaptations for securing an +occasional cross between two individuals; and that the advantages from +such a cross depend altogether on the beings which are united, or their +progenitors, having had their sexual elements somewhat differentiated, +so that the embryo is benefited in the same manner as is a mature plant +or animal by a slight change in its conditions of life, although in a +much higher degree. + +Another and more important result may be deduced from my observations. +Eggs and seeds are highly serviceable as a means of dissemination, but +we now know that fertile eggs can be produced without the aid of the +male. There are also many other methods by which organisms can be +propagated asexually. Why then have the two sexes been developed, and +why do males exist which cannot themselves produce offspring? The answer +lies, as I can hardly doubt, in the great good which is derived from the +fusion of two somewhat differentiated individuals; and with the +exception of the lowest organisms this is possible only by means of the +sexual elements, these consisting of cells separated from the body, +containing the germs of every part, and capable of being fused +completely together. + +It has been shown in the present volume that the offspring from the +union of two distinct individuals, especially if their progenitors have +been subjected to very different conditions, have an immense advantage +in height, weight, constitutional vigour and fertility over the +self-fertilised offspring from one of the same parents. And this fact is +amply sufficient to account for the development of the sexual elements, +that is, for the genesis of the two sexes. + +It is a different question why the two sexes are sometimes combined in +the same individual and are sometimes separated. As with many of the +lowest plants and animals the conjugation of two individuals which are +either quite similar or in some degree different, is a common +phenomenon, it seems probable, as remarked in the last chapter, that the +sexes were primordially separate. The individual which receives the +contents of the other, may be called the female; and the other, which is +often smaller and more locomotive, may be called the male; though these +sexual names ought hardly to be applied as long as the whole contents of +the two forms are blended into one. The object gained by the two sexes +becoming united in the same hermaphrodite form probably is to allow of +occasional or frequent self-fertilisation, so as to ensure the +propagation of the species, more especially in the case of organisms +affixed for life to the same spot. There does not seem to be any great +difficulty in understanding how an organism, formed by the conjugation +of two individuals which represented the two incipient sexes, might have +given rise by budding first to a monoecious and then to an hermaphrodite +form; and in the case of animals even without budding to an +hermaphrodite form, for the bilateral structure of animals perhaps +indicates that they were aboriginally formed by the fusion of two +individuals. + +It is a more difficult problem why some plants and apparently all the +higher animals, after becoming hermaphrodites, have since had their +sexes re-separated. This separation has been attributed by some +naturalists to the advantages which follow from a division of +physiological labour. The principle is intelligible when the same organ +has to perform at the same time diverse functions; but it is not obvious +why the male and female glands when placed in different parts of the +same compound or simple individual, should not perform their functions +equally well as when placed in two distinct individuals. In some +instances the sexes may have been re-separated for the sake of +preventing too frequent self-fertilisation; but this explanation does +not seem probable, as the same end might have been gained by other and +simpler means, for instance dichogamy. It may be that the production of +the male and female reproductive elements and the maturation of the +ovules was too great a strain and expenditure of vital force for a +single individual to withstand, if endowed with a highly complex +organisation; and that at the same time there was no need for all the +individuals to produce young, and consequently that no injury, on the +contrary, good resulted from half of them, or the males, failing to +produce offspring. + +There is another subject on which some light is thrown by the facts +given in this volume, namely, hybridisation. It is notorious that when +distinct species of plants are crossed, they produce with the rarest +exceptions fewer seeds than the normal number. This unproductiveness +varies in different species up to sterility so complete that not even an +empty capsule is formed; and all experimentalists have found that it is +much influenced by the conditions to which the crossed species are +subjected. The pollen of each species is strongly prepotent over that of +any other species, so that if a plant's own pollen is placed on the +stigma some time after foreign pollen has been applied to it, any effect +from the latter is quite obliterated. It is also notorious that not only +the parent species, but the hybrids raised from them are more or less +sterile; and that their pollen is often in a more or less aborted +condition. The degree of sterility of various hybrids does not always +strictly correspond with the degree of difficulty in uniting the parent +forms. When hybrids are capable of breeding inter se, their descendants +are more or less sterile, and they often become still more sterile in +the later generations; but then close interbreeding has hitherto been +practised in all such cases. The more sterile hybrids are sometimes much +dwarfed in stature, and have a feeble constitution. Other facts could be +given, but these will suffice for us. Naturalists formerly attributed +all these results to the difference between species being fundamentally +distinct from that between the varieties of the same species; and this +is still the verdict of some naturalists. + +The results of my experiments in self-fertilising and cross-fertilising +the individuals or the varieties of the same species, are strikingly +analogous with those just given, though in a reversed manner. With the +majority of species flowers fertilised with their own pollen yield +fewer, sometimes much fewer seeds, than those fertilised with pollen +from another individual or variety. Some self-fertilised flowers are +absolutely sterile; but the degree of their sterility is largely +determined by the conditions to which the parent plants have been +exposed, as was well exemplified in the case of Eschscholtzia and +Abutilon. The effects of pollen from the same plant are obliterated by +the prepotent influence of pollen from another individual or variety, +although the latter may have been placed on the stigma some hours +afterwards. The offspring from self-fertilised flowers are themselves +more or less sterile, sometimes highly sterile, and their pollen is +sometimes in an imperfect condition; but I have not met with any case of +complete sterility in self-fertilised seedlings, as is so common with +hybrids. The degree of their sterility does not correspond with that of +the parent-plants when first self-fertilised. The offspring of +self-fertilised plants suffer in stature, weight, and constitutional +vigour more frequently and in a greater degree than do the hybrid +offspring of the greater number of crossed species. Decreased height is +transmitted to the next generation, but I did not ascertain whether this +applies to decreased fertility. + +I have elsewhere shown that by uniting in various ways dimorphic or +trimorphic heterostyled plants, which belong to the same undoubted +species, we get another series of results exactly parallel with those +from crossing distinct species. (12/18. 'Journal of the Linnean Society +Botany' volume 10 1867 page 393.) Plants illegitimately fertilised with +pollen from a distinct plant belonging to the same form, yield fewer, +often much fewer seeds, than they do when legitimately fertilised with +pollen from a plant belonging to a distinct form. They sometimes yield +no seed, not even an empty capsule, like a species fertilised with +pollen from a distinct genus. The degree of sterility is much affected +by the conditions to which the plants have been subjected. (12/19. +'Journal of the Linnean Society Botany' volume 8 1864 page 180.) The +pollen from a distinct form is strongly prepotent over that from the +same form, although the former may have been placed on the stigma many +hours afterwards. The offspring from a union between plants of the same +form are more or less sterile, like hybrids, and have their pollen in a +more or less aborted condition; and some of the seedlings are as barren +and as dwarfed as the most barren hybrid. They also resemble hybrids in +several other respects, which need not here be specified in +detail,--such as their sterility not corresponding in degree with that +of the parent plants,--the unequal sterility of the latter, when +reciprocally united,--and the varying sterility of the seedlings raised +from the same seed-capsule. + +We thus have two grand classes of cases giving results which correspond +in the most striking manner with those which follow from the crossing of +so-called true and distinct species. With respect to the difference +between seedlings raised from cross and self-fertilised flowers, there +is good evidence that this depends altogether on whether the sexual +elements of the parents have been sufficiently differentiated, by +exposure to different conditions or by spontaneous variation. It is +probable that nearly the same conclusion may be extended to heterostyled +plants; but this is not the proper place for discussing the origin of +the long-styled, short-styled and mid-styled forms, which all belong to +the same species as certainly as do the two sexes of the same species. +We have therefore no right to maintain that the sterility of species +when first crossed and of their hybrid offspring, is determined by some +cause fundamentally different from that which determines the sterility +of the individuals both of ordinary and of heterostyled plants when +united in various ways. Nevertheless, I am aware that it will take many +years to remove this prejudice. + +There is hardly anything more wonderful in nature than the sensitiveness +of the sexual elements to external influences, and the delicacy of their +affinities. We see this in slight changes in the conditions of life +being favourable to the fertility and vigour of the parents, while +certain other and not great changes cause them to be quite sterile +without any apparent injury to their health. We see how sensitive the +sexual elements of those plants must be, which are completely sterile +with their own pollen, but are fertile with that of any other individual +of the same species. Such plants become either more or less self-sterile +if subjected to changed conditions, although the change may be far from +great. The ovules of a heterostyled trimorphic plant are affected very +differently by pollen from the three sets of stamens belonging to the +same species. With ordinary plants the pollen of another variety or +merely of another individual of the same variety is often strongly +prepotent over its own pollen, when both are placed at the same time on +the same stigma. In those great families of plants containing many +thousand allied species, the stigma of each distinguishes with unerring +certainty its own pollen from that of every other species. + +There can be no doubt that the sterility of distinct species when first +crossed, and of their hybrid offspring, depends exclusively on the +nature or affinities of their sexual elements. We see this in the want +of any close correspondence between the degree of sterility and the +amount of external difference in the species which are crossed; and +still more clearly in the wide difference in the results of crossing +reciprocally the same two species;--that is, when species A is crossed +with pollen from B, and then B is crossed with pollen from A. Bearing in +mind what has just been said on the extreme sensitiveness and delicate +affinities of the reproductive system, why should we feel any surprise +at the sexual elements of those forms, which we call species, having +been differentiated in such a manner that they are incapable or only +feebly capable of acting on one another? We know that species have +generally lived under the same conditions, and have retained their own +proper characters, for a much longer period than varieties. +Long-continued domestication eliminates, as I have shown in my +'Variation under Domestication,' the mutual sterility which distinct +species lately taken from a state of nature almost always exhibit when +intercrossed; and we can thus understand the fact that the most +different domestic races of animals are not mutually sterile. But +whether this holds good with cultivated varieties of plants is not +known, though some facts indicate that it does. The elimination of +sterility through long-continued domestication may probably be +attributed to the varying conditions to which our domestic animals have +been subjected; and no doubt it is owing to this same cause that they +withstand great and sudden changes in their conditions of life with far +less loss of fertility than do natural species. From these several +considerations it appears probable that the difference in the affinities +of the sexual elements of distinct species, on which their mutual +incapacity for breeding together depends, is caused by their having been +habituated for a very long period each to its own conditions, and to the +sexual elements having thus acquired firmly fixed affinities. However +this may be, with the two great classes of cases before us, namely, +those relating to the self-fertilisation and cross-fertilisation of the +individuals of the same species, and those relating to the illegitimate +and legitimate unions of heterostyled plants, it is quite unjustifiable +to assume that the sterility of species when first crossed and of their +hybrid offspring, indicates that they differ in some fundamental manner +from the varieties or individuals of the same species. + + + +INDEX. + +Abutilon darwinii, self-sterile in Brazil. +moderately self-fertile in England. +fertilised by birds. + +Acacia sphaerocephala. + +Acanthaceae. + +Aconitum napellus. + +Adlumia cirrhosa. + +Adonis aestivalis. +measurements. +relative heights of crossed and self-fertilised plants. +self-fertile. + +Ajuga reptans. + +Allium cepa (blood-red var.) + +Anagallis collina (var. grandiflora). +measurements. +seeds. + +Anderson, J., on the Calceolaria. +removing the corollas. + +Anemone. + +Anemophilous plants. +often diclinous. + +Antirrhinum majus (red var.) +perforated corolla. +--(white var.). +--(peloric var.). + +Apium petroselinum. +result of experiments. + +Argemone ochroleuca. + +Aristolochia. + +Aristotle on bees frequenting flowers of the same species. + +Arum maculatum. + +Bailey, Mr., perforation of corolla. + +Bartonia aurea. +measurements. +result of experiments. + +Bartsia odontites. + +Beal, W.J., sterility of Kalmia latifolia. +on nectar in Ribes aureum. + +Bean, the common. + +Bees distinguish colours. +frequent the flowers of the same species. +guided by coloured corolla. +powers of vision and discrimination. +memory. +unattracted by odour of certain flowers. +industry. +profit by the corolla perforated by humble-bees. +skill in working. +habit. +foresight. + +Bees, humble, recognise varieties as of one species. +colour not the sole guide. +rate of flying. +number of flowers visited. +corolla perforated by. +skill and judgment. + +Belt, Mr., the hairs of Digitalis purpurea. +Phaseolus multiflorus. +not visited by bees in Nicaragua. +humming-birds carrying pollen. +secretion of nectar. +in Acacia sphaerocephalus and passion-flower. +perforation of corolla. + +Bennett, A.W., on Viola tricolor. +structure of Impatiens fulva. +plants flowering in winter. +bees frequenting flowers of same species. + +Bentham, on protection of the stigma in Synaphea. + +Beta vulgaris. +measurements. +crossed not exceeded by self-fertilised. +prepotency of other pollen. + +Bignonia. + +Birds, means of fertilisation. + +Blackley, Mr., on anthers of rye. +pollen carried by wind, experiments with a kite. + +Boraginaceae. + +Borago officinalis. +measurements. +early flowering of crossed. +seeds. +partially self-sterile. + +Brackenridge, Mr., organism of animals affected by temperature and food. +different effect of changed conditions. + +Brassica oleracea. +measurements. +weight. +remarks on experiments. +superiority of crossed. +period of flowering. +seeds. +self-fertile. +--napus. +--rapa. + +Brisout, M., insects frequenting flowers of same species. + +Broom. + +Brugmansia. +humming-birds boring the flower. + +Bulrush, weight of pollen produced by one plant. + +Bundy, Mr., Ribes perforated by bees. + +Butschli, O., sexual relations. + +Cabbage. +affected by pollen of purple bastard. +prepotency of other pollen. +--, Ragged Jack. + +Calceolaria. + +Calluna vulgaris. + +Campanula carpathica. + +Campanulaceae. + +Candolle, A. de, on ascending a mountain the flowers of the same species +disappear abruptly. + +Canna warscewiczi. +result of crossed and self-fertilised. +period of flowering. +seeds. +highly self-fertile. + +Cannaceae. + +Carduus arctioides. + +Carnation. + +Carriere, relative period of the maturity of the sexual elements on same +flower. + +Caryophyllaceae. + +Caspary, Professor, on Corydalis cava. +Nymphaeaceae. +Euryale ferox. + +Cecropia, food-bodies of. + +Centradenia floribunda. + +Cereals, grains of. + +Cheeseman, Mr., on Orchids in New Zealand. + +Chenopodiaceae. + +Cineraria. + +Clarkia elegans. +measurements. +early flowering of self-fertilised. +seeds. + +Cleistogene flowers. + +Coe, Mr., crossing Phaseolus vulgaris. + +Colgate, R., red clover never sucked by hive-bees in New Zealand. + +Colour, uniform, of flowers on plants self-fertilised and grown under +similar conditions for several generations. + +Colours of flowers attractive to insects. +not the sole guide to bees. + +Compositae. + +Coniferae. + +Convolvulus major. +-- tricolor. + +Corolla, removal of. +perforation by bees. + +Coronilla. + +Corydalis cava. +-- halleri. +-- intermedia. +-- lutea. +-- ochroleuca. +-- solida. + +Corylus avellana. + +Cowslip. + +Crinum. + +Crossed plants, greater constitutional vigour of. + +Cross-fertilisation. +see Fertilisation. + +Crossing flowers on same plant, effects of. + +Cruciferae. + +Cruger, Dr., secretion of sweet fluid in Marcgraviaceae. + +Cuphea purpurea. + +Cycadiae. + +Cyclamen persicum. +measurements. +early flowering of crossed. +seeds. +self-sterile. +-- repandum. + +Cytisus laburnum. + +Dandelion, number of pollen grains. + +Darwin, C., self-fertilisation in Pisum sativum. +sexual affinities. +on Primula. +bud variation. +constitutional vigour from cross parentage in common pea. +hybrids of Gladiolus and Cistus. +Phaseolus multiflorus. +nectar in Orchids. +on cross-fertilisation. +inheritance of acquired modifications. +change in the conditions of life beneficial to plants and animals. + +Darwin, F., structure of Phaseolus multiflorus. +Pteris aquilina. +perforation of Lathyrus sylvestris. + +Darwin, G., on marriages with first cousins. + +Decaisne on Delphinium consolida. + +De Candolle, nectar as an excretion. + +Delphinium consolida. +measurements. +seeds. +partially sterile. +corolla removed. + +Delpino, Professor, Viola tricolor. +Phaseolus multiflorus. +intercrossing of sweet-pea. +Lobelia ramosa. +structure of the Cannaceae. +wind and water carrying pollen. +Juglans regia. +anemophilous plants. +fertilisation of Plantago. +excretion of nectar. +secretion of nectar to defend the plant. +anemophilous and entomophilous plants. +dioecious plants. + +Denny, Pelargonium zonale. + +Diagram showing mean height of Ipomoea purpurea. + +Dianthus caryophyllus. +crossed and self-fertilised. +measurements. +cross with fresh stock. +weight of seed. +colour of flowers. +remarks on experiments. +early flowering of crossed. +uniform colour of self-fertilised. +seeds. +few capsules. + +Dickie, Dr., self-fertilisation in Cannaceae. + +Dictamnus fraxinella. + +Digitalis purpurea. +measurements. +effects of intercrossing. +superiority of crossed. +self-sterile. + +Dipsaceae. + +Dobbs, bees frequenting flowers of same species. + +Dodel, Dr. A., sexual reproduction. + +Duhamel on Raphanus sativus. + +Dunal, nectar as an excretion. + +Dyer, Mr., on Lobelia ramosa. +on Cineraria. + +Earley, W., self-fertilisation of Lathyrus odoratus. + +Eaton, Reverend A.E., on Pringlea. + +Engelmann, development of sexual forms. + +Engler, Dr., on dichogamous Saxifraga. + +Entomophilous plants. + +Epipactis latifolia, attractive only to wasps. + +Erica tetralix. +perforated corolla. + +Erythrina. + +Eschscholtzia californica. +measurements. +plants raised from Brazilian seed. +weight. +seeds. +experiments on. +superiority of self-fertilised over crossed. +early flowering. +artificially self-fertilised. +pollen from other flowers more effective. +self-sterile in Brazil. + +Euphrasia officinalis. + +Euryale amazonica. +-- ferox. + +Fabricius on Aristolochia. + +Fagopyrum esculentum. +early flowering of crossed plant. + +Faivre, Professor, self-fertilisation of Cannaceae. + +Farrer, T.H., papilionaceous flowers. +Lupinus luteus. +Phaseolus multiflorus. +Pisum sativum. +cross-fertilisation of Lobelia ramosa. +on Coronilla. + +Fermond, M., Phaseolus multiflorus. +Phaseolus coccineus hybridus. + +Fertilisation, means of. +plants sterile, or partially so without insect-aid. +plants fertile without insect-aid. +means of cross-fertilisation. +humming-birds. +Australian flowers fertilised by honey-sucking birds. +in New Zealand by the Anthornis melanura. +attraction of bright colours. +of odours. +flowers adapted to certain kinds of insects. +large amount of pollen-grains. +transport of pollen by insects. +structure and conspicuousness of flowers. +pollen from a distinct plant. +prepotent pollen. + +Fertility, heights and weights, relative, of plants crossed by a fresh +stock, self-fertilised, or intercrossed (Table 7/C). + +Fertility of plants as influenced by cross and self-fertilisation (Table +9/D). +relative, of crossed and self-fertilised parents (Table 9/E). +innate, from a cross with fresh stock (Table 9/F). +relative, of flowers crossed with pollen from a distinct plant and their +own pollen (Table 9/G). +of crossed and self-fertilised flowers. + +Flowering, period of, superiority of crossed over self-fertilised. + +Flowers, white, larger proportion smelling sweetly. +structure and conspicuousness of. +conspicuous and inconspicuous. +papilionaceous. +fertilised with pollen from a distinct plant. + +Forsythia viridissima. + +Foxglove. +Frankland, Dr., chemical affinity. + +Fraxinus ornus. + +Fumaria capreolata. +-- officinalis. + +Galium aparine. + +Gallesio, spontaneous crossing of oranges. + +Galton, Mr., Limnanthes douglasii. +report on the tables of measurements. +self-fertilised plants. +superior vigour of crossed seedlings in Lathyrus odoratus. + +Gartner, excess of pollen injurious. +plants fertilising one another at a considerable distance. +Lobelia fulgens. +sterility of Verbascum nigrum. +number of pollen-grains to fertilise Geum urbanum. +experiments with pollen. + +Gentry, Mr., perforation of corolla. + +Geraniaceae. + +Geranium phaeum. + +Gerardia pedicularia. + +Germination, period of, and relative weight of seeds from crossed and +self-fertilised flowers. + +Gesneria pendulina. +measurements. +seeds. + +Gesneriaceae. + +Geum urbanum, number of pollen-grains for fertilisation. + +Glaucium luteum. + +Godron, intercrossing of carrot. +Primula grandiflora affected by pollen of Primula officinalis. +tulips. + +Gould, humming-birds frequenting Impatiens. + +Graminaceae. + +Grant, Mr., bees of different hives visiting different kinds of flowers. + +Gray, Asa, sexual relations of trees in United States. +on sexual reproduction. + +Hallet, Major, on selection of grains of cereals. + +Hassall, Mr., number of pollen-grains in Paeony and Dandelion. +weight of pollen produced by one plant of Bulrush. + +Heartsease. + +Hedychium. + +Hedysarum onobrychis. + +Heights, relative, of crossed and self-fertilised plants (Table 7/A). + +Heights, weights, and fertility, summary. + +Henschel's experiments with pollen. + +Henslow, Reverend G., cross-fertilisation in Sarothamnus scoparius. + +Herbert on cross-fertilisation. +pollen brought from distant plants. +spontaneous crossing of rhododendrons. + +Hero, descendants of the plant. +its self-fertilisation. + +Heterocentron mexicanum. + +Hibiscus africanus. +measurements. +result of experiments. +early flowering of crossed plant. +number of pollen-grains for fertilisation. + +Hildebrand on pollen of Digitalis purpurea. +Thunbergia alata. +experiments on Eschscholtzia californica. +Viola tricolor. +Lobelia ramosa. +Fagopyrum esculentum. +self-fertilisation of Zea mays. +Corydalis cava. +Hypecoum grandiflorum. +and Hypecoum procumbens. +sterility of Eschscholtzia. +experiments on self-fertilisation. +Corydalis lutea. +spontaneously self-fertilised flowers. +various mechanical structure to check self-fertilisation. +early separation of the sexes. +on Aristolochia. +fertilisation of the Gramineae. +wide dissemination of seeds. + +Hoffmann, Professor H., self-fertilised capsules of Papaver somniferum. +Adonis aestivalis. +spontaneous variability of Phaseolus multiflorus. +self-fertilisation of kidney-bean. +Papaver alpinum. +sterility of Corydalis solida. +Linum usitatissimum. + +Honey-dew. + +Hooker, Dr., Euryale ferox and Victoria regia, each producing several +flowers at once. +on sexual relation of trees in New Zealand. + +Horse-chestnut. + +Humble-bees, see Bees. + +Humboldt, on the grains of cereals. + +Humming-Birds a means of cross-fertilisation. + +Hyacinth. + +Hybrid plants, tendency to revert to their parent forms. + +Hypecoum grandiflorum. +-- procumbens. + +Iberis umbellata (var. kermesiana). +measurement. +cross by fresh stocks. +remarks on experiments. +superiority of crossed over self-fertilised seedlings. +early flowering. +number of seeds. +highly self-fertile. +prepotency of other pollen. +-- amara. + +Impatiens frequented by humming-birds. +-- barbigera. +-- fulva. +-- noli-me-tangere. +-- pallida. + +Inheritance, force of, in plants. + +Insects, means of cross-fertilisation. +attracted by bright colours. +by odours. +by conspicuous flowers. +dark streaks and marks as guides for. +flowers adapted to certain kinds. + +Ipomoea purpurea. +measurements. +flowers on same plant crossed. +cross with fresh stock. +descendants of Hero. +summary of measurements. +diagram showing mean heights. +summary of observations. +of experiments. +superiority of crossed. +early flowering. +effects of intercrossing. +uniform colour of self-fertilised. +seeds. +highly self-fertile. +prepotency of other pollen. + +Iris, secretion of saccharine matter from calyx. + +Isotoma. + +Juglans regia. + +Kalmia latifolia. + +Kerner, on protection of the pollen. +on the single daily flower of Villarsia parnassifolia. +pollen carried by wind. + +Kidney-bean. + +Kitchener, Mr., on the action of the stigma. +on Viola tricolor. + +Knight, A., on the sexual intercourse of plants. +crossing varieties of peas. +sexual reproduction. + +Kohl-rabi, prepotency of pollen. + +Kolreuter on cross-fertilisation. +number of pollen-grains necessary for fertilisation. +sexual affinities of Nicotiana. +Verbascum phoeniceum. +experiments with pollen of Hibiscus vesicarius. + +Kuhn adopts the term cleistogene. + +Kurr, on excretion of nectar. +removal of corolla. + +Labiatae. + +Lactuca sativa. +measurement. +prepotency of other pollen. + +Lamium album. +-- purpureum. + +Lathyrus odoratus. +measurements. +remarks on experiments. +period of flowering. +cross-fertilisation. +seeds. +self-fertile. +-- grandiflorus. +-- nissolia. +-- sylvestris, perforation of corolla. + +Lawes and Gilbert, Messrs., consumption of inorganic matter by plants. + +Laxton, Mr., crossing varieties of peas. + +Lecoq, Cyclamen repandum. +on Fumariaceae. +annual plants rarely dioecious. + +Leersia oryzoides. + +Leguminosae. +summary on the. + +Leighton, Reverend W.A., on Phaseolus multiflorus. +Acacia magnifica. + +Leptosiphon androsaceus. + +Leschenaultia formosa. + +Lettuce. + +Lilium auratum. + +Limnanthes douglasii. +measurements. +early flowering of crossed. +seeds. +highly self-fertile. +prepotency of other pollen. + +Linaria vulgaris. +seeds. +self-sterile. +-- cymbalaria. + +Lindley on Fumariaceae. + +Link, hypopetalous nectary in Chironia decussata. + +Linum grandiflorum. +-- usitatissimum. + +Loasaceae. + +Lobelia erinus. +secretion of nectar in sunshine. +experiments with bees. + +Lobelia fulgens. +measurements. +summary of experiments. +early flowering of self-fertilised. +seeds. +sterile unless visited by humble-bees. +-- ramosa. +measurements. +early flowering of crossed. +seeds. +self-sterile. +-- tenuior. + +Loiseleur-Deslongchamp, on the grains of cereals. + +Lotus corniculatus. + +Lubbock, Sir J., cross-fertilisation of flowers. +on Viola tricolor. +bees distinguishing colours. +instinct of bees and insects sucking nectar. + +Lupinus luteus. +measurements. +early flowering of self-fertilised. +self-fertile. +prepotency of other pollen. +-- pilosus. +self-fertile. + +Lychnis dioica. + +MacNab, Mr., on the shorter or longer stamens of rhododendrons. + +Mahonia aquifolium. +-- repens. + +Malvaceae. + +Marcgraviaceae. + +Masters, Mr., cross-fertilisation in Pisum sativum. +cabbages affected by pollen at a distance. + +Masters, Dr. Maxwell, on honey-dew. + +Measurements, summary of. +Table 7/A. +Table 7/B. +Table 7/C. + +Medicago lupulina. + +Meehan, Mr., fertilising Petunia violacea by night moth. + +Melastomaceae. + +Melilotus officinalis. + +Mercurialis annua. + +Miller, Professor, on chemical affinity. + +Mimulus luteus, effects of crossing. +crossed and self-fertilised plants. +measurements. +cross with a distinct stock. +intercrossed on same plant. +summary of observations. +of experiments. +superiority of crossed plants. +simultaneous flowering. +effects of intercrossing. +uniform colour of self-fertilised. +seeds. +highly self-fertile. +prepotency of other pollen. +-- roseus. + +Miner, Mr., red clover never sucked by hive-bees in the United States. + +Mirabilis, dwarfed plants raised by using too few pollen-grains. +number of grains necessary for fertilisation. + +Mitchell, Dr., on first cousins inter-marrying. + +Monochaetum ensiferum. + +Moore, Mr., on Cinerarias. + +Muller, Fritz, on Posoqueria fragrans. +experiments on hybrid Abutilons and Bignonias. +large number of Orchidaceous genera sterile in their native home, also +Bignonia and Tabernaemontana echinata. +sterility of Eschscholtzia californica. +Abutilon darwinii. +experiments in self-fertilisation. +self-sterile plants. +incapacity of pollen-tubes to penetrate the stigma. +cross-fertilisation by means of birds. +imperfectly developed male and female Termites. +food-bodies in Cecropia. + +Muller, Hermann, fertilisation of flowers by insects. +on Digitalis purpurea. +Calceolaria. +Linaria vulgaris. +Verbascum nigrum. +the common cabbage. +Papaver dubium. +Viola tricolor. +structure of Delphinium consolida. +of Lupinus lutea. +flowers of Pisum sativum. +on Sarothamnus scoparius not secreting nectar. +Apium petroselinum. +Borago officinalis. +red clover visited by hive-bees in Germany. +insects rarely visiting Fumaria officinalis. +comparison of lowland and alpine species. +structure of plants adapted to cross and self-fertilisation. +large conspicuous flowers more frequently visited by insects than small +inconspicuous ones. +Solanum generally unattractive to insects. +Lamium album. +on anemophilous plants. +fertilisation of Plantago. +secretion of nectar. +instinct of bees sucking nectar. +bees frequenting flowers of the same species. +cause of it. +powers of vision and discrimination of bees. + +Muller, Dr. H., hive-bees occasionally perforate the flower of Erica +tetralix. +calyx and corolla of Rhinanthus alecterolophus bored by Bombus +mastrucatus. + +Munro, Mr., some species of Oncidium and Maxillaria sterile with own +pollen. + +Myrtaceae. + +Nageli on odours attracting insects. +sexual relations. + +Natural selection, effect upon self-sterility and self-fertilisation. + +Naudin on number of pollen-grains necessary for fertilisation. +Petunia violacea. + +Nectar regarded as an excretion. + +Nemophila insignis. +measurements. +early flowering of crossed plant. +effects of cross and self-fertilisation. +seeds. + +Nepeta glechoma. + +Nicotiana glutinosa. +-- tabacum. +measurements. +cross with fresh stock. +measurements. +summary of experiments. +superiority of crossed plants. +early flowering. +seeds. +experiments on. +self-fertile. + +Nolana prostrata. +measurements. +crossed and self-fertilised plants. +number of capsules and seeds. +self-fertile. + +Nolanaceae. + +Nymphaea. + +Odours emitted by flowers attractive to insects. + +Ogle, Dr., on Digitalis purpurea. +Gesneria. +Phaseolus multiflorus. +perforation of corolla. +case of the Monkshood. + +Onagraceae. + +Onion, prepotency of other pollen. + +Ononis minutissima. +measurements. +seeds. +self-fertile. + +Ophrys apifera. +-- muscifera. + +Oranges, spontaneous crossing. + +Orchideae. +excretion of saccharine matter. + +Orchis, fly. + +Origanum vulgare. +measurements. +early flowering of crossed plant. +effects of intercrossing. + +Paeony, number of pollen-grains. + +Papaveraceae. + +Papaver alpinum. +-- argemonoides. +-- bracteatum. +-- dubium. +-- orientale. +-- rhoeas. +-- somniferum. +-- vagum. +measurements. +number of capsules. +seeds. +prepotency of other pollen. + +Papillae of the Viola tricolor attractive to insects. + +Parsley. + +Passiflora alata. +-- gracilis. +measurements. +crossed and self-fertilised. +seeds. +self-fertile. + +Passifloraceae. + +Pea, common. + +Pelargonium zonale. +measurements. +effects of intercrossing. +almost self-sterile. + +Pentstemon argutus, perforated corolla. + +Petunia violacea. +measurements. +weight of seed. +cross with fresh stock. +relative fertility. +colour. +summary of experiments. +superiority of crossed over self-fertilised. +early flowering. +uniform colour of self-fertilised. +seeds. +self-sterile. + +Phalaris canariensis. +measurements. +early flowering of crossed. + +Phaseolus coccineus. +-- multiflorus. +measurement. +partially sterile. +crossed and self-fertilised. +early flowering of crossed. +seeds. +perforated by humble-bees. +-- vulgaris. +self-fertile. + +Pisum sativum. +measurements. +seldom intercross. +summary of experiments. +self-fertile. + +Plants, crossed, greater constitutional vigour. + +Pleroma. + +Polemoniaceae. + +Pollen, relative fertility of flowers crossed from a distinct plant, or +with their own. +difference of results in Nolana prostrata. +crossed and self-fertilised plants, again crossed from a distinct plant +and their own pollen. +sterile with their own. +semi-self-sterile. +loss of. +number of grains in Dandelion, Paeony, and Wistaria sinensis. +number necessary for fertilisation. +transported from flower to flower. +prepotency. +aboriginally the sole attraction to insects. +quantity produced by anemophilous plants. + +Polyanthus, prepotency over cowslip. + +Polygoneae. + +Posoqueria fragrans. + +Potato. + +Poterium sanguisorba. + +Potts, heads of Anthornis melanura covered with pollen. + +Primrose, Chinese. + +Primula elatior. +-- grandiflora. +-- mollis. +-- officinalis. +-- scotica. +-- sinensis. +measurements. +early flowering of crossed. +-- veris (var. officinalis). +measurements. +result of experiments. +early flowering of crossed. +seeds. +self-fertility. +prepotency of dark red polyanthus. + +Primulaceae. + +Pringlea. + +Proteaceae of Australia. + +Prunus avium. +-- laurocerasus. + +Pteris aquilina. + +Radish. + +Ranunculaceae. + +Ranunculus acris. + +Raphanus sativus. + +Reinke, nectar-secreting glands of Prunus avium. + +Reseda lutea. +measurements. +result of experiments. +self-fertile. +-- odorata. +measurements. +self-fertilised scarcely exceeded by crossed. +seeds. +want of correspondence between seeds and vigour of offspring. +result of experiments. +sterile and self-fertile. + +Resedaceae. + +Rheum rhaponticum. + +Rhexia glandulosa. + +Rhododendron, spontaneous crossing. + +Rhododendron azaloides. + +Rhubarb. + +Ribes aureum. + +Riley, Mr., pollen carried by wind. +Yucca moth. + +Rodgers, Mr., secretion of nectar in Vanilla. + +Rye, experiment on pollen of. + +Salvia coccinea. +measurements. +early flowering of crossed. +seeds. +partially self-sterile. +-- glutinosa. +-- grahami. +-- tenori. + +Sarothamnus scoparius. +measurements. +superiority of crossed seedlings. +seeds. +self-sterile. + +Scabiosa atro-purpurea. +measurements. + +Scarlet-runner. + +Scott, J., Papaver somniferum. +sterility of Verbascum. +Oncidium and Maxillaria. +on Primula scotica and Cortusa matthioli. + +Scrophulariaceae. + +Self-sterile varieties, appearance of. + +Self-fertilisation, mechanical structure to check. + +Self-sterile plants. +wide distribution throughout the vegetable kingdom. +difference in plants. +cause of self-sterility. +affected by changed conditions. +necessity of differentiation in the sexual elements. + +Senecio cruentus. +-- heritieri. +-- maderensis +-- populifolius. +-- tussilaginis. + +Sharpe, Messrs., precautions against intercrossing. + +Snow-flake. + +Solanaceae. + +Solanum tuberosum. + +Specularia perfoliata. +-- speculum. +measurements. +crossed and self-fertilised. +early flowering of crossed. +seeds. +self-fertile. + +Spencer, Herbert, chemical affinity. + +Spiranthes autumnalis. + +Sprengel, C.K., fertilisation of flowers by insects. +Viola tricolor. +colours in flowers attract and guide insects. +on Aristolochia. +Aconitum napellus. +importance of insects in fertilising flowers. + +Stachys coccinea. + +Stellaria media. + +Strachey, General, perforated flowers in the Himalaya. + +Strawberry. + +Strelitzia fertilised by the Nectarinideae. + +Structure of plants adapted to cross and self-fertilisation. + +Swale, Mr., garden lupine not visited by bees in New Zealand. + +Sweet-pea. + +Tabernaemontana echinata. + +Tables of measurements of heights, weights, and fertility of plants. + +Termites, imperfectly developed males and females. + +Thunbergia alata. + +Thyme. + +Tinzmann, on Solanum tuberosum. + +Tobacco. + +Transmission of the good effects of a cross to later generations. + +Trees, separated sexes. + +Trifolium arvense. +-- incarnatum. +-- minus. +-- pratense. +-- procumbens. +-- repens. + +Tropaeolum minus. +measurements. +early flowering of crossed. +seeds. +-- tricolor. +seeds. + +Tulips. + +Typha. + +Umbelliferae. + +Urban, Ig., fertilisation of Medicago lupulina. + +Vandellia nummularifolia. +seeds. +self-fertile. + +Vanilla, secretion of nectar. + +Verbascum lychnitis. +-- nigrum. +-- phoeniceum. +-- thapsus. +measurements. +self-fertile. + +Verlot on Convolvulus tricolor. +intercrossing of Nemophila. +of Leptosiphon. + +Veronica agrestis. +-- chamaedrys. +-- hederaefolia. + +Vicia faba. +-- hirsuta. +-- sativa. + +Victoria regia. + +Villarsia parnassifolia. + +Vilmorin on transmitting character to offspring. + +Vinca major. +-- rosea. + +Viola canina. +-- tricolor. +measurements. +superiority of crossed plants. +period of flowering. +effects of cross-fertilisation. +seeds. +partially sterile. +corolla removed. + +Violaceae. + +Viscaria oculata. +measurement. +average height of crossed and self-fertilised. +simultaneous flowering. +seeds. +self-fertile. + +Wallace, Mr., the beaks and faces of brush-tongued lories covered with +pollen. + +Wasps attracted by Epipactis latifolia. + +Weights, relative, of crossed and self-fertilised plants. +and period of germination of seeds. + +Wilder, Mr., fertilisation of flowers with their own pollen. + +Wilson, A.J., superior vigour of crossed seedlings in Brassica +campestris ruta baga. + +Wistaria sinensis. + +Yucca moth. + +Zea mays. +measurements. +difference of height between crossed and self-fertilised. +early flowering of crossed. +self-fertile. +prepotency of other pollen. + + + + + + + + + +End of the Project Gutenberg Etext of The Effects of Cross & Self-Fertilisation +in the Vegetable Kingdom, by Charles Darwin + |