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diff --git a/old/cplnt10.txt b/old/cplnt10.txt new file mode 100644 index 0000000..4609cad --- /dev/null +++ b/old/cplnt10.txt @@ -0,0 +1,5626 @@ +The Project Gutenberg Etext of Climbing Plants by Charles Darwin +#10 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 posting these files!! + +Please take a look at the important information in this header. +We encourage you to keep this file on your own disk, keeping an +electronic path open for the next readers. Do not remove this. + + +**Welcome To The World of Free Plain Vanilla Electronic Texts** + +**Etexts Readable By Both Humans and By Computers, Since 1971** + +*These Etexts Prepared By Hundreds of Volunteers and Donations* + +Information on contacting Project Gutenberg to get Etexts, and +further information is included below. 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FOR PUBLIC DOMAIN ETEXTS*Ver.04.29.93*END* + + + + + +This etext was prepared by David Price, email ccx074@coventry.ac.uk +from the 1906 John Murray edition. + + + + + +THE MOVEMENTS AND HABITS OF CLIMBING PLANTS + + + + +PREFACE + + + +This Essay first appeared in the ninth volume of the 'Journal of the +Linnean Society,' published in 1865. It is here reproduced in a +corrected and, I hope, clearer form, with some additional facts. The +illustrations were drawn by my son, George Darwin. Fritz Muller, +after the publication of my paper, sent to the Linnean Society +(Journal, vol. ix., p. 344) some interesting observations on the +climbing plants of South Brazil, to which I shall frequently refer. +Recently two important memoirs, chiefly on the difference in growth +between the upper and lower sides of tendrils, and on the mechanism +of the movements of twining-plants, by Dr. Hugo de Vries, have +appeared in the 'Arbeiten des Botanischen Instituts in Wurzburg,' +Heft. iii., 1873. These memoirs ought to be carefully studied by +every one interested in the subject, as I can here give only +references to the more important points. This excellent observer, as +well as Professor Sachs, {1} attributes all the movements of tendrils +to rapid growth along one side; but, from reasons assigned towards +the close of my fourth chapter, I cannot persuade myself that this +holds good with respect to those due to a touch. In order that the +reader may know what points have interested me most, I may call his +attention to certain tendril-bearing plants; for instance, Bignonia +capreolata, Cobaea, Echinocystis, and Hanburya, which display as +beautiful adaptations as can be found in any part of the kingdom of +nature. It is, also, an interesting fact that intermediate states +between organs fitted for widely different functions, may be observed +on the same individual plant of Corydalis claviculata and the common +vine; and these cases illustrate in a striking manner the principle +of the gradual evolution of species. + + + +APPENDIX TO PREFACE (1882). + + + +Since the publication of this Edition two papers by eminent botanists +have appeared; Schwendener, 'Das Winden der Pflanzen' (Monatsberichte +der Berliner Akademie, Dec. 1881), and J. Sachs, 'Notiz uber +Schlingpflanzen' (Arbeiten des botanischen Instituts in Wurzburg, Bd. +ii. p. 719, 1882). The view "that the capacity of revolving, on +which most climbers depend, is inherent, though undeveloped, in +almost every plant in the vegetable kingdom" ('Climbing Plants,' p. +205), has been confirmed by the observations on circumnutation since +given in 'The Power of Movement in Plants.' + + + +ERRATA. + + + +On pp. 28, 32, 40, 53, statements are made with reference to the +supposed acceleration of the revolving movement towards the light. +It appears from the observations given in 'The Power of Movement in +Plants,' p. 451, that these conclusions were drawn from insufficient +observations, and are erroneous. + + + + +THE MOVEMENTS AND HABITS OF CLIMBING PLANTS. + + + + +CHAPTER I.--TWINING PLANTS. + + + +Introductory remarks--Description of the twining of the Hop--Torsion +of the stems--Nature of the revolving movement, and manner of ascent- +-Stems not irritable--Rate of revolution in various plants--Thickness +of the support round which plants can twine--Species which revolve in +an anomalous manner. + +I was led to this subject by an interesting, but short paper by +Professor Asa Gray on the movements of the tendrils of some +Cucurbitaceous plants. {2} My observations were more than half +completed before I learnt that the surprising phenomenon of the +spontaneous revolutions of the stems and tendrils of climbing plants +had been long ago observed by Palm and by Hugo von Mohl, {3} and had +subsequently been the subject of two memoirs by Dutrochet. {4} +Nevertheless, I believe that my observations, founded on the +examination of above a hundred widely distinct living species, +contain sufficient novelty to justify me in publishing them. + +Climbing plants may be divided into four classes. First, those which +twine spirally round a support, and are not aided by any other +movement. Secondly, those endowed with irritable organs, which when +they touch any object clasp it; such organs consisting of modified +leaves, branches, or flower-peduncles. But these two classes +sometimes graduate to a certain extent into one another. Plants of +the third class ascend merely by the aid of hooks; and those of the +fourth by rootlets; but as in neither class do the plants exhibit any +special movements, they present little interest, and generally when I +speak of climbing plants I refer to the two first great classes. + + +TWINING PLANTS. + + +This is the largest subdivision, and is apparently the primordial and +simplest condition of the class. My observations will be best given +by taking a few special cases. When the shoot of a Hop (Humulus +lupulus) rises from the ground, the two or three first-formed joints +or internodes are straight and remain stationary; but the next- +formed, whilst very young, may be seen to bend to one side and to +travel slowly round towards all points of the compass, moving, like +the hands of a watch, with the sun. The movement very soon acquires +its full ordinary velocity. From seven observations made during +August on shoots proceeding from a plant which had been cut down, and +on another plant during April, the average rate during hot weather +and during the day is 2 hrs. 8 m. for each revolution; and none of +the revolutions varied much from this rate. The revolving movement +continues as long as the plant continues to grow; but each separate +internode, as it becomes old, ceases to move. + +To ascertain more precisely what amount of movement each internode +underwent, I kept a potted plant, during the night and day, in a +well-warmed room to which I was confined by illness. A long shoot +projected beyond the upper end of the supporting stick, and was +steadily revolving. I then took a longer stick and tied up the +shoot, so that only a very young internode, 1.75 of an inch in +length, was left free. This was so nearly upright that its +revolution could not be easily observed; but it certainly moved, and +the side of the internode which was at one time convex became +concave, which, as we shall hereafter see, is a sure sign of the +revolving movement. I will assume that it made at least one +revolution during the first twenty-four hours. Early the next +morning its position was marked, and it made a second revolution in 9 +hrs.; during the latter part of this revolution it moved much +quicker, and the third circle was performed in the evening in a +little over 3 hrs. As on the succeeding morning I found that the +shoot revolved in 2 hrs. 45 m., it must have made during the night +four revolutions, each at the average rate of a little over 3 hrs. I +should add that the temperature of the room varied only a little. +The shoot had now grown 3.5 inches in length, and carried at its +extremity a young internode 1 inch in length, which showed slight +changes in its curvature. The next or ninth revolution was effected +in 2 hrs. 30 m. From this time forward, the revolutions were easily +observed. The thirty-sixth revolution was performed at the usual +rate; so was the last or thirty-seventh, but it was not completed; +for the internode suddenly became upright, and after moving to the +centre, remained motionless. I tied a weight to its upper end, so as +to bow it slightly and thus detect any movement; but there was none. +Some time before the last revolution was half performed, the lower +part of the internode ceased to move. + +A few more remarks will complete all that need be said about this +internode. It moved during five days; but the more rapid movements, +after the performance of the third revolution, lasted during three +days and twenty hours. The regular revolutions, from the ninth to +thirty-sixth inclusive, were effected at the average rate of 2 hrs. +31 m.; but the weather was cold, and this affected the temperature of +the room, especially during the night, and consequently retarded the +rate of movement a little. There was only one irregular movement, +which consisted in the stem rapidly making, after an unusually slow +revolution, only the segment of a circle. After the seventeenth +revolution the internode had grown from 1.75 to 6 inches in length, +and carried an internode 1.875 inch long, which was just perceptibly +moving; and this carried a very minute ultimate internode. After the +twenty-first revolution, the penultimate internode was 2.5 inches +long, and probably revolved in a period of about three hours. At the +twenty-seventh revolution the lower and still moving internode was +8.375, the penultimate 3.5, and the ultimate 2.5 inches in length; +and the inclination of the whole shoot was such, that a circle 19 +inches in diameter was swept by it. When the movement ceased, the +lower internode was 9 inches, and the penultimate 6 inches in length; +so that, from the twenty-seventh to thirty-seventh revolutions +inclusive, three internodes were at the same time revolving. + +The lower internode, when it ceased revolving, became upright and +rigid; but as the whole shoot was left to grow unsupported, it became +after a time bent into a nearly horizontal position, the uppermost +and growing internodes still revolving at the extremity, but of +course no longer round the old central point of the supporting stick. +From the changed position of the centre of gravity of the extremity, +as it revolved, a slight and slow swaying movement was given to the +long horizontally projecting shoot; and this movement I at first +thought was a spontaneous one. As the shoot grew, it hung down more +and more, whilst the growing and revolving extremity turned itself up +more and more. + +With the Hop we have seen that three internodes were at the same time +revolving; and this was the case with most of the plants observed by +me. With all, if in full health, two internodes revolved; so that by +the time the lower one ceased to revolve, the one above was in full +action, with a terminal internode just commencing to move. With Hoya +carnosa, on the other hand, a depending shoot, without any developed +leaves, 32 inches in length, and consisting of seven internodes (a +minute terminal one, an inch in length, being counted), continually, +but slowly, swayed from side to side in a semicircular course, with +the extreme internodes making complete revolutions. This swaying +movement was certainly due to the movement of the lower internodes, +which, however, had not force sufficient to swing the whole shoot +round the central supporting stick. The case of another +Asclepiadaceous plant, viz., Ceropegia Gardnerii, is worth briefly +giving. I allowed the top to grow out almost horizontally to the +length of 31 inches; this now consisted of three long internodes, +terminated by two short ones. The whole revolved in a course opposed +to the sun (the reverse of that of the Hop), at rates between 5 hrs. +15 m. and 6 hrs. 45 m. for each revolution. The extreme tip thus +made a circle of above 5 feet (or 62 inches) in diameter and 16 feet +in circumference, travelling at the rate of 32 or 33 inches per hour. +The weather being hot, the plant was allowed to stand on my study- +table; and it was an interesting spectacle to watch the long shoot +sweeping this grand circle, night and day, in search of some object +round which to twine. + +If we take hold of a growing sapling, we can of course bend it to all +sides in succession, so as to make the tip describe a circle, like +that performed by the summit of a spontaneously revolving plant. By +this movement the sapling is not in the least twisted round its own +axis. I mention this because if a black point be painted on the +bark, on the side which is uppermost when the sapling is bent towards +the holder's body, as the circle is described, the black point +gradually turns round and sinks to the lower side, and comes up again +when the circle is completed; and this gives the false appearance of +twisting, which, in the case of spontaneously revolving plants, +deceived me for a time. The appearance is the more deceitful because +the axes of nearly all twining-plants are really twisted; and they +are twisted in the same direction with the spontaneous revolving +movement. To give an instance, the internode of the Hop of which the +history has been recorded, was at first, as could be seen by the +ridges on its surface, not in the least twisted; but when, after the +37th revolution, it had grown 9 inches long, and its revolving +movement had ceased, it had become twisted three times round its own +axis, in the line of the course of the sun; on the other hand, the +common Convolvulus, which revolves in an opposite course to the Hop, +becomes twisted in an opposite direction. + +Hence it is not surprising that Hugo von Mohl (p. 105, 108, &c.) +thought that the twisting of the axis caused the revolving movement; +but it is not possible that the twisting of the axis of the Hop three +times should have caused thirty-seven revolutions. Moreover, the +revolving movement commenced in the young internode before any +twisting of its axis could be detected. The internodes of a young +Siphomeris and Lecontea revolved during several days, but became +twisted only once round their own axes. The best evidence, however, +that the twisting does not cause the revolving movement is afforded +by many leaf-climbing and tendril-bearing plants (as Pisum sativum, +Echinocystis lobata, Bignonia capreolata, Eccremocarpus scaber, and +with the leaf-climbers, Solanum jasminoides and various species of +Clematis), of which the internodes are not twisted, but which, as we +shall hereafter see, regularly perform revolving movements like those +of true twining-plants. Moreover, according to Palm (pp. 30, 95) and +Mohl (p. 149), and Leon, {5} internodes may occasionally, and even +not very rarely, be found which are twisted in an opposite direction +to the other internodes on the same plant, and to the course of their +revolutions; and this, according to Leon (p. 356), is the case with +all the internodes of a certain variety of Phaseolus multiflorus. +Internodes which have become twisted round their own axes, if they +have not ceased to revolve, are still capable of twining round a +support, as I have several times observed. + +Mohl has remarked (p. 111) that when a stem twines round a smooth +cylindrical stick, it does not become twisted. {6} Accordingly I +allowed kidney-beans to run up stretched string, and up smooth rods +of iron and glass, one-third of an inch in diameter, and they became +twisted only in that degree which follows as a mechanical necessity +from the spiral winding. The stems, on the other hand, which had +ascended ordinary rough sticks were all more or less and generally +much twisted. The influence of the roughness of the support in +causing axial twisting was well seen in the stems which had twined up +the glass rods; for these rods were fixed into split sticks below, +and were secured above to cross sticks, and the stems in passing +these places became much twisted. As soon as the stems which had +ascended the iron rods reached the summit and became free, they also +became twisted; and this apparently occurred more quickly during +windy than during calm weather. Several other facts could be given, +showing that the axial twisting stands in some relation to +inequalities in the support, and likewise to the shoot revolving +freely without any support. Many plants, which are not twiners, +become in some degree twisted round their own axes; {7} but this +occurs so much more generally and strongly with twining-plants than +with other plants, that there must be some connexion between the +capacity for twining and axial twisting. The stem probably gains +rigidity by being twisted (on the same principle that a much twisted +rope is stiffer than a slackly twisted one), and is thus indirectly +benefited so as to be enabled to pass over inequalities in its spiral +ascent, and to carry its own weight when allowed to revolve freely. +{8} + +I have alluded to the twisting which necessarily follows on +mechanical principles from the spiral ascent of a stem, namely, one +twist for each spire completed. This was well shown by painting +straight lines on living stems, and then allowing them to twine; but, +as I shall have to recur to this subject under Tendrils, it may be +here passed over. + +The revolving movement of a twining plant has been compared with that +of the tip of a sapling, moved round and round by the hand held some +way down the stem; but there is one important difference. The upper +part of the sapling when thus moved remains straight; but with +twining plants every part of the revolving shoot has its own separate +and independent movement. This is easily proved; for when the lower +half or two-thirds of a long revolving shoot is tied to a stick, the +upper free part continues steadily revolving. Even if the whole +shoot, except an inch or two of the extremity, be tied up, this part, +as I have seen in the case of the Hop, Ceropegia, Convolvulus, &c., +goes on revolving, but much more slowly; for the internodes, until +they have grown to some little length, always move slowly. If we +look to the one, two, or several internodes of a revolving shoot, +they will be all seen to be more or less bowed, either during the +whole or during a large part of each revolution. Now if a coloured +streak be painted (this was done with a large number of twining +plants) along, we will say, the convex surface, the streak will after +a time (depending on the rate of revolution) be found to be running +laterally along one side of the bow, then along the concave side, +then laterally on the opposite side, and, lastly, again on the +originally convex surface. This clearly proves that during the +revolving movement the internodes become bowed in every direction. +The movement is, in fact, a continuous self-bowing of the whole +shoot, successively directed to all points of the compass; and has +been well designated by Sachs as a revolving nutation. + +As this movement is rather difficult to understand, it will be well +to give an illustration. Take a sapling and bend it to the south, +and paint a black line on the convex surface; let the sapling spring +up and bend it to the east, and the black line will be seen to run +along the lateral face fronting the north; bend it to the north, the +black line will be on the concave surface; bend it to the west, the +line will again be on the lateral face; and when again bent to the +south, the line will be on the original convex surface. Now, instead +of bending the sapling, let us suppose that the cells along its +northern surface from the base to the tip were to grow much more +rapidly than on the three other sides, the whole shoot would then +necessarily be bowed to the south; and let the longitudinal growing +surface creep round the shoot, deserting by slow degrees the northern +side and encroaching on the western side, and so round by the south, +by the east, again to the north. In this case the shoot would remain +always bowed with the painted line appearing on the several above +specified surfaces, and with the point of the shoot successively +directed to each point of the compass. In fact, we should have the +exact kind of movement performed by the revolving shoots of twining +plants. {9} + +It must not be supposed that the revolving movement is as regular as +that given in the above illustration; in very many cases the tip +describes an ellipse, even a very narrow ellipse. To recur once +again to our illustration, if we suppose only the northern and +southern surfaces of the sapling alternately to grow rapidly, the +summit would describe a simple arc; if the growth first travelled a +very little to the western face, and during the return a very little +to the eastern face, a narrow ellipse would be described; and the +sapling would be straight as it passed to and fro through the +intermediate space; and a complete straightening of the shoot may +often be observed in revolving plants. The movement is frequently +such that three of the sides of the shoot seem to be growing in due +order more rapidly than the remaining side; so that a semi-circle +instead of a circle is described, the shoot becoming straight and +upright during half of its course. + +When a revolving shoot consists of several internodes, the lower ones +bend together at the same rate, but one or two of the terminal ones +bend at a slower rate; hence, though at times all the internodes are +in the same direction, at other times the shoot is rendered slightly +serpentine. The rate of revolution of the whole shoot, if judged by +the movement of the extreme tip, is thus at times accelerated or +retarded. One other point must be noticed. Authors have observed +that the end of the shoot in many twining plants is completely +hooked; this is very general, for instance, with the Asclepiadaceae. +The hooked tip, in all the cases observed by me, viz, in Ceropegia, +Sphaerostemma, Clerodendron, Wistaria, Stephania, Akebia, and +Siphomeris, has exactly the same kind of movement as the other +internodes; for a line painted on the convex surface first becomes +lateral and then concave; but, owing to the youth of these terminal +internodes, the reversal of the hook is a slower process than that of +the revolving movement. {10} This strongly marked tendency in the +young, terminal and flexible internodes, to bend in a greater degree +or more abruptly than the other internodes, is of service to the +plant; for not only does the hook thus formed sometimes serve to +catch a support, but (and this seems to be much more important) it +causes the extremity of the shoot to embrace the support much more +closely than it could otherwise have done, and thus aids in +preventing the stem from being blown away during windy weather, as I +have many times observed. In Lonicera brachypoda the hook only +straightens itself periodically, and never becomes reversed. I will +not assert that the tips of all twining plants when hooked, either +reverse themselves or become periodically straight, in the manner +just described; for the hooked form may in some cases be permanent, +and be due to the manner of growth of the species, as with the tips +of the shoots of the common vine, and more plainly with those of +Cissus discolor--plants which are not spiral twiners. + +The first purpose of the spontaneous revolving movement, or, more +strictly speaking, of the continuous bowing movement directed +successively to all points of the compass, is, as Mohl has remarked, +to favour the shoot finding a support. This is admirably effected by +the revolutions carried on night and day, a wider and wider circle +being swept as the shoot increases in length. This movement likewise +explains how the plants twine; for when a revolving shoot meets with +a support, its motion is necessarily arrested at the point of +contact, but the free projecting part goes on revolving. As this +continues, higher and higher points are brought into contact with the +support and are arrested; and so onwards to the extremity; and thus +the shoot winds round its support. When the shoot follows the sun in +its revolving course, it winds round the support from right to left, +the support being supposed to stand in front of the beholder; when +the shoot revolves in an opposite direction, the line of winding is +reversed. As each internode loses from age its power of revolving, +it likewise loses its power of spirally twining. If a man swings a +rope round his head, and the end hits a stick, it will coil round the +stick according to the direction of the swinging movement; so it is +with a twining plant, a line of growth travelling round the free part +of the shoot causing it to bend towards the opposite side, and this +replaces the momentum of the free end of the rope. + +All the authors, except Palm and Mohl, who have discussed the spiral +twining of plants, maintain that such plants have a natural tendency +to grow spirally. Mohl believes (p. 112) that twining stems have a +dull kind of irritability, so that they bend towards any object which +they touch; but this is denied by Palm. Even before reading Mohl's +interesting treatise, this view seemed to me so probable that I +tested it in every way that I could, but always with a negative +result. I rubbed many shoots much harder than is necessary to excite +movement in any tendril or in the foot-stalk of any leaf climber, but +without any effect. I then tied a light forked twig to a shoot of a +Hop, a Ceropegia, Sphaerostemma, and Adhatoda, so that the fork +pressed on one side alone of the shoot and revolved with it; I +purposely selected some very slow revolvers, as it seemed most likely +that these would profit most from possessing irritability; but in no +case was any effect produced. {11} Moreover, when a shoot winds +round a support, the winding movement is always slower, as we shall +immediately see, than whilst it revolves freely and touches nothing. +Hence I conclude that twining stems are not irritable; and indeed it +is not probable that they should be so, as nature always economizes +her means, and irritability would have been superfluous. +Nevertheless I do not wish to assert that they are never irritable; +for the growing axis of the leaf-climbing, but not spirally twining, +Lophospermum scandens is, certainly irritable; but this case gives me +confidence that ordinary twiners do not possess any such quality, for +directly after putting a stick to the Lophopermum, I saw that it +behaved differently from a true twiner or any other leaf-climber. +{12} + +The belief that twiners have a natural tendency to grow spirally, +probably arose from their assuming a spiral form when wound round a +support, and from the extremity, even whilst remaining free, +sometimes assuming this form. The free internodes of vigorously +growing plants, when they cease to revolve, become straight, and show +no tendency to be spiral; but when a shoot has nearly ceased to grow, +or when the plant is unhealthy, the extremity does occasionally +become spiral. I have seen this in a remarkable manner with the ends +of the shoots of the Stauntonia and of the allied Akebia, which +became wound up into a close spire, just like a tendril; and this was +apt to occur after some small, ill-formed leaves had perished. The +explanation, I believe, is, that in such cases the lower parts of the +terminal internodes very gradually and successively lose their power +of movement, whilst the portions just above move onwards and in their +turn become motionless; and this ends in forming an irregular spire. + +When a revolving shoot strikes a stick, it winds round it rather more +slowly than it revolves. For instance, a shoot of the Ceropegia, +revolved in 6 hrs., but took 9 hrs. 30 m. to make one complete spire +round a stick; Aristolochia gigas revolved in about 5 hrs., but took +9 hrs. 15 m. to complete its spire. This, I presume, is due to the +continued disturbance of the impelling force by the arrestment of the +movement at successive points; and we shall hereafter see that even +shaking a plant retards the revolving movement. The terminal +internodes of a long, much-inclined, revolving shoot of the +Ceropegia, after they had wound round a stick, always slipped up it, +so as to render the spire more open than it was at first; and this +was probably in part due to the force which caused the revolutions, +being now almost freed from the constraint of gravity and allowed to +act freely. With the Wistaria, on the other hand, a long horizontal +shoot wound itself at first into a very close spire, which remained +unchanged; but subsequently, as the shoot twined spirally up its +support, it made a much more open spire. With all the many plants +which were allowed freely to ascend a support, the terminal +internodes made at first a close spire; and this, during windy +weather, served to keep the shoots in close contact with their +support; but as the penultimate internodes grew in length, they +pushed themselves up for a considerable space (ascertained by +coloured marks on the shoot and on the support) round the stick, and +the spire became more open. {13} + +It follows from this latter fact that the position occupied by each +leaf with respect to the support depends on the growth of the +internodes after they have become spirally wound round it. I mention +this on account of an observation by Palm (p. 34), who states that +the opposite leaves of the Hop always stand in a row, exactly over +one another, on the same side of the supporting stick, whatever its +thickness may be. My sons visited a hop-field for me, and reported +that though they generally found the points of insertion of the +leaves standing over each other for a space of two or three feet in +height, yet this never occurred up the whole length of the pole; the +points of insertion forming, as might have been expected, an +irregular spire. Any irregularity in the pole entirely destroyed the +regularity of position of the leaves. From casual inspection, it +appeared to me that the opposite leaves of Thunbergia alata were +arranged in lines up the sticks round which they had twined; +accordingly, I raised a dozen plants, and gave them sticks of various +thicknesses, as well as string, to twine round; and in this case one +alone out of the dozen had its leaves arranged in a perpendicular +line: I conclude, therefore, Palm's statement is not quite accurate. + +The leaves of different twining-plants are arranged on the stem +(before it has twined) alternately, or oppositely, or in a spire. In +the latter case the line of insertion of the leaves and the course of +the revolutions coincide. This fact has been well shown by +Dutrochet, {14} who found different individuals of Solanum dulcamara +twining in opposite directions, and these had their leaves in each +case spirally arranged in the same direction. A dense whorl of many +leaves would apparently be incommodious for a twining plant, and some +authors assert that none have their leaves thus arranged; but a +twining Siphomeris has whorls of three leaves. + +If a stick which has arrested a revolving shoot, but has not as yet +been encircled, be suddenly taken away, the shoot generally springs +forward, showing that it was pressing with some force against the +stick. After a shoot has wound round a stick, if this be withdrawn, +it retains for a time its spiral form; it then straightens itself, +and again commences to revolve. The long, much-inclined shoot of the +Ceropegia previously alluded to offered some curious peculiarities. +The lower and older internodes, which continued to revolve, were +incapable, on repeated trials, of twining round a thin stick; showing +that, although the power of movement was retained, this was not +sufficient to enable the plant to twine. I then moved the stick to a +greater distance, so that it was struck by a point 2.5 inches from +the extremity of the penultimate internode; and it was then neatly +encircled by this part of the penultimate and by the ultimate +internode. After leaving the spirally wound shoot for eleven hours, +I quietly withdrew the stick, and in the course of the day the curled +portion straightened itself and recommenced revolving; but the lower +and not curled portion of the penultimate internode did not move, a +sort of hinge separating the moving and the motionless part of the +same internode. After a few days, however, I found that this lower +part had likewise recovered its revolving power. These several facts +show that the power of movement is not immediately lost in the +arrested portion of a revolving shoot; and that after being +temporarily lost it can be recovered. When a shoot has remained for +a considerable time round a support, it permanently retains its +spiral form even when the support is removed. + +When a tall stick was placed so as to arrest the lower and rigid +internodes of the Ceropegia, at the distance at first of 15 and then +of 21 inches from the centre of revolution, the straight shoot slowly +and gradually slid up the stick, so as to become more and more highly +inclined, but did not pass over the summit. Then, after an interval +sufficient to have allowed of a semi-revolution, the shoot suddenly +bounded from the stick and fell over to the opposite side or point of +the compass, and reassumed its previous slight inclination. It now +recommenced revolving in its usual course, so that after a semi- +revolution it again came into contact with the stick, again slid up +it, and again bounded from it and fell over to the opposite side. +This movement of the shoot had a very odd appearance, as if it were +disgusted with its failure but was resolved to try again. We shall, +I think, understand this movement by considering the former +illustration of the sapling, in which the growing surface was +supposed to creep round from the northern by the western to the +southern face; and thence back again by the eastern to the northern +face, successively bowing the sapling in all directions. Now with +the Ceropegia, the stick being placed to the south of the shoot and +in contact with it, as soon as the circulatory growth reached the +western surface, no effect would be produced, except that the shoot +would be pressed firmly against the stick. But as soon as growth on +the southern surface began, the shoot would be slowly dragged with a +sliding movement up the stick; and then, as soon as the eastern +growth commenced, the shoot would be drawn from the stick, and its +weight coinciding with the effects of the changed surface of growth, +would cause it suddenly to fall to the opposite side, reassuming its +previous slight inclination; and the ordinary revolving movement +would then go on as before. I have described this curious case with +some care, because it first led me to understand the order in which, +as I then thought, the surfaces contracted; but in which, as we now +know from Sachs and II. de Vries, they grow for a time rapidly, thus +causing the shoot to bow towards the opposite side. + +The view just given further explains, as I believe, a fact observed +by Mohl (p. 135), namely, that a revolving shoot, though it will +twine round an object as thin as a thread, cannot do so round a thick +support. I placed some long revolving shoots of a Wistaria close to +a post between 5 and 6 inches in diameter, but, though aided by me in +many ways, they could not wind round it. This apparently was due to +the flexure of the shoot, whilst winding round an object so gently +curved as this post, not being sufficient to hold the shoot to its +place when the growing surface crept round to the opposite surface of +the shoot; so that it was withdrawn at each revolution from its +support. + +When a free shoot has grown far beyond its support, it sinks +downwards from its weight, as already explained in the case of the +Hop, with the revolving extremity turned upwards. If the support be +not lofty, the shoot falls to the ground, and resting there, the +extremity rises up. Sometimes several shoots, when flexible, twine +together into a cable, and thus support one another. Single thin +depending shoots, such as those of the Sollya Drummondii, will turn +abruptly backwards and wind up on themselves. The greater number of +the depending shoots, however, of one twining plant, the Hibbertia +dentata, showed but little tendency to turn upwards. In other cases, +as with the Cryptostegia grandiflora, several internodes which were +at first flexible and revolved, if they did not succeed in twining +round a support, become quite rigid, and supporting themselves +upright, carried on their summits the younger revolving internodes. + +Here will be a convenient place to give a Table showing the direction +and rate of movement of several twining plants, with a few appended +remarks. These plants are arranged according to Lindley's 'Vegetable +Kingdom' of 1853; and they have been selected from all parts of the +series so as to show that all kinds behave in a nearly uniform +manner. {15} + + +The Rate of Revolution of various Twining Plants. + +(ACOTYLEDONS.) + +Lygodium scandens (Polypodiaceae) moves against the sun. + + H. M. +June 18, 1st circle was made in 6 0 + 18, 2nd 6 15 (late in evening) + 19, 3rd 5 32 (very hot day) + 19, 4th 5 0 (very hot day) + 20, 5th 6 0 + +Lygodium articulatum moves against the sun. + + H. M. +July 19, 1st circle was made in 16 30 (shoot very young) + 20, 2nd 15 0 + 21, 3rd 8 0 + 22, 4th 10 30 + +(MONOCOTYLEDONS.) + +Ruscus androgynus (Liliaceae), placed in the hot-house, moves against +the sun. + + H. M. +May 24, 1st circle was made in 6 14 (shoot very young) + 25, 2nd 2 21 + 25, 3rd 3 37 + 25, 4th 3 22 + 26, 5th 2 50 + 27, 6th 3 52 + 27, 7th 4 11 + +Asparagus (unnamed species from Kew) (Liliaceae) moves against the +sun, placed in hothouse. + + H. M. +Dec. 26, 1st circle was made in 5 0 + 27, 2nd 5 40 + +Tamus communis (Dioscoreaceae). A young shoot from a tuber in a pot +placed in the greenhouse: follows the sun. + + H. M. +July, 7, 1st circle was made in 3 10 + 7, 2nd 2 38 + 8, 3rd 3 5 + 8, 4th 2 56 + 8, 5th 2 30 + 8, 6th 2 30 + +Lapagerea rosea (Philesiaceae), in greenhouse, follows the sun. + + H. M. +March 9, 1st circle was made in 26 15 (shoot young) + 10, semicircle 8 15 + 11, 2nd circle 11 0 + 12, 3rd 15 30 + 13, 4th 14 15 + 16, 5th 8 40 when placed in the +hothouse; but the next day the shoot remained stationary. + +Roxburghia viridiflora (Roxburghiaceae) moves against the sun; it +completed a circle in about 24 hours. + +(DICOTYLEDONS.) + +Humulus Lupulus (Urticaceae) follows the sun. The plant was kept in +a room during warm weather. + + H. M. +April 9, 2 circles were made in 4 16 +Aug. 13, 3rd circle was 2 0 + 14, 4th 2 20 + 14, 5th 2 16 + 14, 6th 2 2 + 14, 7th 2 0 + 14, 8th 2 4 + +With the Hop a semicircle was performed, in travelling from the +light, in 1 hr. 33 m.; in travelling to the light, in 1 hr. 13 m.; +difference of rate, 20 m. + +Akebia quinata (Lardizabalaceae), placed in hothouse, moves against +the sun. + + H. M. +March 17, 1st circle was made in 4 0 (shoot young) + 18, 2nd 1 40 + 18, 3rd 1 30 + 19, 4th 1 45 + +Stauntonia latifolia (Lardizabalaceae), placed in hothouse, moves +against the sun. + + H. M. +March 28, 1st circle was made in 3 30 + 29, 2nd 3 45 + +Sphaerostemma marmoratum (Schizandraceae) follows the sun. + + H. M. +August 5th, 1st circle was made in about 24 0 + 5th, 2nd circle was made in 18 30 + +Stephania rotunda (Menispermaceae) moves against the sun + + H. M. +May 27, 1st circle was made in 5 5 + 30, 2nd 7 6 +June 2, 3rd 5 15 + 3, 4th 6 28 + +Thryallis brachystachys (Malpighiaceae) moves against the sun: one +shoot made a circle in 12 hrs., and another in 10 hrs. 30 m.; but the +next day, which was much colder, the first shoot took 10 hrs. to +perform only a semicircle. + +Hibbertia dentata (Dilleniaceae), placed in the hothouse, followed +the sun, and made (May 18th) a circle in 7 hrs. 20 m.; on the 19th, +reversed its course, and moved against the sun, and made a circle in +7 hrs.; on the 20th, moved against the sun one-third of a circle, and +then stood still; on the 26th, followed the sun for two-thirds of a +circle, and then returned to its starting-point, taking for this +double course 11 hrs. 46 m. + +Sollya Drummondii (Pittosporaceae) moves against the sun kept in +greenhouse. + + H. M. +April 4, 1st circle was made in 4 25 + 5, 2nd 8 0 (very cold day) + 6, 3rd 6 25 + 7, 4th 7 5 + +Polygonum dumetorum (Polygonaceae). This case is taken from +Dutrochet (p. 299), as I observed, no allied plant: follows the +sun. Three shoots, cut off a plant, and placed in water made circles +in 3 hrs. 10 m., 5 hrs. 20 m., and 7 hrs. 15 m. + +Wistaria Chinensis (Leguminosae), in greenhouse, moves against the +sun. + + H. M. +May 13, 1st circle was made in 3 5 + 13, 2nd 3 20 + 16, 3rd 2 5 + 24, 4th 3 21 + 25, 5th 2 37 + 25, 6th 2 35 + +Phaseolus vulgaris (Leguminosae), in greenhouse, moves against the +sun. + + H. M. +May, 1st circle was made in 2 0 + 2nd 1 55 + 3rd 1 55 + +Dipladenia urophylla (Apocynaceae) moves against the sun. + + H. M. +April 18, 1st circle was made in 8 0 + 19, 2nd 9 15 + 30, 3rd 9 40 + +Dipladenia crassinoda moves against the sun. + + H. M. +May 16, 1st circle was made in 9 5 +July 20, 2nd 8 0 + 21, 3rd 8 5 + +Ceropegia Gardnerii (Asclepiadaceae) moves against the sun. + + H. M. +Shoot very young, 2 inches } + in length } 1st circle was performed in 7 55 +Shoot still young 2nd 7 0 +Long shoot 3rd 6 33 +Long shoot 4th 5 15 +Long shoot 5th 6 45 + +Stephanotis floribunda (Asclepiadaceae) moves against the sun and +made a circle in 6 hrs. 40 m., a second circle in about 9 hrs. + +Hoya carnosa (Asclepiadaceae) made several circles in from 16 hrs. to +22 hrs. or 24 hrs. + +Ipomaea purpurea (Convolvulaceae) moves against the sun. Plant +placed in room with lateral light. + + {Semicircle, from the light in +1st circle was made in 2 hrs. 42 m. { 1 hr. 14 m., to the light + { 1 hr. 28 m.: difference 14 m. + + {Semicircle, from the light in +2nd circle was made in 2 hrs. 47 m. { 1 hr. 17 m., to the light 1 hr. + { 30 m.: difference 13 m. + +Ipomaea jucunda (Convolvulaceae) moves against the sun, placed in my +study, with windows facing the north-east. Weather hot. + + {Semicircle, from the light in +1st circle was made in 5 hrs. 30 m. { 4 hrs. 30 m., to the light 1 +hr. + { 0 m.: difference 3 hrs. 30 m. + +2nd circle was made in 5 hrs. {Semicircle, from the light in + 20 m. (Late in afternoon: { 3 hrs. 50 m., to the light 1 +hr. + circle completed at 6 hrs. 40 m. { 30 m.: difference 2 hrs. 20 m. + P.M.) + +We have here a remarkable instance of the power of light in retarding +and hastening the revolving movement. (See ERRATA.) + +Convolvulus sepium (large-flowered cultivated var.) moves against the +sun. Two circles, were made each in 1 hr. 42 m.: difference in +semicircle from and to the light 14 m. + +Rivea tiliaefolia (Convolvulaceae) moves against the sun, made four +revolutions in 9 hrs.; so that, on an average, each was performed in +2 hrs. 15 m. + +Plumbago rosea (Plumbaginaceae) follows the sun. The shoot did not +begin to revolve until nearly a yard in height; it then made a fine +circle in 10 hrs. 45 m. During the next few days it continued to +move, but irregularly. On August 15th the shoot followed, during a +period of 10 hrs. 40 m., a long and deeply zigzag course and then +made a broad ellipse. The figure apparently represented three +ellipses, each of which averaged 3 hrs. 38 m. for its completion. + +Jasminum pauciflorum, Bentham (Jasminaceae), moves against the sun. +A circle was made in 7 hrs. 15 m., and a second rather more quickly. + +Clerodendrum Thomsonii (Verbenaceae) follows the sun. + + H. M. +April 12, 1st circle was made in 5 45 (shoot very young) + 14, 2nd 3 30 + {(directly after the + 18, a semicircle 5 0 { plant was shaken + { on being moved) + 19, 3rd circle 3 0 + 20, 4th 4 20 + +Tecoma jasminoides (Bignoniaceae) moves against the sun. + + H. M. +March 17, 1st circle was made in 6 30 + 19, 2nd 7 0 + 22, 3rd 8 30 (very cold day) + 24, 4th 6 45 + +Thunbergia alata (Acanthaceae) moves against sun. + + H. M. +April 14, 1st circle was made in 3 20 + 18, 2nd 2 50 + 18, 3rd 2 55 + 18, 4th 3 55 (late in afternoon) + +Adhadota cydonaefolia (Acanthaceae) follows the sun. A young shoot +made a semicircle in 24 hrs.; subsequently it made a circle in +between 40 hrs. and 48 hrs. Another shoot, however, made a circle in +26 hrs. 30 m. + +Mikania scandens (Compositae) moves against the sun. + + H. M. +March 14, 1st circle was made in 3 10 + 15, 2nd 3 0 + 16, 3rd 3 0 + 17, 4th 3 33 +April 7, 5th 2 50 + 7, 6th 2 40 {This circle was +made + { after a copious +water- + { ing with cold +water at + { 47 degrees Fahr. + +Combretum argenteum (Combretaceae) moves against the sun. Kept in +hothouse. + + H. M. + {Early in morning, +when +Jan. 24, 1st circle was made in 2 55 { the temperature of +the + { house had fallen a + { little. + + 24, 2 circles each at an } + average of } 2 20 + 25, 4th circle was made in 2 25 + +Combretum purpureum revolves not quite so quickly as C. argenteum. + +Loasa aurantiaca (Loasaceae). Revolutions variable in their course: +a plant which moved against the sun. + + H. M. +June 20, 1st circle was made in 2 37 + 20, 2nd 2 13 + 20, 3rd 4 0 + 21, 4th 2 35 + 22, 5th 3 26 + 23, 6th 3 5 + +Another plant which followed the sun in its revolutions. + + H. M. +July 11, 1st circle was made in 1 51 } + 11, 2nd 1 46 } Very hot day. + 11, 3rd 1 41 } + 11, 4th 1 48 } + 12, 5th 2 35 } + +Scyphanthus elegans (Loasaceae) follows the sun. + + H. M. +June 13, 1st circle was made in 1 45 + 13, 2nd 1 17 + 14, 3rd 1 36 + 14, 4th 1 59 + 14, 5th 2 3 + +Siphomeris or Lecontea (unnamed sp.) (Cinchonaceae) follows the sun. + + H. M. + {(shoot extremely +May 25, semicircle was made in 10 27 { young) + 26, 1st circle 10 15 (shoot still young) + 30, 2nd 8 55 +June 2, 3rd 8 11 + 6, 4th 6 8 + { Taken from the + 8, 5th 7 20 { hothouse, and + 9, 6th 8 36 { placed in a room + { in my house. + +Manettia bicolor (Cinchonaceae), young plant, follows the sun. + + H. M. +July 7, 1st circle was made in 6 18 + 8, 2nd 6 53 + 9, 3rd 6 30 + +Lonicera brachypoda (Caprifoliaceae) follows the sun, kept in a warm +room in the house. + + H. M. +April, 1st circle was made in 9 10 (about) + {(a distinct shoot, +very +April, 2nd circle was made in 12 20 { young, on same +plant) + 3rd 7 30 + {In this latter +circle, + { the semicircle from + { the light took 5 +hrs. + 4th 8 0 { 23 m., and to the + { light 2 hrs. 37 +min.: + { difference 2 hrs +46m. + +Aristolochia gigas (Aristolochiaceae) moves against the sun. + + H. M. + July 22, 1st circle was made in 8 0 (rather young shoot) + 23, 2nd 7 15 + 24, 3rd 5 0 (about) + +In the foregoing Table, which includes twining plants belonging to +widely different orders, we see that the rate at which growth travels +or circulates round the axis (on which the revolving movement +depends), differs much. As long as a plant remains under the same +conditions, the rate is often remarkably uniform, as with the Hop, +Mikania, Phaseolus, &c. The Scyphanthus made one revolution in 1 hr. +17 m., and this is the quickest rate observed by me; but we shall +hereafter see a tendril-bearing Passiflora revolving more rapidly. A +shoot of the Akebia quinata made a revolution in 1 hr. 30 m., and +three revolutions at the average rate of 1 hr. 38 m.; a Convolvulus +made two revolutions at the average of 1 hr. 42 m., and Phaseolus +vulgaris three at the average of 1 hr. 57 m. On the other hand, some +plants take 24 hrs. for a single revolution, and the Adhadota +sometimes required 48 hrs.; yet this latter plant is an efficient +twiner. Species of the same genus move at different rates. The rate +does not seem governed by the thickness of the shoots: those of the +Sollya are as thin and flexible as string, but move more slowly than +the thick and fleshy shoots of the Ruscus, which seem little fitted +for movement of any kind. The shoots of the Wistaria, which become +woody, move faster than those of the herbaceous Ipomoea or +Thunbergia. + +We know that the internodes, whilst still very young, do not acquire +their proper rate of movement; hence the several shoots on the same +plant may sometimes be seen revolving at different rates. The two or +three, or even more, internodes which are first formed above the +cotyledons, or above the root-stock of a perennial plant, do not +move; they can support themselves, and nothing superfluous is +granted. + +A greater number of twiners revolve in a course opposed to that of +the sun, or to the hands of a watch, than in the reversed course, +and, consequently, the majority, as is well known, ascend their +supports from left to right. Occasionally, though rarely, plants of +the same order twine in opposite directions, of which Mohl (p. 125) +gives a case in the Leguminosae, and we have in the table another in +the Acanthaceae. I have seen no instance of two species of the same +genus twining in opposite directions, and such cases must be rare; +but Fritz Muller {16} states that although Mikania scandens twines, +as I have described, from left to right, another species in South +Brazil twines in an opposite direction. It would have been an +anomalous circumstance if no such cases had occurred, for different +individuals of the same species, namely, of Solanum dulcamara +(Dutrochet, tom. xix. p. 299), revolve and twine in two directions: +this plant, however; is a most feeble twiner. Loasa aurantiaca +(Leon, p. 351) offers a much more curious case: I raised seventeen +plants: of these eight revolved in opposition to the sun and +ascended from left to right; five followed the sun and ascended from +right to left; and four revolved and twined first in one direction, +and then reversed their course, {17} the petioles of the opposite +leaves affording a point d'appui for the reversal of the spire. One +of these four plants made seven spiral turns from right to left, and +five turns from left to right. Another plant in the same family, the +Scyphanthus elegans, habitually twines in this same manner. I raised +many plants of it, and the stems of all took one turn, or +occasionally two or even three turns in one direction, and then, +ascending for a short space straight, reversed their course and took +one or two turns in an opposite direction. The reversal of the +curvature occurred at any point in the stem, even in the middle of an +internode. Had I not seen this case, I should have thought its +occurrence most improbable. It would be hardly possible with any +plant which ascended above a few feet in height, or which lived in an +exposed situation; for the stem could be pulled away easily from its +support, with but little unwinding; nor could it have adhered at all, +had not the internodes soon become moderately rigid. With leaf- +climbers, as we shall soon see, analogous cases frequently occur; but +these present no difficulty, as the stem is secured by the clasping +petioles. + +In the many other revolving and twining plants observed by me, I +never but twice saw the movement reversed; once, and only for a short +space, in Ipomoea jucunda; but frequently with Hibbertia dentata. +This plant at first perplexed me much, for I continually observed its +long and flexible shoots, evidently well fitted for twining, make a +whole, or half, or quarter circle in one direction and then in an +opposite direction; consequently, when I placed the shoots near thin +or thick sticks, or perpendicularly stretched string, they seemed as +if constantly trying to ascend, but always failed. I then surrounded +the plant with a mass of branched twigs; the shoots ascended, and +passed through them, but several came out laterally, and their +depending extremities seldom turned upwards as is usual with twining +plants. Finally, I surrounded a second plant with many thin upright +sticks, and placed it near the first one with twigs; and now both had +got what they liked, for they twined up the parallel sticks, +sometimes winding round one and sometimes round several; and the +shoots travelled laterally from one to the other pot; but as the +plants grew older, some of the shoots twined regularly up thin +upright sticks. Though the revolving movement was sometimes in one +direction and sometimes in the other, the twining was invariably from +left to right; {18} so that the more potent or persistent movement of +revolution must have been in opposition to the course of the sun. It +would appear that this Hibbertia is adapted both to ascend by +twining, and to ramble laterally through the thick Australian scrub. + +I have described the above case in some detail, because, as far as I +have seen, it is rare to find any special adaptations with twining +plants, in which respect they differ much from the more highly +organized tendril-bearers. The Solanum dulcamara, as we shall +presently see, can twine only round stems which are both thin and +flexible. Most twining plants are adapted to ascend supports of +moderate though of different thicknesses. Our English twiners, as +far as I have seen, never twine round trees, excepting the +honeysuckle (Lonicera periclymenum), which I have observed twining up +a young beech-tree nearly 4.5 inches in diameter. Mohl (p. 134) +found that the Phaseolus multiflorus and Ipomoea purpurea could not, +when placed in a room with the light entering on one side, twine +round sticks between 3 and 4 inches in diameter; for this interfered, +in a manner presently to be explained, with the revolving movement. +In the open air, however, the Phaseolus twined round a support of the +above thickness, but failed in twining round one 9 inches in +diameter. Nevertheless, some twiners of the warmer temperate regions +can manage this latter degree of thickness; for I hear from Dr. +Hooker that at Kew the Ruscus androgynus has ascended a column 9 +inches in diameter; and although a Wistaria grown by me in a small +pot tried in vain for weeks to get round a post between 5 and 6 +inches in thickness, yet at Kew a plant ascended a trunk above 6 +inches in diameter. The tropical twiners, on the other hand, can +ascend thicker trees; I hear from Drs. Thomson and Hooker that this +is the case with the Butea parviflora, one of the Menispermaceae, and +with some Dalbergias and other Leguminosae. {19} This power would be +necessary for any species which had to ascend by twining the large +trees of a tropical forest; otherwise they would hardly ever be able +to reach the light. In our temperate countries it would be injurious +to the twining plants which die down every year if they were enabled +to twine round trunks of trees, for they could not grow tall enough +in a single season to reach the summit and gain the light. + +By what means certain twining plants are adapted to ascend only thin +stems, whilst others can twine round thicker ones, I do not know. It +appeared to me probable that twining plants with very long revolving +shoots would be able to ascend thick supports; accordingly I placed +Ceropegia Gardnerii near a post 6 inches in diameter, but the shoots +entirely failed to wind round it; their great length and power of +movement merely aid them in finding a distant stem round which to +twine. The Sphaerostemma marmoratum is a vigorous tropical twiner; +and as it is a very slow revolver, I thought that this latter +circumstance might help it in ascending a thick support; but though +it was able to wind round a 6-inch post, it could do this only on the +same level or plane, and did not form a spire and thus ascend. + +As ferns differ so much in structure from phanerogamic plants, it may +be worth while here to show that twining ferns do not differ in their +habits from other twining plants. In Lygodium articulatum the two +internodes of the stem (properly the rachis) which are first formed +above the root-stock do not move; the third from the ground revolves, +but at first very slowly. This species is a slow revolver: but L. +scandens made five revolutions, each at the average rate of 5 hrs. 45 +m.; and this represents fairly well the usual rate, taking quick and +slow movers, amongst phanerogamic plants. The rate was accelerated +by increased temperature. At each stage of growth only the two upper +internodes revolved. A line painted along the convex surface of a +revolving internode becomes first lateral, then concave, then lateral +and ultimately again convex. Neither the internodes nor the petioles +are irritable when rubbed. The movement is in the usual direction, +namely, in opposition to the course of the sun; and when the stem +twines round a thin stick, it becomes twisted on its own axis in the +same direction. After the young internodes have twined round a +stick, their continued growth causes them to slip a little upwards. +If the stick be soon removed, they straighten themselves, and +recommence revolving. The extremities of the depending shoots turn +upwards, and twine on themselves. In all these respects we have +complete identity with twining phanerogamic plants; and the above +enumeration may serve as a summary of the leading characteristics of +all twining plants. + +The power of revolving depends on the general health and vigour of +the plant, as has been laboriously shown by Palm. But the movement +of each separate internode is so independent of the others, that +cutting off an upper one does not affect the revolutions of a lower +one. When, however, Dutrochet cut off two whole shoots of the Hop, +and placed them in water, the movement was greatly retarded; for one +revolved in 20 hrs. and the other in 23 hrs., whereas they ought to +have revolved in between 2 hrs. and 2 hrs. 30 m. Shoots of the +Kidney-bean, cut off and placed in water, were similarly retarded, +but in a less degree. I have repeatedly observed that carrying a +plant from the greenhouse to my room, or from one part to another of +the greenhouse, always stopped the movement for a time; hence I +conclude that plants in a state of nature and growing in exposed +situations, would not make their revolutions during very stormy +weather. A decrease in temperature always caused a considerable +retardation in the rate of revolution; but Dutrochet (tom. xvii. pp. +994, 996) has given such precise observations on this head with +respect to the common pea that I need say nothing more. When twining +plants are placed near a window in a room, the light in some cases +has a remarkable power (as was likewise observed by Dutrochet, p. +998, with the pea) on the revolving movement, but this differs in +degree with different plants; thus Ipomoea jucunda made a complete +circle in 5 hrs. 30 m.; the semicircle from the light taking 4 hrs. +80 m., and that towards the light only 1 hr. Lonicera brachypoda +revolved, in a reversed direction to the Ipomoea, in 8 hrs.; the +semicircle from the light taking 5 hrs. 23 m., and that to the light +only 2 hrs. 37 m. From the rate of revolution in all the plants +observed by me, being nearly the same during the night and the day, I +infer that the action of the light is confined to retarding one +semicircle and accelerating the other, so as not to modify greatly +the rate of the whole revolution. This action of the light is +remarkable, when we reflect how little the leaves are developed on +the young and thin revolving internodes. It is all the more +remarkable, as botanists believe (Mohl, p. 119) that twining plants +are but little sensitive to the action of light. + +I will conclude my account of twining plants by giving a few +miscellaneous and curious cases. With most twining plants all the +branches, however many there may be, go on revolving together; but, +according to Mohl (p. 4), only the lateral branches of Tamus +elephantipes twine, and not the main stem. On the other hand, with a +climbing species of Asparagus, the leading shoot alone, and not the +branches, revolved and twined; but it should be stated that the plant +was not growing vigorously. My plants of Combretum argenteum and C. +purpureum made numerous short healthy shoots; but they showed no +signs of revolving, and I could not conceive how these plants could +be climbers; but at last C. argenteum put forth from the lower part +of one of its main branches a thin shoot, 5 or 6 feet in length, +differing greatly in appearance from the previous shoots, owing to +its leaves being little developed, and this shoot revolved vigorously +and twined. So that this plant produces shoots of two kinds. With +Periploca Graeca (Palm, p. 43) the uppermost shoots alone twine. +Polygonum convolvulus twines only during the middle of the summer +(Palm, p. 43, 94); and plants growing vigorously in the autumn show +no inclination to climb. The majority of Asclepiadaceae are twiners; +but Asclepias nigra only "in fertiliori solo incipit scandere +subvolubili caule" (Willdenow, quoted and confirmed by Palm, p. 41). +Asclepias vincetoxicum does not regularly twine, but occasionally +does so (Palm, p. 42; Mohl, p. 112) when growing under certain +conditions. So it is with two species of Ceropegia, as I hear from +Prof. Harvey, for these plants in their native dry South African +home generally grow erect, from 6 inches to 2 feet in height,--a very +few taller specimens showing some inclination to curve; but when +cultivated near Dublin, they regularly twined up sticks 5 or 6 feet +in height. Most Convolvulaceae are excellent twiners; but in South +Africa Ipomoea argyraeoides almost always grows erect and compact, +from about 12 to 18 inches in height, one specimen alone in Prof. +Harvey's collection showing an evident disposition to twine. On the +other hand, seedlings raised near Dublin twined up sticks above 8 +feet in height. These facts are remarkable; for there can hardly be +a doubt that in the dryer provinces of South Africa these plants have +propagated themselves for thousands of generations in an erect +condition; and yet they have retained during this whole period the +innate power of spontaneously revolving and twining, whenever their +shoots become elongated under proper conditions of life. Most of the +species of Phaseolus are twiners; but certain varieties of the P. +multiflorus produce (Leon, p. 681) two kinds of shoots, some upright +and thick, and others thin and twining. I have seen striking +instances of this curious case of variability in "Fulmer's dwarf +forcing-bean," which occasionally produced a single long twining +shoot. + +Solanum dulcamara is one of the feeblest and poorest of twiners: it +may often be seen growing as an upright bush, and when growing in the +midst of a thicket merely scrambles up between the branches without +twining; but when, according to Dutrochet (tom. xix. p. 299), it +grows near a thin and flexible support, such as the stem of a nettle, +it twines round it. I placed sticks round several plants, and +vertically stretched strings close to others, and the strings alone +were ascended by twining. The stem twines indifferently to the right +or left. Some others species of Solanum, and of another genus, viz. +Habrothamnus, belonging to the same family, are described in +horticultural works as twining plants, but they seem to possess this +faculty in a very feeble degree. We may suspect that the species of +these two genera have as yet only partially acquired the habit of +twining. On the other hand with Tecoma radicans, a member of a +family abounding with twiners and tendril-bearers, but which climbs, +like the ivy, by the aid of rootlets, we may suspect that a former +habit of twining has been lost, for the stem exhibited slight +irregular movements which could hardly be accounted for by changes in +the action of the light. There is no difficulty in understanding how +a spirally twining plant could graduate into a simple root-climber; +for the young internodes of Bignonia Tweedyana and of Hoya carnosa +revolve and twine, but likewise emit rootlets which adhere to any +fitting surface, so that the loss of twining would be no great +disadvantage and in some respects an advantage to these species, as +they would then ascend their supports in a more direct line. {20} + + + +CHAPTER II.--LEAF-CLIMBERS. + + + +Plants which climb by the aid of spontaneously revolving and +sensitive petioles--Clematis--Tropaeolum--Maurandia, flower-peduncles +moving spontaneously and sensitive to a touch--Rhodochiton-- +Lophospermum--internodes sensitive--Solanum, thickening of the +clasped petioles--Fumaria--Adlumia--Plants which climb by the aid of +their produced midribs--Gloriosa--Flagellaria--Nepenthes--Summary on +leaf-climbers. + +We now come to our second class of climbing plants, namely, those +which ascend by the aid of irritable or sensitive organs. For +convenience' sake the plants in this class have been grouped under +two sub-divisions, namely, leaf-climbers, or those which retain their +leaves in a functional condition, and tendril-bearers. But these +sub-divisions graduate into each other, as we shall see under +Corydalis and the Gloriosa lily. + +It has long been observed that several plants climb by the aid of +their leaves, either by their petioles (foot-stalks) or by their +produced midribs; but beyond this simple fact they have not been +described. Palm and Mohl class these plants with those which bear +tendrils; but as a leaf is generally a defined object, the present +classification, though artificial, has at least some advantages. +Leaf-climbers are, moreover, intermediate in many respects between +twiners and tendril-bearers. Eight species of Clematis and seven of +Tropaeolum were observed, in order to see what amount of difference +in the manner of climbing existed within the same genus; and the +differences are considerable. + +CLEMATIS.--C. glandulosa.--The thin upper internodes revolve, moving +against the course of the sun, precisely like those of a true twiner, +at an average rate, judging from three revolutions, of 3 hrs. 48 m. +The leading shoot immediately twined round a stick placed near it; +but, after making an open spire of only one turn and a half, it +ascended for a short space straight, and then reversed its course and +wound two turns in an opposite direction. This was rendered possible +by the straight piece between the opposed spires having become rigid. +The simple, broad, ovate leaves of this tropical species, with their +short thick petioles, seem but ill-fitted for any movement; and +whilst twining up a vertical stick, no use is made of them. +Nevertheless, if the footstalk of a young leaf be rubbed with a thin +twig a few times on any side, it will in the course of a few hours +bend to that side; afterwards becoming straight again. The under +side seemed to be the most sensitive; but the sensitiveness or +irritability is slight compared to that which we shall meet with in +some of the following species; thus, a loop of string, weighing 1.64 +grain (106.2 mg.) and hanging for some days on a young footstalk, +produced a scarcely perceptible effect. A sketch is here given of +two young leaves which had naturally caught hold of two thin +branches. A forked twig placed so as to press lightly on the under +side of a young footstalk caused it, in 12 hrs., to bend greatly, and +ultimately to such an extent that the leaf passed to the opposite +side of the stem; the forked stick having been removed, the leaf +slowly recovered its former position. + +The young leaves spontaneously and gradually change their position: +when first developed the petioles are upturned and parallel to the +stem; they then slowly bend downwards, remaining for a short time at +right angles to the stem, and then become so much arched downwards +that the blade of the leaf points to the ground with its tip curled +inwards, so that the whole petiole and leaf together form a hook. +They are thus enabled to catch hold of any twig with which they may +be brought into contact by the revolving movement of the internodes. +If this does not happen, they retain their hooked shape for a +considerable time, and then bending upwards reassume their original +upturned position, which is preserved ever afterwards. The petioles +which have clasped any object soon become much thickened and +strengthened, as may be seen in the drawing. + +Clematis montana.--The long, thin petioles of the leaves, whilst +young, are sensitive, and when lightly rubbed bend to the rubbed +side, subsequently becoming straight. They are far more sensitive +than the petioles of C. glandulosa; for a loop of thread weighing a +quarter of a grain (16.2 mg.) caused them to bend; a loop weighing +only one-eighth of a grain (8.1 mg.) sometimes acted and sometimes +did not act. The sensitiveness extends from the blade of the leaf to +the stem. I may here state that I ascertained in all cases the +weights of the string and thread used by carefully weighing 50 inches +in a chemical balance, and then cutting off measured lengths. The +main petiole carries three leaflets; but their short, sub-petioles +are not sensitive. A young, inclined shoot (the plant being in the +greenhouse) made a large circle opposed to the course of the sun in 4 +hrs. 20 m., but the next day, being very cold, the time was 5 hrs. 10 +m. A stick placed near a revolving stem was soon struck by the +petioles which stand out at right angles, and the revolving movement +was thus arrested. The petioles then began, being excited by the +contact, to slowly wind round the stick. When the stick was thin, a +petiole sometimes wound twice round it. The opposite leaf was in no +way affected. The attitude assumed by the stem after the petiole had +clasped the stick, was that of a man standing by a column, who throws +his arm horizontally round it. With respect to the stem's power of +twining, some remarks will be made under C. calycina. + +Clematis Sieboldi.--A shoot made three revolutions against the sun at +an average rate of 3 hrs. 11 m. The power of twining is like that of +the last species. Its leaves are nearly similar in structure and in +function, excepting that the sub-petioles of the lateral and terminal +leaflets are sensitive. A loop of thread, weighing one-eighth of a +grain, acted on the main petiole, but not until two or three days had +elapsed. The leaves have the remarkable habit of spontaneously +revolving, generally in vertical ellipses, in the same manner, but in +a less degree, as will be described under C. microphylla. + +Clematis calycina.--The young shoots are thin and flexible: one +revolved, describing a broad oval, in 5 hrs. 30 m., and another in 6 +hrs. 12 m. They followed the course of the sun; but the course, if +observed long enough, would probably be found to vary in this +species, as well as in all the others of the genus. It is a rather +better twiner than the two last species: the stem sometimes made two +spiral turns round a thin stick, if free from twigs; it then ran +straight up for a space, and reversing its course took one or two +turns in an opposite direction. This reversal of the spire occurred +in all the foregoing species. The leaves are so small compared with +those of most of the other species, that the petioles at first seem +ill-adapted for clasping. Nevertheless, the main service of the +revolving movement is to bring them into contact with surrounding +objects, which are slowly but securely seized. The young petioles, +which alone are sensitive, have their ends bowed a little downwards, +so as to be in a slight degree hooked; ultimately the whole leaf, if +it catches nothing, becomes level. I gently rubbed with a thin twig +the lower surfaces of two young petioles; and in 2 hrs. 30 m. they +were slightly curved downwards; in 5 hrs., after being rubbed, the +end of one was bent completely back, parallel to the basal portion; +in 4 hrs. subsequently it became nearly straight again. To show how +sensitive the young petioles are, I may mention that I just touched +the under sides of two with a little water-colour, which when dry +formed an excessively thin and minute crust; but this sufficed in 24 +hrs. to cause both to bend downwards. Whilst the plant is young, +each leaf consists of three divided leaflets, which barely have +distinct petioles, and these are not sensitive; but when the plant is +well grown, the petioles of the two lateral and terminal leaflets are +of considerable length, and become sensitive so as to be capable of +clasping an object in any direction. + +When a petiole has clasped a twig, it undergoes some remarkable +changes, which may be observed with the other species, but in a less +strongly marked manner, and will here be described once for all. The +clasped petiole in the course of two or three days swells greatly, +and ultimately becomes nearly twice as thick as the opposite one +which has clasped nothing. When thin transverse slices of the two +are placed under the microscope their difference is conspicuous: the +side of the petiole which has been in contact with the support, is +formed of a layer of colourless cells with their longer axes directed +from the centre, and these are very much larger than the +corresponding cells in the opposite or unchanged petiole; the central +cells, also, are in some degree enlarged, and the whole is much +indurated. The exterior surface generally becomes bright red. But a +far greater change takes place in the nature of the tissues than that +which is visible: the petiole of the unclasped leaf is flexible and +can be snapped easily, whereas the clasped one acquires an +extraordinary degree of toughness and rigidity, so that considerable +force is required to pull it into pieces. With this change, great +durability is probably acquired; at least this is the case with the +clasped petioles of Clematis vitalba. The meaning of these changes +is obvious, namely, that the petioles may firmly and durably support +the stem. + +Clematis microphylla, var. leptophylla.--The long and thin internodes +of this Australian species revolve sometimes in one direction and +sometimes in an opposite one, describing long, narrow, irregular +ellipses or large circles. Four revolutions were completed within +five minutes of the same average rate of 1 hr. 51 m.; so that this +species moves more quickly than the others of the genus. The shoots, +when placed near a vertical stick, either twine round it, or clasp it +with the basal portions of their petioles. The leaves whilst young +are nearly of the same shape as those of C. viticella, and act in the +same manner like a hook, as will be described under that species. +But the leaflets are more divided, and each segment whilst young +terminates in a hardish point, which is much curved downwards and +inwards; so that the whole leaf readily catches hold of any +neighbouring object. The petioles of the young terminal leaflets are +acted on by loops of thread weighing 0.125th and even 0.0625th of a +grain. The basal portion of the main petiole is much less sensitive, +but will clasp a stick against which it presses. + +The leaves, whilst young, are continually and spontaneously moving +slowly. A bell-glass was placed over a shoot secured to a stick, and +the movements of the leaves were traced on it during several days. A +very irregular line was generally formed; but one day, in the course +of eight hours and three quarters, the figure clearly represented +three and a half irregular ellipses, the most perfect one of which +was completed in 2 hrs. 35 m. The two opposite leaves moved +independently of each other. This movement of the leaves would aid +that of the internodes in bringing the petioles into contact with +surrounding objects. I discovered this movement too late to be +enabled to observe it in the other species; but from analogy I can +hardly doubt that the leaves of at least C. viticella, C. flammula, +and C. vitalba move spontaneously; and, judging from C Sieboldi, this +probably is the case with C. montana and C. calycina. I ascertained +that the simple leaves of C. glandulosa exhibited no spontaneous +revolving movement. + +Clematis viticella, var. venosa.--In this and the two following +species the power of spirally twining is completely lost, and this +seems due to the lessened flexibility of the internodes and to the +interference caused by the large size of the leaves. But the +revolving movement, though restricted, is not lost. In our present +species a young internode, placed in front of a window, made three +narrow ellipses, transversely to the direction of the light, at an +average rate of 2 hrs. 40 m. When placed so that the movements were +to and from the light, the rate was greatly accelerated in one half +of the course, and retarded in the other, as with twining plants. +The ellipses were small; the longer diameter, described by the apex +of a shoot bearing a pair of not expanded leaves, was only 4.625 +inches, and that by the apex of the penultimate internode only 1.125 +inch. At the most favourable period of growth each leaf would hardly +be carried to and fro by the movement of the internodes more than two +or three inches, but, as above stated, it is probable that the leaves +themselves move spontaneously. The movement of the whole shoot by +the wind and by its rapid growth, would probably be almost equally +efficient as these spontaneous movements, in bringing the petioles +into contact with surrounding objects. + +The leaves are of large size. Each bears three pairs of lateral +leaflets and a terminal one, all supported on rather long sub- +petioles. The main petiole bends a little angularly downwards at +each point where a pair of leaflets arises (see fig. 2), and the +petiole of the terminal leaflet is bent downwards at right angles; +hence the whole petiole, with its rectangularly bent extremity, acts +as a hook. This hook, the lateral petioles being directed a little +upwards; forms an excellent grappling apparatus, by which the leaves +readily become entangled with surrounding objects. If they catch +nothing, the whole petiole ultimately grows straight. The main +petiole, the sub-petioles, and the three branches into which each +basi-lateral sub-petiole is generally subdivided, are all sensitive. +The basal portion of the main petiole, between the stem and the first +pair of leaflets, is less sensitive than the remainder; it will, +however, clasp a stick with which it is left in contact. The +inferior surface of the rectangularly bent terminal portion (carrying +the terminal leaflet), which forms the inner side of the end of the +hook, is the most sensitive part; and this portion is manifestly best +adapted to catch a distant support. To show the difference in +sensibility, I gently placed loops of string of the same weight (in +one instance weighing only 0.82 of a grain or 53.14 mg.) on the +several lateral sub-petioles and on the terminal one; in a few hours +the latter was bent, but after 24 hrs. no effect was produced on the +other sub-petioles. Again, a terminal sub-petiole placed in contact +with a thin stick became sensibly curved in 45 m., and in 1 hr. 10m. +moved through ninety degrees; whilst a lateral sub-petiole did not +become sensibly curved until 3 hrs. 30 m. had elapsed. In all cases, +if the sticks are taken away, the petioles continue to move during +many hours afterwards; so they do after a slight rubbing; but they +become straight again, after about a day's interval, that is if the +flexure has not been very great or long continued. + +The graduated difference in the extension of the sensitiveness in the +petioles of the above-described species deserves notice. In C. +montana it is confined to the main petiole, and has not spread to the +sub-petioles of the three leaflets; so it is with young plants of C. +calycina, but in older plants it spreads to the three sub-petioles. +In C. viticella the sensitiveness has spread to the petioles of the +seven leaflets, and to the subdivisions of the basi-lateral sub- +petioles. But in this latter species it has diminished in the basal +part of the main petiole, in which alone it resided in C. montana; +whilst it has increased in the abruptly bent terminal portion. + +Clematis flammula.--The rather thick, straight, and stiff shoots, +whilst growing vigorously in the spring, make small oval revolutions, +following the sun in their course. Four were made at an average rate +of 3 hrs. 45 m. The longer axis of the oval, described by the +extreme tip, was directed at right angles to the line joining the +opposite leaves; its length was in one case only 1.375, and in +another case 1.75 inch; so that the young leaves were moved a very +short distance. The shoots of the same plant observed in midsummer, +when growing not so quickly, did not revolve at all. I cut down +another plant in the early summer, so that by August 1st it had +formed new and moderately vigorous shoots; these, when observed under +a bell-glass, were on some days quite stationary, and on other days +moved to and fro only about the eighth of an inch. Consequently the +revolving power is much enfeebled in this species, and under +unfavourable circumstances is completely lost. The shoot must depend +for coming into contact with surrounding objects on the probable, +though not ascertained spontaneous movement of the leaves, on rapid +growth, and on movement from the wind. Hence, perhaps, it is that +the petioles have acquired a high degree of sensitiveness as a +compensation for the little power of movement in the shoots. + +The petioles are bowed downwards, and have the same general hook-like +form as in C. viticella. The medial petiole and the lateral sub- +petioles are sensitive, especially the much bent terminal portion. +As the sensitiveness is here greater than in any other species of the +genus observed by me, and is in itself remarkable, I will give fuller +details. The petioles, when so young that they have not separated +from one another, are not sensitive; when the lamina of a leaflet has +grown to a quarter of an inch in length (that is, about one-sixth of +its full size), the sensitiveness is highest; but at this period the +petioles are relatively much more fully developed than are the blades +of the leaves. Full-grown petioles are not in the least sensitive. +A thin stick placed so as to press lightly against a petiole, having +a leaflet a quarter of an inch in length, caused the petiole to bend +in 3 hrs. 15 m. In another case a petiole curled completely round a +stick in 12 hrs. These petioles were left curled for 24 hrs., and the +sticks were then removed; but they never straightened themselves. I +took a twig, thinner than the petiole itself, and with it lightly +rubbed several petioles four times up and down; these in 1 hr. 45 m. +became slightly curled; the curvature increased during some hours and +then began to decrease, but after 25 hrs. from the time of rubbing a +vestige of the curvature remained. Some other petioles similarly +rubbed twice, that is, once up and once down, became perceptibly +curved in about 2 hrs. 30 m., the terminal sub-petiole moving more +than the lateral sub-petioles; they all became straight again in +between 12 hrs. and 14 hrs. Lastly, a length of about one-eighth of +an inch of a sub-petiole, was lightly rubbed with the same twig only +once; it became slightly curved in 3 hrs., remaining so during 11 +hrs., but by the next morning was quite straight. + +The following observations are more precise. After trying heavier +pieces of string and thread, I placed a loop of fine string, weighing +1.04 gr. (67.4 mg.) on a terminal sub-petiole: in 6 hrs. 40 m. a +curvature could be seen; in 24 hrs. the petiole formed an open ring +round the string; in 48 hrs. the ring had almost closed on the +string, and in 72 hrs. seized it so firmly, that some force was +necessary for its withdrawal. A loop weighing 0.52 of a grain (33.7 +mg.) caused in 14 hrs. a lateral sub-petiole just perceptibly to +curve, and in 24 hrs. it moved through ninety degrees. These +observations were made during the summer: the following were made in +the spring, when the petioles apparently are more sensitive:- A loop +of thread, weighing one-eighth of a grain (8.1 mg.), produced no +effect on the lateral sub-petioles, but placed on a terminal one, +caused it, after 24 hrs., to curve moderately; the curvature, though +the loop remained suspended, was after 48 hrs. diminished, but never +disappeared; showing that the petiole had become partially accustomed +to the insufficient stimulus. This experiment was twice repeated +with nearly the same result. Lastly, a loop of thread, weighing only +one-sixteenth of a grain (4.05 mg.) was twice gently placed by a +forceps on a terminal sub-petiole (the plant being, of course, in a +still and closed room), and this weight certainly caused a flexure, +which very slowly increased until the petiole moved through nearly +ninety degrees: beyond this it did not move; nor did the petiole, +the loop remaining suspended, ever become perfectly straight again. + +When we consider, on the one hand, the thickness and stiffness of the +petioles, and, on the other hand, the thinness and softness of fine +cotton thread, and what an extremely small weight one-sixteenth of a +grain (4.05 mg.) is, these facts are remarkable. But I have reason +to believe that even a less weight excites curvature when pressing +over a broader surface than that acted on by a thread. Having +noticed that the end of a suspended string which accidentally touched +a petiole, caused it to bend, I took two pieces of thin twine, 10 +inches in length (weighing 1.64 gr.), and, tying them to a stick, let +them hang as nearly perpendicularly downwards as their thinness and +flexuous form, after being stretched, would permit; I then quietly +placed their ends so as just to rest on two petioles, and these +certainly became curved in 36 hrs. One of the ends touched the angle +between a terminal and lateral sub-petiole, and it was in 48 hours +caught between them as by a forceps. In these cases the pressure, +though spread over a wider surface than that touched by the cotton +thread, must have been excessively slight. + +Clematis vitalba.--The plants were in pots and not healthy, so that I +dare not trust my observations, which indicate much similarity in +habits with C. flammula. I mention this species only because I have +seen many proofs that the petioles in a state of nature are excited +to movement by very slight pressure. For instance, I have found them +embracing thin withered blades of grass, the soft young leaves of a +maple, and the flower-peduncles of the quaking-grass or Briza. The +latter are about as thick as the hair of a man's beard, but they were +completely surrounded and clasped. The petioles of a leaf, so young +that none of the leaflets were expanded, had partially seized a twig. +Those of almost all the old leaves, even when unattached to any +object, are much convoluted; but this is owing to their having come, +whilst young, into contact during several hours with some object +subsequently removed. With none of the above-described species, +cultivated in pots and carefully observed, was there any permanent +bending of the petioles without the stimulus of contact. In winter, +the blades of the leaves of C. vitalba drop off; but the petioles (as +was observed by Mohl) remain attached to the branches, sometimes +during two seasons; and, being convoluted, they curiously resemble +true tendrils, such as those possessed by the allied genus Naravelia. +The petioles which have clasped some object become much more stiff, +hard, and polished than those which have failed in this their proper +function. + +TROPAEOLUM.--I observed T. tricolorum, T. azureum, T. pentaphyllum, +T. peregrinum, T. elegans, T. tuberosum, and a dwarf variety of, as I +believe, T. minus. + +Tropaeolum tricolorum, var. grandiflorum.--The flexible shoots, which +first rise from the tubers, are as thin as fine twine. One such +shoot revolved in a course opposed to the sun, at an average rate, +judging from three revolutions, of 1 hr. 23 m.; but no doubt the +direction of the revolving movement is variable. When the plants +have grown tall and are branched, all the many lateral shoots +revolve. The stem, whilst young, twines regularly round a thin +vertical stick, and in one case I counted eight spiral turns in the +same direction; but when grown older, the stem often runs straight up +for a space, and, being arrested by the clasping petioles, makes one +or two spires in a reversed direction. Until the plant grows to a +height of two or three feet, requiring about a month from the time +when the first shoot appears above ground, no true leaves are +produced, but, in their place, filaments coloured like the stem. The +extremities of these filaments are pointed, a little flattened, and +furrowed on the upper surface. They never become developed into +leaves. As the plant grows in height new filaments are produced with +slightly enlarged tips; then others, bearing on each side of the +enlarged medial tip a rudimentary segment of a leaf; soon other +segments appear, and at last a perfect leaf is formed, with seven +deep segments. So that on the same plant we may see every step, from +tendril-like clasping filaments to perfect leaves with clasping +petioles. After the plant has grown to a considerable height, and is +secured to its support by the petioles of the true leaves, the +clasping filaments on the lower part of the stem wither and drop off; +so that they perform only a temporary service. + +These filaments or rudimentary leaves, as well as the petioles of the +perfect leaves, whilst young, are highly sensitive on all sides to a +touch. The slightest rub caused them to curve towards the rubbed +side in about three minutes, and one bent itself into a ring in six +minutes; they subsequently became straight. When, however, they have +once completely clasped a stick, if this is removed, they do not +straighten themselves. The most remarkable fact, and one which I +have observed in no other species of the genus, is that the filaments +and the petioles of the young leaves, if they catch no object, after +standing for some days in their original position, spontaneously and +slowly oscillate a little from side to side, and then move towards +the stem and clasp it. They likewise often become, after a time, in +some degree spirally contracted. They therefore fully deserve to be +called tendrils, as they are used for climbing, are sensitive to a +touch, move spontaneously, and ultimately contract into a spire, +though an imperfect one. The present species would have been classed +amongst the tendril-bearers, had not these characters been confined +to early youth. During maturity it is a true leaf-climber. + +Tropaeolum azureum.--An upper internode made four revolutions, +following the sun, at an average rate of 1 hr. 47 m. The stem twined +spirally round a support in the same irregular manner as that of the +last species. Rudimentary leaves or filaments do not exist. The +petioles of the young leaves are very sensitive: a single light rub +with a twig caused one to move perceptibly in 5 m., and another in 6 +m. The former became bent at right angles in 15 min., and became +straight again in between 5 hrs. and 6 hrs. A loop of thread +weighing 0.125th of a grain caused another petiole to curve. + +Tropaeolum pentaphyllum.--This species has not the power of spirally +twining, which seems due, not so much to a want of flexibility in the +stem, as to continual interference from the clasping petioles. An +upper internode made three revolutions, following the sun, at an +average rate of 1 hr. 46 m. The main purpose of the revolving +movement in all the species of Tropaeolum manifestly is to bring the +petioles into contact with some supporting object. The petiole of a +young leaf, after a slight rub, became curved in 6 m.; another, on a +cold day, in 20 m., and others in from 8 m. to 10 m. Their curvature +usually increased greatly in from 15 m. to 20 m., and they became +straight again in between 5 hrs. and 6 hrs., but on one occasion in 3 +hrs. When a petiole has fairly clasped a stick, it is not able, on +the removal of the stick, to straighten itself. The free upper part +of one, the base of which had already clasped a stick, still retained +the power of movement. A loop of thread weighing 0.125th of a grain +caused a petiole to curve; but the stimulus was not sufficient, the +loop remaining suspended, to cause a permanent flexure. If a much +heavier loop be placed in the angle between the petiole and the stem, +it produces no effect; whereas we have seen with Clematis montana +that the angle between the stem and petiole is sensitive. + +Tropaeolum peregrinum.--The first-formed internodes of a young plant +did not revolve, resembling in this respect those of a twining plant. +In an older plant the four upper internodes made three irregular +revolutions, in a course opposed to the sun, at an average rate of 1 +hr. 48 min. It is remarkable that the average rate of revolution +(taken, however, but from few observations) is very nearly the same +in this and the two last species, namely, 1 hr. 47 m., 1 hr. 46 m., +and 1 hr. 48 m. The present species cannot twine spirally, which +seems mainly due to the rigidity of the stem. In a very young plant, +which did not revolve, the petioles were not sensitive. In older +plants the petioles of quite young leaves, and of leaves as much as +an inch and a quarter in diameter, are sensitive. A moderate rub +caused one to curve in 10 m., and others in 20 m. They became +straight again in between 5 hrs. 45m. and 8 hrs. Petioles which have +naturally come into contact with a stick, sometimes take two turns +round it. After they have clasped a support, they become rigid and +hard. They are less sensitive to a weight than in the previous +species; for loops of string weighing 0.82 of a grain (53.14 mg.), +did not cause any curvature, but a loop of double this weight (1.64 +gr.) acted. + +Tropaeolum elegans.--I did not make many observations on this +species. The short and stiff internodes revolve irregularly, +describing small oval figures. One oval was completed in 3 hrs. A +young petiole, when rubbed, became slightly curved in 17 m.; and +afterwards much more so. It was nearly straight again in 8 hrs. + +Tropaeolum tuberosum.--On a plant nine inches in height, the +internodes did not move at all; but on an older plant they moved +irregularly and made small imperfect ovals. These movements could be +detected only by being traced on a bell-glass placed over the plant. +Sometimes the shoots stood still for hours; during some days they +moved only in one direction in a crooked line; on other days they +made small irregular spires or circles, one being completed in about +4 hrs. The extreme points reached by the apex of the shoot were only +about one or one and a half inches asunder; yet this slight movement +brought the petioles into contact with some closely surrounding +twigs, which were then clasped. With the lessened power of +spontaneously revolving, compared with that of the previous species, +the sensitiveness of the petioles is also diminished. These, when +rubbed a few times, did not become curved until half an hour had +elapsed; the curvature increased during the next two hours, and then +very slowly decreased; so that they sometimes required 24 hrs. to +become straight again. Extremely young leaves have active petioles; +one with the lamina only 0.15 of an inch in diameter, that is, about +a twentieth of the full size, firmly clasped a thin twig. But leaves +grown to a quarter of their full size can likewise act. + +Tropaeolum minus (?).--The internodes of a variety named "dwarf +crimson Nasturtium" did not revolve, but moved in a rather irregular +course during the day to the light, and from the light at night. The +petioles, when well rubbed, showed no power of curving; nor could I +see that they ever clasped any neighbouring object. We have seen in +this genus a gradation from species such as T. tricolorum, which have +extremely sensitive petioles, and internodes which rapidly revolve +and spirally twine up a support, to other species such as T. elegans +and T. tuberosum, the petioles of which are much less sensitive, and +the internodes of which have very feeble revolving powers and cannot +spirally twine round a support, to this last species, which has +entirely lost or never acquired these faculties. From the general +character of the genus, the loss of power seems the more probable +alternative. + +In the present species, in T. elegans, and probably in others, the +flower-peduncle, as soon as the seed-capsule begins to swell, +spontaneously bends abruptly downwards and becomes somewhat +convoluted. If a stick stands in the way, it is to a certain extent +clasped; but, as far as I have been able to observe, this clasping +movement is independent of the stimulus from contact. + +ANTIRRHINEAE.--In this tribe (Lindley) of the Scrophulariaceae, at +least four of the seven included genera have leaf-climbing species. + +Maurandia Barclayana.--A thin, slightly bowed shoot made two +revolutions, following the sun, each in 3 hrs. 17 min.; on the +previous day this same shoot revolved in an opposite direction. The +shoots do not twine spirally, but climb excellently by the aid of +their young and sensitive petioles. These petioles, when lightly +rubbed, move after a considerable interval of time, and subsequently +become straight again. A loop of thread weighing 0.125th of a grain +caused them to bend. + +Maurandia semperflorens.--This freely growing species climbs exactly +like the last, by the aid of its sensitive petioles. A young +internode made two circles, each in 1 hr. 46 mm.; so that it moved +almost twice as rapidly as the last species. The internodes are not +in the least sensitive to a touch or pressure. I mention this +because they are sensitive in a closely allied genus, namely, +Lophospermum. The present species is unique in one respect. Mohl +asserts (p. 45) that "the flower-peduncles, as well as the petioles, +wind like tendrils;" but he classes as tendrils such objects as the +spiral flower-stalks of the Vallisneria. This remark, and the fact +of the flower-peduncles being decidedly flexuous, led me carefully to +examine them. They never act as true tendrils; I repeatedly placed +thin sticks in contact with young and old peduncles, and I allowed +nine vigorous plants to grow through an entangled mass of branches; +but in no one instance did they bend round any object. It is indeed +in the highest degree improbable that this should occur, for they are +generally developed on branches which have already securely clasped a +support by the petioles of their leaves; and when borne on a free +depending branch, they are not produced by the terminal portion of +the internode which alone has the power of revolving; so that they +could be brought only by accident into contact with any neighbouring +object. Nevertheless (and this is the remarkable fact) the flower- +peduncles, whilst young, exhibit feeble revolving powers, and are +slightly sensitive to a touch. Having selected some stems which had +firmly clasped a stick by their petioles, and having placed a bell- +glass over them, I traced the movements of the young flower- +peduncles. The tracing generally formed a short and extremely +irregular line, with little loops in its course. A young peduncle +1.5 inch in length was carefully observed during a whole day, and it +made four and a half narrow, vertical, irregular, and short ellipses- +-each at an average rate of about 2 hrs. 25 m. An adjoining peduncle +described during the same time similar, though fewer, ellipses. As +the plant had occupied for some time exactly the same position, these +movements could not be attributed to any change in the action of the +light. Peduncles, old enough for the coloured petals to be just +visible, do not move. With respect to irritability, {21} I rubbed +two young peduncles (1.5 inch in length) a few times very lightly +with a thin twig; one was rubbed on the upper, and the other on the +lower side, and they became in between 4 hrs. and 5 hrs. distinctly +bowed towards these sides; in 24 hrs. subsequently, they straightened +themselves. Next day they were rubbed on the opposite sides, and +they became perceptibly curved towards these sides. Two other and +younger peduncles (three-fourths of an inch in length) were lightly +rubbed on their adjoining sides, and they became so much curved +towards one another, that the arcs of the bows stood at nearly right +angles to their previous direction; and this was the greatest +movement seen by me. Subsequently they straightened themselves. +Other peduncles, so young as to be only three-tenths of an inch in +length, became curved when rubbed. On the other hand, peduncles +above 1.5 inch in length required to be rubbed two or three times, +and then became only just perceptibly bowed. Loops of thread +suspended on the peduncles produced no effect; loops of string, +however, weighing 0.82 and 1.64 of a grain sometimes caused a slight +curvature; but they were never closely clasped, as were the far +lighter loops of thread by the petioles. + +In the nine vigorous plants observed by me, it is certain that +neither the slight spontaneous movements nor the slight sensitiveness +of the flower-peduncles aided the plants in climbing. If any member +of the Scrophulariaceae had possessed tendrils produced by the +modification of flower-peduncles, I should have thought that this +species of Maurandia had perhaps retained a useless or rudimentary +vestige of a former habit; but this view cannot be maintained. We +may suspect that, owing to the principle of correlation, the power of +movement has been transferred to the flower-peduncles from the young +internodes, and sensitiveness from the young petioles. But to +whatever cause these capacities are due, the case is interesting; +for, by a little increase in power through natural selection, they +might easily have been rendered as useful to the plant in climbing, +as are the flower-peduncles (hereafter to be described) of Vitis or +Cardiospermum. + +Rhodochiton volubile.--A long flexible shoot swept a large circle, +following the sun, in 5 hrs. 30 m.; and, as the day became warmer, a +second circle was completed in 4 hrs. 10 m. The shoots sometimes +make a whole or a half spire round a vertical stick, they then run +straight up for a space, and afterwards turn spirally in an opposite +direction. The petioles of very young leaves about one-tenth of +their full size, are highly sensitive, and bend towards the side +which is touched; but they do not move quickly. One was perceptibly +curved in 1 hr. 10 m., after being lightly rubbed, and became +considerably curved in 5 hrs. 40 m.; some others were scarcely curved +in 5 hrs. 30 m., but distinctly so in 6 hrs. 30 m. A curvature was +perceptible in one petiole in between 4 hrs. 30 m. and 5 hrs., after +the suspension of a little loop of string. A loop of fine cotton +thread, weighing one sixteenth of a grain (4.05 mg.), not only caused +a petiole slowly to bend, but was ultimately so firmly clasped that +it could be withdrawn only by some little force. The petioles, when +coming into contact with a stick, take either a complete or half a +turn round it, and ultimately increase much in thickness. They do +not possess the power of spontaneously revolving. + +Lophospermum scandens, var. purpureum.--Some long, moderately thin +internodes made four revolutions at an average rate of 3 hrs. 15 m. +The course pursued was very irregular, namely, an extremely narrow +ellipse, a large circle, an irregular spire or a zigzag line, and +sometimes the apex stood still. The young petioles, when brought by +the revolving movement into contact with sticks, clasped them, and +soon increased considerably in thickness. But they are not quite so +sensitive to a weight as those of the Rhodochiton, for loops of +thread weighing one-eighth of a grain did not always cause them to +bend. + +This plant presents a case not observed by me in any other leaf- +climber or twiner, {22} namely, that the young internodes of the stem +are sensitive to a touch. When a petiole of this species clasps a +stick, it draws the base of the internode against it; and then the +internode itself bends towards the stick, which is caught between the +stem and the petiole as by a pair of pincers. The internode +afterwards straightens itself, excepting the part in actual contact +with the stick. Young internodes alone are sensitive, and these are +sensitive on all sides along their whole length. I made fifteen +trials by twice or thrice lightly rubbing with a thin twig several +internodes; and in about 2 hrs., but in one case in 3 hrs., all were +bent: they became straight again in about 4 hrs. afterwards. An +internode, which was rubbed as often as six or seven times, became +just perceptibly curved in 1 hr. 15 m., and in 3 hrs. the curvature +increased much; it became straight again in the course of the +succeeding night. I rubbed some internodes one day on one side, and +the next day either on the opposite side or at right angles to the +first side; and the curvature was always towards the rubbed side. + +According to Palm (p. 63), the petioles of Linaria cirrhosa and, to a +limited degree, those of L. elatine have the power of clasping a +support. + +SOLANACEAE.--Solanum jasminoides.--Some of the species in this large +genus are twiners; but the present species is a true leaf-climber. A +long, nearly upright shoot made four revolutions, moving against the +sun, very regularly at an average rate of 3 hrs. 26 m. The shoots, +however, sometimes stood still. It is considered a greenhouse plant; +but when kept there, the petioles took several days to clasp a stick: +in the hothouse a stick was clasped in 7 hrs. In the greenhouse a +petiole was not affected by a loop of string, suspended during +several days and weighing 2.5 grains (163 mg.); but in the hothouse +one was made to curve by a loop weighing 1.64 gr. (106.27 mg.); and, +on the removal of the string, it became straight again. Another +petiole was not at all acted on by a loop weighing only 0.82 of a +grain (53.14 mg.) We have seen that the petioles of some other leaf- +climbing plants are affected by one-thirteenth of this latter weight. +In this species, and in no other leaf-climber seen by me, a full- +grown leaf is capable of clasping a stick; but in the greenhouse the +movement was so extraordinarily slow that the act required several +weeks; on each succeeding week it was clear that the petiole had +become more and more curved, until at last it firmly clasped the +stick. + +The flexible petiole of a half or a quarter grown leaf which has +clasped an object for three or four days increases much in thickness, +and after several weeks becomes so wonderfully hard and rigid that it +can hardly be removed from its support. On comparing a thin +transverse slice of such a petiole with one from an older leaf +growing close beneath, which had not clasped anything, its diameter +was found to be fully doubled, and its structure greatly changed. In +two other petioles similarly compared, and here represented, the +increase in diameter was not quite so great. In the section of the +petiole in its ordinary state (A), we see a semilunar band of +cellular tissue (not well shown in the woodcut) differing slightly in +appearance from that outside it, and including three closely +approximate groups of dark vessels. Near the upper surface of the +petiole, beneath two exterior ridges, there are two other small +circular groups of vessels. In the section of the petiole (B) which +had clasped during several weeks a stick, the two exterior ridges +have become much less prominent, and the two groups of woody vessels +beneath them much increased in diameter. The semilunar band has been +converted into a complete ring of very hard, white, woody tissue, +with lines radiating from the centre. The three groups of vessels, +which, though near together, were before distinct, are now completely +blended. The upper part of this ring of woody vessels, formed by the +prolongation of the horns of the original semilunar band, is narrower +than the lower part, and slightly less compact. This petiole after +clasping the stick had actually become thicker than the stem from +which it arose; and this was chiefly due to the increased thickness +of the ring of wood. This ring presented, both in a transverse and +longitudinal section, a closely similar structure to that of the +stem. It is a singular morphological fact that the petiole should +thus acquire a structure almost identically the same with that of the +axis; and it is a still more singular physiological fact that so +great a change should have been induced by the mere act of clasping a +support. {23} + +FUMARIACEAE.--Fumaria officinalis.--It could not have been +anticipated that so lowly a plant as this Fumaria should have been a +climber. It climbs by the aid of the main and lateral petioles of +its compound leaves; and even the much-flattened terminal portion of +the petiole can seize a support. I have seen a substance as soft as +a withered blade of grass caught. Petioles which have clasped any +object ultimately become rather thicker and more cylindrical. On +lightly rubbing several petioles with a twig, they became perceptibly +curved in 1 hr. 15 m., and subsequently straightened themselves. A +stick gently placed in the angle between two sub-petioles excited +them to move, and was almost clasped in 9 hrs. A loop of thread, +weighing one-eighth of a grain, caused, after 12 hrs. and before 20 +hrs, had elapsed, a considerable curvature; but it was never fairly +clasped by the petiole. The young internodes are in continual +movement, which is considerable in extent, but very irregular; a +zigzag line, or a spire crossing itself; or a figure of 8 being +formed. The course during 12 hrs., when traced on a bell-glass, +apparently represented about four ellipses. The leaves themselves +likewise move spontaneously, the main petioles curving themselves in +accordance with the movements of the internodes; so that when the +latter moved to one side, the petioles moved to the same side, then, +becoming straight, reversed their curvature. The petioles, however, +do not move over a wide space, as could be seen when a shoot was +securely tied to a stick. The leaf in this case followed an +irregular course, like that made by the internodes. + +Adlumia cirrhosa.--I raised some plants late in the summer; they +formed very fine leaves, but threw up no central stem. The first- +formed leaves were not sensitive; some of the later ones were so, but +only towards their extremities, which were thus enabled to clasp +sticks. This could be of no service to the plant, as these leaves +rose from the ground; but it showed what the future character of the +plant would have been, had it grown tall enough to climb. The tip of +one of these basal leaves, whilst young, described in 1 hr. 36 m. a +narrow ellipse, open at one end, and exactly three inches in length; +a second ellipse was broader, more irregular, and shorter, viz., only +2.5 inches in length, and was completed in 2 hrs. 2 m. From the +analogy of Fumaria and Corydalis, I have no doubt that the internodes +of Adlumia have the power of revolving. + +Corydalis claviculata.--This plant is interesting from being in a +condition so exactly intermediate between a leaf-climber and a +tendril-bearer, that it might have been described under either head; +but, for reasons hereafter assigned, it has been classed amongst +tendril-bearers. + +Besides the plants already described, Bignonia unguis and its close +allies, though aided by tendrils, have clasping petioles. According +to Mohl (p. 40), Cocculus Japonicus (one of the Menispermaceae) and a +fern, the Ophioglossum Japonicum (p. 39), climb by their leaf-stalks. + + +We now come to a small section of plants which climb by means of the +produced midribs or tips of their leaves. + +LILIACEAE.--Gloriosa Plantii.--The stem of a half-grown plant +continually moved, generally describing an irregular spire, but +sometimes oval figures with the longer axes directed in different +lines. It either followed the sun, or moved in an opposite course, +and sometimes stood still before reversing its direction. One oval +was completed in 3 hrs. 40 m.; of two horseshoe-shaped figures, one +was completed in 4 hrs. 35 m. and the other in 3 hrs. The shoots, in +their movements, reached points between four and five inches asunder. +The young leaves, when first developed, stand up nearly vertically; +but by the growth of the axis, and by the spontaneous bending down of +the terminal half of the leaf, they soon become much inclined, and +ultimately horizontal. The end of the leaf forms a narrow, ribbon- +like, thickened projection, which at first is nearly straight, but by +the time the leaf gets into an inclined position, the end bends +downwards into a well-formed hook. This hook is now strong and rigid +enough to catch any object, and, when caught, to anchor the plant and +stop the revolving movement. Its inner surface is sensitive, but not +in nearly so high a degree as that of the many before-described +petioles; for a loop of string, weighing 1.64 grain, produced no +effect. When the hook has caught a thin twig or even a rigid fibre, +the point may be perceived in from 1 hr. to 3 hrs. to have curled a +little inwards; and, under favourable circumstances, it curls round +and permanently seizes an object in from 8 hrs. to 10 hrs. The hook +when first formed, before the leaf has bent downwards, is but little +sensitive. If it catches hold of nothing, it remains open and +sensitive for a long time; ultimately the extremity spontaneously and +slowly curls inwards, and makes a button-like, flat, spiral coil at +the end of the leaf. One leaf was watched, and the hook remained +open for thirty-three days; but during the last week the tip had +curled so much inwards that only a very thin twig could have been +inserted within it. As soon as the tip has curled so much inwards +that the hook is converted into a ring, its sensibility is lost; but +as long as it remains open some sensibility is retained. + +Whilst the plant was only about six inches in height, the leaves, +four or five in number, were broader than those subsequently +produced; their soft and but little-attenuated tips were not +sensitive, and did not form hooks; nor did the stem then revolve. At +this early period of growth, the plant can support itself; its +climbing powers are not required, and consequently are not developed. +So again, the leaves on the summit of a full-grown flowering plant, +which would not require to climb any higher, were not sensitive and +could not clasp a stick. We thus see how perfect is the economy of +nature. + +COMMELYNACEAE.--Flagellaria Indica.--From dried specimens it is +manifest that this plant climbs exactly like the Gloriosa. A young +plant 12 inches in height, and bearing fifteen leaves, had not a +single leaf as yet produced into a hook or tendril-like filament; nor +did the stem revolve. Hence this plant acquires its climbing powers +later in life than does the Gloriosa lily. According to Mohl (p. +41), Uvularia (Melanthaceae) also climbs like Gloriosa. + +These three last-named genera are Monocotyledons; but there is one +Dicotyledon, namely Nepenthes, which is ranked by Mohl (p. 41) +amongst tendril-bearers; and I hear from Dr. Hooker that most of the +species climb well at Kew. This is effected by the stalk or midrib +between the leaf and the pitcher coiling round any support. The +twisted part becomes thicker; but I observed in Mr. Veitch's hothouse +that the stalk often takes a turn when not in contact with any +object, and that this twisted part is likewise thickened. Two +vigorous young plants of N. laevis and N. distillatoria, in my +hothouse, whilst less than a foot in height, showed no sensitiveness +in their leaves, and had no power of climbing. But when N. laevis +had grown to a height of 16 inches, there were signs of these powers. +The young leaves when first formed stand upright, but soon become +inclined; at this period they terminate in a stalk or filament, with +the pitcher at the extremity hardly at all developed. The leaves now +exhibited slight spontaneous movements; and when the terminal +filaments came into contact with a stick, they slowly bent round and +firmly seized it. But owing to the subsequent growth of the leaf, +this filament became after a time quite slack, though still remaining +firmly coiled round the stick. Hence it would appear that the chief +use of the coiling, at least whilst the plant is young, is to support +the pitcher with its load of secreted fluid. + + +Summary on Leaf-climbers.--Plants belonging to eight families are +known to have clasping petioles, and plants belonging to four +families climb by the tips of their leaves. In all the species +observed by me, with one exception, the young internodes revolve more +or less regularly, in some cases as regularly as those of a twining +plant. They revolve at various rates, in most cases rather rapidly. +Some few can ascend by spirally twining round a support. Differently +from most twiners, there is a strong tendency in the same shoot to +revolve first in one and then in an opposite direction. The object +gained by the revolving movement is to bring the petioles or the tips +of the leaves into contact with surrounding objects; and without this +aid the plant would be much less successful in climbing. With rare +exceptions, the petioles are sensitive only whilst young. They are +sensitive on all sides, but in different degrees in different plants; +and in some species of Clematis the several parts of the same petiole +differ much in sensitiveness. The hooked tips of the leaves of the +Gloriosa are sensitive only on their inner or inferior surfaces. The +petioles are sensitive to a touch and to excessively slight continued +pressure, even from a loop of soft thread weighing only the one- +sixteenth of a grain (4.05 mg.); and there is reason to believe that +the rather thick and stiff petioles of Clematis flammula are +sensitive to even much less weight if spread over a wide surface. +The petioles always bend towards the side which is pressed or +touched, at different rates in different species, sometimes within a +few minutes, but generally after a much longer period. After +temporary contact with any object, the petiole continues to bend for +a considerable time; afterwards it slowly becomes straight again, and +can then re-act. A petiole excited by an extremely slight weight +sometimes bends a little, and then becomes accustomed to the +stimulus, and either bends no more or becomes straight again, the +weight still remaining suspended. Petioles which have clasped an +object for some little time cannot recover their original position. +After remaining clasped for two or three days, they generally +increase much in thickness either throughout their whole diameter or +on one side alone; they subsequently become stronger and more woody, +sometimes to a wonderful degree; and in some cases they acquire an +internal structure like that of the stem or axis. + +The young internodes of the Lophospermum as well as the petioles are +sensitive to a touch, and by their combined movement seize an object. +The flower-peduncles of the Maurandia semperflorens revolve +spontaneously and are sensitive to a touch, yet are not used for +climbing. The leaves of at least two, and probably of most, of the +species of Clematis, of Fumaria and Adlumia, spontaneously curve from +side to side, like the internodes, and are thus better adapted to +seize distant objects. The petioles of the perfect leaves of +Tropaeolum tricolorum, as well as the tendril-like filaments of the +plants whilst young, ultimately move towards the stem or the +supporting stick, which they then clasp. These petioles and +filaments also show some tendency to contract spirally. The tips of +the uncaught leaves of the Gloriosa, as they grow old, contract into +a flat spire or helix. These several facts are interesting in +relation to true tendrils. + +With leaf climbers, as with twining plants, the first internodes +which rise from the ground do not, at least in the cases observed by +me, spontaneously revolve; nor are the petioles or tips of the first- +formed leaves sensitive. In certain species of Clematis, the large +size of the leaves, together with their habit of revolving, and the +extreme sensitiveness of their petioles, appear to render the +revolving movement of the internodes superfluous; and this latter +power has consequently become much enfeebled. In certain species of +Tropaeolum, both the spontaneous movements of the internodes and the +sensitiveness of the petioles have become much enfeebled, and in one +species have been completely lost. + + + +CHAPTER III.--TENDRIL-BEARERS. + + + +Nature of tendrils--BIGNONIACEAE, various species of, and their +different modes of climbing--Tendrils which avoid the light and creep +into crevices--Development of adhesive discs--Excellent adaptations +for seizing different kinds of supports.--POLEMONIACEAE--Cobaea +scandens much branched and hooked tendrils, their manner of action-- +LEGUMINOSAE--COMPOSITAE--SMILACEAE--Smilax aspera, its inefficient +tendrils--FUMARIACEAE--Corydalis claviculata, its state intermediate +between that of a leaf-climber and a tendril-bearer. + +By tendrils I mean filamentary organs, sensitive to contact and used +exclusively for climbing. By this definition, spines, hooks and +rootlets, all of which are used for climbing, are excluded. True +tendrils are formed by the modification of leaves with their +petioles, of flower-peduncles, branches, {24} and perhaps stipules. +Mohl, who includes under the name of tendrils various organs having a +similar external appearance, classes them according to their +homological nature, as being modified leaves, flower-peduncles, &c. +This would be an excellent scheme; but I observe that botanists are +by no means unanimous on the homological nature of certain tendrils. +Consequently I will describe tendril-bearing plants by natural +families, following Lindley's classification; and this will in most +cases keep those of the same nature together. The species to be +described belong to ten families, and will be given in the following +order: --Bignoniaceae, Polemoniaceae, Leguminosae, Compositae, +Smilaceae, Fumariaceae, Cucurbitaceae, Vitaceae, Sapindaceae, +Passifloraceae. {25} + +BIGNONIACEAE.--This family contains many tendril-bearers, some +twiners, and some root-climbers. The tendrils always consist of +modified leaves. Nine species of Bignonia, selected by hazard, are +here described, in order to show what diversity of structure and +action there may be within the same genus, and to show what +remarkable powers some tendrils possess. The species, taken +together, afford connecting links between twiners, leaf-climbers, +tendril-bearers, and root-climbers. + +Bignonia (an unnamed species from Kew, closely allied to B. unguis, +but with smaller and rather broader leaves).--A young shoot from a +cut-down plant made three revolutions against the sun, at an average +rate of 2 hrs. 6m. The stem is thin and flexible; it twined round a +slender vertical stick, ascending from left to right, as perfectly +and as regularly as any true twining-plant. When thus ascending, it +makes no use of its tendrils or petioles; but when it twined round a +rather thick stick, and its petioles were brought into contact with +it, these curved round the stick, showing that they have some degree +of irritability. The petioles also exhibit a slight degree of +spontaneous movement; for in one case they certainly described +minute, irregular, vertical ellipses. The tendrils apparently curve +themselves spontaneously to the same side with the petioles; but from +various causes, it was difficult to observe the movement of either +the tendrils or petioles, in this and the two following species. The +tendrils are so closely similar in all respects to those of B. +unguis, that one description will suffice. + +Bignonia unguis.--The young shoots revolve, but less regularly and +less quickly than those of the last species. The stem twines +imperfectly round a vertical stick, sometimes reversing its +direction, in the same manner as described in so many leaf-climbers; +and this plant though possessing tendrils, climbs to a certain extent +like a leaf-climber. Each leaf consists of a petiole bearing a pair +of leaflets, and terminates in a tendril, which is formed by the +modification of three leaflets, and closely resembles that above +figured (fig. 5). But it is a little larger, and in a young plant +was about half an inch in length. It is curiously like the leg and +foot of a small bird, with the hind toe cut off. The straight leg or +tarsus is longer than the three toes, which are of equal length, and +diverging, lie in the same plane. The toes terminate in sharp, hard +claws, much curved downwards, like those on a bird's foot. The +petiole of the leaf is sensitive to contact; even a small loop of +thread suspended for two days caused it to bend upwards; but the sub- +petioles of the two lateral leaflets are not sensitive. The whole +tendril, namely, the tarsus and the three toes, are likewise +sensitive to contact, especially on their under surfaces. When a +shoot grows in the midst of thin branches, the tendrils are soon +brought by the revolving movement of the internodes into contact with +them; and then one toe of the tendril or more, commonly all three, +bend, and after several hours seize fast hold of the twigs, like a +bird when perched. If the tarsus of the tendril comes into contact +with a twig, it goes on slowly bending, until the whole foot is +carried quite round, and the toes pass on each side of the tarsus and +seize it. In like manner, if the petiole comes into contact with a +twig, it bends round, carrying the tendril, which then seizes its own +petiole or that of the opposite leaf. The petioles move +spontaneously, and thus, when a shoot attempts to twine round an +upright stick, those on both sides after a time come into contact +with it, and are excited to bend. Ultimately the two petioles clasp +the stick in opposite directions, and the foot-like tendrils, seizing +on each other or on their own petioles, fasten the stem to the +support with surprising security. The tendrils are thus brought into +action, if the stem twines round a thin vertical stick; and in this +respect the present species differs from the last. Both species use +their tendrils in the same manner when passing through a thicket. +This plant is one of the most efficient climbers which I have +observed; and it probably could ascend a polished stem incessantly +tossed by heavy storms. To show how important vigorous health is for +the action of all the parts, I may mention that when I first examined +a plant which was growing moderately well, though not vigorously, I +concluded that the tendrils acted only like the hooks on a bramble, +and that it was the most feeble and inefficient of all climbers! + +Bignonia Tweedyana.--This species is closely allied to the last, and +behaves in the same manner; but perhaps twines rather better round a +vertical stick. On the same plant, one branch twined in one +direction and another in an opposite direction. The internodes in +one case made two circles, each in 2 hrs. 33 m. I was enabled to +observe the spontaneous movements of the petioles better in this than +in the two preceding species: one petiole described three small +vertical ellipses in the course of 11 hrs., whilst another moved in +an irregular spire. Some little time after a stem has twined round +an upright stick, and is securely fastened to it by the clasping +petioles and tendrils, it emits aerial roots from the bases of its +leaves; and these roots curve partly round and adhere to the stick. +This species of Bignonia, therefore, combines four different methods +of climbing generally characteristic of distinct plants, namely, +twining, leaf-climbing, tendril-climbing, and root-climbing. + +In the three foregoing species, when the foot-like tendril has caught +an object, it continues to grow and thicken, and ultimately becomes +wonderfully strong, in the same manner as the petioles of leaf- +climbers. If the tendril catches nothing, it first slowly bends +downwards, and then its power of clasping is lost. Very soon +afterwards it disarticulates itself from the petiole, and drops off +like a leaf in autumn. I have seen this process of disarticulation +in no other tendrils, for these, when they fail to catch an object, +merely wither away. + +Bignonia venusta.--The tendrils differ considerably from those of the +previous species. The lower part, or tarsus, is four times as long +as the three toes; these are of equal length and diverge equally, but +do not lie in the same plane; their tips are bluntly hooked, and the +whole tendril makes an excellent grapnel. The tarsus is sensitive on +all sides; but the three toes are sensitive only on their outer +surfaces. The sensitiveness is not much developed; for a slight +rubbing with a twig did not cause the tarsus or the toes to become +curved until an hour had elapsed, and then only in a slight degree. +Subsequently they straightened themselves. Both the tarsus and toes +can seize well hold of sticks. If the stem is secured, the tendrils +are seen spontaneously to sweep large ellipses; the two opposite +tendrils moving independently of one another. I have no doubt, from +the analogy of the two following allied species, that the petioles +also move spontaneously; but they are not irritable like those of B. +unguis and B. Tweedyana. The young internodes sweep large circles, +one being completed in 2 hrs. 15 m., and a second in 2 hrs. 55 m. By +these combined movements of the internodes, petioles, and grapnel- +like tendrils, the latter are soon brought into contact with +surrounding objects. When a shoot stands near an upright stick, it +twines regularly and spirally round it. As it ascends, it seizes the +stick with one of its tendrils, and, if the stick be thin, the right- +and left-hand tendrils are alternately used. This alternation +follows from the stem necessarily taking one twist round its own axis +for each completed circle. + +The tendrils contract spirally a short time after catching any +object; those which catch nothing merely bend slowly downwards. But +the whole subject of the spiral contraction of tendrils will be +discussed after all the tendril-bearing species have been described. + +Bignonia littoralis.--The young internodes revolve in large ellipses. +An internode bearing immature tendrils made two revolutions, each in +3 hrs. 50 m.; but when grown older with the tendrils mature, it made +two ellipses, each at the rate of 2 hrs. 44 m. This species, unlike +the preceding, is incapable of twining round a stick: this does not +appear to be due to any want of flexibility in the internodes or to +the action of the tendrils, and certainly not to any want of the +revolving power; nor can I account for the fact. Nevertheless the +plant readily ascends a thin upright stick by seizing a point above +with its two opposite tendrils, which then contract spirally. If the +tendrils seize nothing, they do not become spiral. + +The species last described, ascended a vertical stick by twining +spirally and by seizing it alternately with its opposite tendrils, +like a sailor pulling himself up a rope, hand over hand; the present +species pulls itself up, like a sailor seizing with both hands +together a rope above his head. + +The tendrils are similar in structure to those of the last species. +They continue growing for some time, even after they have clasped an +object. When fully grown, though borne by a young plant, they are 9 +inches in length. The three divergent toes are shorter relatively to +the tarsus than in the former species; they are blunt at their tips +and but slightly hooked; they are not quite equal in length, the +middle one being rather longer than the others. Their outer surfaces +are highly sensitive; for when lightly rubbed with a twig, they +became perceptibly curved in 4 m. and greatly curved in 7 m. In 7 +hrs. they became straight again and were ready to re-act. The +tarsus, for the space of one inch close to the toes, is sensitive, +but in a rather less degree than the toes; for the latter after a +slight rubbing, became curved in about half the time. Even the +middle part of the tarsus is sensitive to prolonged contact, as soon +as the tendril has arrived at maturity. After it has grown old, the +sensitiveness is confined to the toes, and these are only able to +curl very slowly round a stick. A tendril is perfectly ready to act, +as soon as the three toes have diverged, and at this period their +outer surfaces first become irritable. The irritability spreads but +little from one part when excited to another: thus, when a stick was +caught by the part immediately beneath the three toes, these seldom +clasped it, but remained sticking straight out. + +The tendrils revolve spontaneously. The movement begins before the +tendril is converted into a three-pronged grapnel by the divergence +of the toes, and before any part has become sensitive; so that the +revolving movement is useless at this early period. The movement is, +also, now slow, two ellipses being completed conjointly in 24 hrs. 18 +m. A mature tendril made an ellipse in 6 hrs.; so that it moved much +more slowly than the internodes. The ellipses which were swept, both +in a vertical and horizontal plane, were of large size. The petioles +are not in the least sensitive, but revolve like the tendrils. We +thus see that the young internodes, the petioles, and the tendrils +all continue revolving together, but at different rates. The +movements of the tendrils which rise opposite one another are quite +independent. Hence, when the whole shoot is allowed freely to +revolve, nothing can be more intricate than the course followed by +the extremity of each tendril. A wide space is thus irregularly +searched for some object to be grasped. + +One other curious point remains to be mentioned. In the course of a +few days after the toes have closely clasped a stick, their blunt +extremities become developed, though not invariably, into irregular +disc-like balls which have the power of adhering firmly to the wood. +As similar cellular outgrowths will be fully described under B. +capreolata, I will here say nothing more about them. + +Bignonia aequinoctialis, var. Chamberlaynii.--The internodes, the +elongated non-sensitive petioles, and the tendrils all revolve. The +stem does not twine, but ascends a vertical stick in the same manner +as the last species. The tendrils also resemble those of the last +species, but are shorter; the three toes are more unequal in length, +the two outer ones being about one-third shorter and rather thinner +than the middle toe; but they vary in this respect. They terminate +in small hard points; and what is important, cellular adhesive discs +are not developed. The reduced size of two of the toes as well as +their lessened sensitiveness, seem to indicate a tendency to +abortion; and on one of my plants the first-formed tendrils were +sometimes simple, that is, were not divided into three toes. We are +thus naturally led to the three following species with undivided +tendrils + +Bignonia speciosa.--The young shoots revolve irregularly, making +narrow ellipses, spires or circles, at rates varying from 3 hrs. 30 +m. to 4 hrs. 40 m.; but they show no tendency to twine. Whilst the +plant is young and does not require a support, tendrils are not +developed. Those borne by a moderately young plant were five inches +in length. They revolve spontaneously, as do the short and non- +sensitive petioles. When rubbed, they slowly bend to the rubbed side +and subsequently straighten themselves; but they are not highly +sensitive. There is something strange in their behaviour: I +repeatedly placed close to them, thick and thin, rough and smooth +sticks and posts, as well as string suspended vertically, but none of +these objects were well seized. After clasping an upright stick, +they repeatedly loosed it again, and often would not seize it at all, +or their extremities did not coil closely round. I have observed +hundreds of tendrils belonging to various Cucurbitaceous, +Passifloraceous, and Leguminous plants, and never saw one behave in +this manner. When, however, my plant had grown to a height of eight +or nine feet, the tendrils acted much better. They now seized a +thin, upright stick horizontally, that is, at a point on their own +level, and not some way up the stick as in the case of all the +previous species. Nevertheless, the non-twining stem was enabled by +this means to ascend the stick. + +The extremity of the tendril is almost straight and sharp. The whole +terminal portion exhibits a singular habit, which in an animal would +be called an instinct; for it continually searches for any little +crevice or hole into which to insert itself. I had two young plants; +and, after having observed this habit, I placed near them posts, +which had been bored by beetles, or had become fissured by drying. +The tendrils, by their own movement and by that of the internodes, +slowly travelled over the surface of the wood, and when the apex came +to a hole or fissure it inserted itself; in order to effect this the +extremity for a length of half or quarter of an inch, would often +bend itself at right angles to the basal part. I have watched this +process between twenty and thirty times. The same tendril would +frequently withdraw from one hole and insert its point into a second +hole. I have also seen a tendril keep its point, in one case for 20 +hrs. and in another for 36 hrs., in a minute hole, and then withdraw +it. Whilst the point is thus temporarily inserted, the opposite +tendril goes on revolving. + +The whole length of a tendril often fits itself closely to any +surface of wood with which it has come into contact; and I have +observed one bent at right angles, from having entered a wide and +deep fissure, with its apex abruptly re-bent and inserted into a +minute lateral hole. After a tendril has clasped a stick, it +contracts spirally; if it remains unattached it hangs straight +downwards. If it has merely adapted itself to the inequalities of a +thick post, though it has clasped nothing, or if it has inserted its +apex into some little fissure, this stimulus suffices to induce +spiral contraction; but the contraction always draws the tendril away +from the post. So that in every case these movements, which seem so +nicely adapted for some purpose, were useless. On one occasion, +however, the tip became permanently jammed into a narrow fissure. I +fully expected, from the analogy of B. capreolata and B. littoralis, +that the tips would have been developed into adhesive discs; but I +could never detect even a trace of this process. There is therefore +at present something unintelligible about the habits of this plant. + +Bignonia picta.--This species closely resembles the last in the +structure and movements of its tendrils. I also casually examined a +fine growing plant of the allied B. Lindleyi, and this apparently +behaved in all respects in the same manner. + +Bignonia capreolata.--We now come to a species having tendrils of a +different type; but first for the internodes. A young shoot made +three large revolutions, following the sun, at an average rate of 2 +hrs. 23 m. The stem is thin and flexible, and I have seen one make +four regular spiral turns round a thin upright stick, ascending of +course from right to left, and therefore in a reversed direction +compared with the before described species. Afterwards, from the +interference of the tendrils, it ascended either straight up the +stick or in an irregular spire. The tendrils are in some respects +highly remarkable. In a young plant they were about 2.5 inches in +length and much branched, the five chief branches apparently +representing two pairs of leaflets and a terminal one. Each branch +is, however, bifid or more commonly trifid towards the extremity, +with the points blunt yet distinctly hooked. A tendril bends to any +side which is lightly rubbed, and subsequently becomes straight +again; but a loop of thread weighing 0.25th of a grain produced no +effect. On two occasions the terminal branches became slightly +curved in 10 m. after they had touched a stick; and in 30 m. the tips +were curled quite round it. The basal part is less sensitive. The +tendrils revolved in an apparently capricious manner, sometimes very +slightly or not at all; at other times they described large regular +ellipses. I could detect no spontaneous movement in the petioles of +the leaves. + +Whilst the tendrils are revolving more or less regularly, another +remarkable movement takes place, namely, a slow inclination from the +light towards the darkest side of the house. I repeatedly changed +the position of my plants, and some little time after the revolving +movement had ceased, the successively formed tendrils always ended by +pointing to the darkest side. When I placed a thick post near a +tendril, between it and the light, the tendril pointed in that +direction. In two instances a pair of leaves stood so that one of +the two tendrils was directed towards the light and the other to the +darkest side of the house; the latter did not move, but the opposite +one bent itself first upwards and then right over its fellow, so that +the two became parallel, one above the other, both pointing to the +dark: I then turned the plant half round; and the tendril which had +turned over recovered its original position, and the opposite one +which had not before moved, now turned over to the dark side. +Lastly, on another plant, three pairs of tendrils were produced at +the same time by three shoots, and all happened to be differently +directed: I placed the pot in a box open only on one side, and +obliquely facing the light; in two days all six tendrils pointed with +unerring truth to the darkest corner of the box, though to do this +each had to bend in a different manner. Six wind-vanes could not +have more truly shown the direction of the wind, than did these +branched tendrils the course of the stream of light which entered the +box. I left these tendrils undisturbed for above 24 hrs., and then +turned the pot half round; but they had now lost their power of +movement, and could not any longer avoid the light. + +When a tendril has not succeeded in clasping a support, either +through its own revolving movement or that of the shoot, or by +turning towards any object which intercepts the light, it bends +vertically downwards and then towards its own stem, which it seizes +together with the supporting stick, if there be one. A little aid is +thus given in keeping the stem secure. If the tendril seizes +nothing, it does not contract spirally, but soon withers away and +drops off. If it seizes an object, all the branches contract +spirally. + +I have stated that after a tendril has come into contact with a +stick, it bends round it in about half an hour; but I repeatedly +observed, as in the case of B. speciosa and its allies, that it often +again loosed the stick; sometimes seizing and loosing the same stick +three or four times. Knowing that the tendrils avoided the light, I +gave them a glass tube blackened within, and a well-blackened zinc +plate: the branches curled round the tube and abruptly bent +themselves round the edges of the zinc plate; but they soon recoiled +from these objects with what I can only call disgust, and +straightened themselves. I then placed a post with extremely rugged +bark close to a pair of tendrils; twice they touched it for an hour +or two, and twice they withdrew; at last one of the hooked +extremities curled round and firmly seized an excessively minute +projecting point of bark, and then the other branches spread +themselves out, following with accuracy every inequality of the +surface. I afterwards placed near the plant a post without bark but +much fissured, and the points of the tendrils crawled into all the +crevices in a beautiful manner. To my surprise, I observed that the +tips of the immature tendrils, with the branches not yet fully +separated, likewise crawled just like roots into the minutest +crevices. In two or three days after the tips had thus crawled into +the crevices, or after their hooked ends had seized minute points, +the final process, now to be described, commenced. + +This process I discovered by having accidentally left a piece of wool +near a tendril; and this led me to bind a quantity of flax, moss, and +wool loosely round sticks, and to place them near tendrils. The wool +must not be dyed, for these tendrils are excessively sensitive to +some poisons. The hooked points soon caught hold of the fibres, even +loosely floating fibres, and now there was no recoiling; on the +contrary, the excitement caused the hooks to penetrate the fibrous +mass and to curl inwards, so that each hook caught firmly one or two +fibres, or a small bundle of them. The tips and the inner surfaces +of the hooks now began to swell, and in two or three days were +visibly enlarged. After a few more days the hooks were converted +into whitish, irregular balls, rather above the 0.05th of an inch +(1.27 mm.) in diameter, formed of coarse cellular tissue, which +sometimes wholly enveloped and concealed the hooks themselves. The +surfaces of these balls secrete some viscid resinous matter, to which +the fibres of the flax, &c., adhere. When a fibre has become +fastened to the surface, the cellular tissue does not grow directly +beneath it, but continues to grow closely on each side; so that when +several adjoining fibres, though excessively thin, were caught, so +many crests of cellular matter, each not as thick as a human hair, +grew up between them, and these, arching over on both sides, adhered +firmly together. As the whole surface of the ball continues to grow, +fresh fibres adhere and are afterwards enveloped; so that I have seen +a little ball with between fifty and sixty fibres of flax crossing it +at various angles and all embedded more or less deeply. Every +gradation in the process could be followed--some fibres merely +sticking to the surface, others lying in more or less deep furrows, +or deeply embedded, or passing through the very centre of the +cellular ball. The embedded fibres are so closely clasped that they +cannot be withdrawn. The outgrowing tissue has so strong a tendency +to unite, that two balls produced by distinct tendrils sometimes +unite and grow into a single one. + +On one occasion, when a tendril had curled round a stick, half an +inch in diameter, an adhesive disc was formed; but this does not +generally occur in the case of smooth sticks or posts. If, however, +the tip catches a minute projecting point, the other branches form +discs, especially if they find crevices to crawl into. The tendrils +failed to attach themselves to a brick wall. + +I infer from the adherence of the fibres to the discs or balls, that +these secrete some resinous adhesive matter; and more especially from +such fibres becoming loose if immersed in sulphuric ether. This +fluid likewise removes small, brown, glistening points which can +generally be seen on the surfaces of the older discs. If the hooked +extremities of the tendrils do not touch anything, discs, as far as I +have seen, are never formed; {26} but temporary contact during a +moderate time suffices to cause their development. I have seen eight +discs formed on the same tendril. After their development the +tendrils contract spirally, and become woody and very strong. A +tendril in this state supported nearly seven ounces, and would +apparently have supported a considerably greater weight, had not the +fibres of flax to which the discs were attached yielded. + +From the facts now given, we may infer that though the tendrils of +this Bignonia can occasionally adhere to smooth cylindrical sticks +and often to rugged bark, yet that they are specially adapted to +climb trees clothed with lichens, mosses, or other such productions; +and I hear from Professor Asa Gray that the Polypodium incanum +abounds on the forest-trees in the districts of North America where +this species of Bignonia grows. Finally, I may remark how singular a +fact it is that a leaf should be metamorphosed into a branched organ +which turns from the light, and which can by its extremities either +crawl like roots into crevices, or seize hold of minute projecting +points, these extremities afterwards forming cellular outgrowths +which secrete an adhesive cement, and then envelop by their continued +growth the finest fibres. + +Eccremocarpus scaber (Bignoniaceae).--Plants, though growing pretty +well in my green-house, showed no spontaneous movements in their +shoots or tendrils; but when removed to the hot-house, the young +internodes revolved at rates varying from 3 hrs. 15 m. to 1 hr. 13 m. +One large circle was swept at this latter unusually quick rate; but +generally the circles or ellipses were small, and sometimes the +course pursued was quite irregular. An internode, after making +several revolutions, sometimes stood still for 12 hrs. or 18 hrs., +and then recommenced revolving. Such strongly marked interruptions +in the movements of the internodes I have observed in hardly any +other plant. + +The leaves bear four leaflets, themselves subdivided, and terminate +in much-branched tendrils. The main petiole of the leaf, whilst +young, moves spontaneously, and follows nearly the same irregular +course and at about the same rate as the internodes. The movement to +and from the stem is the most conspicuous, and I have seen the chord +of a curved petiole which formed an angle of 59 degrees with the +stem, in an hour afterwards making an angle of 106 degrees. The two +opposite petioles do not move together, and one is sometimes so much +raised as to stand close to the stem, whilst the other is not far +from horizontal. The basal part of the petiole moves less than the +distal part. The tendrils, besides being carried by the moving +petioles and internodes, themselves move spontaneously; and the +opposite tendrils occasionally move in opposite directions. By these +combined movements of the young internodes, petioles, and tendrils, a +considerable space is swept in search of a support. + +In young plants the tendrils are about three inches in length: they +bear two lateral and two terminal branches; and each branch +bifurcates twice, with the tips terminating in blunt double hooks, +having both points directed to the same side. All the branches are +sensitive on all sides; and after being lightly rubbed, or after +coming into contact with a stick, bend in about 10 m. One which had +become curved in 10 m. after a light rub, continued bending for +between 3 hrs. and 4 hrs., and became straight again in 8 hrs. or 9 +hrs. Tendrils, which have caught nothing, ultimately contract into +an irregular spire, as they likewise do, only much more quickly, +after clasping a support. In both cases the main petiole bearing the +leaflets, which is at first straight and inclined a little upwards, +moves downwards, with the middle part bent abruptly into a right +angle; but this is seen in E. miniatus more plainly than in E. +scaber. The tendrils in this genus act in some respects like those +of Bignonia capreolata; but the whole does not move from the light, +nor do the hooked tips become enlarged into cellular discs. After +the tendrils have come into contact with a moderately thick +cylindrical stick or with rugged bark, the several branches may be +seen slowly to lift themselves up, change their positions, and again +come into contact with the supporting surface. The object of these +movements is to bring the double-hooks at the extremities of the +branches, which naturally face in all directions, into contact with +the wood. I have watched a tendril, half of which had bent itself at +right angles round the sharp corner of a square post, neatly bring +every single hook into contact with both rectangular surfaces. The +appearance suggested the belief, that though the whole tendril is not +sensitive to light, yet that the tips are so, and that they turn and +twist themselves towards any dark surface. Ultimately the branches +arrange themselves very neatly to all the irregularities of the most +rugged bark, so that they resemble in their irregular course a river +with its branches, as engraved on a map. But when a tendril has +wound round a rather thick stick, the subsequent spiral contraction +generally draws it away and spoils the neat arrangement. So it is, +but not in quite so marked a manner, when a tendril has spread itself +over a large, nearly flat surface of rugged bark. We may therefore +conclude that these tendrils are not perfectly adapted to seize +moderately thick sticks or rugged bark. If a thin stick or twig is +placed near a tendril, the terminal branches wind quite round it, and +then seize their own lower branches or the main stem. The stick is +thus firmly, but not neatly, grasped. What the tendrils are really +adapted for, appears to be such objects as the thin culms of certain +grasses, or the long flexible bristles of a brush, or thin rigid +leaves such as those of the Asparagus, all of which they seize in an +admirable manner. This is due to the extremities of the branches +close to the little hooks being extremely sensitive to a touch from +the thinnest object, which they consequently curl round and clasp. +When a small brush, for instance, was placed near a tendril, the tips +of each sub-branch seized one, two, or three of the bristles; and +then the spiral contraction of the several branches brought all these +little parcels close together, so that thirty or forty bristles were +drawn into a single bundle, which afforded an excellent support. + +POLEMONIACEAE.--Cobaea scandens.--This is an excellently constructed +climber. The tendrils on a fine plant were eleven inches long, with +the petiole bearing two pairs of leaflets, only two and a half inches +in length. They revolve more rapidly and vigorously than those of +any other tendril-bearer observed by me, with the exception of one +kind of Passiflora. Three large, nearly circular sweeps, directed +against the sun were completed, each in 1 hr. 15 m.; and two other +circles in 1 hr. 20 m. and 1 hr. 23 m. Sometimes a tendril travels +in a much inclined position, and sometimes nearly upright. The lower +part moves but little and the petiole not at all; nor do the +internodes revolve; so that here we have the tendril alone moving. +On the other hand, with most of the species of Bignonia and the +Eccremocarpus, the internodes, tendrils, and petioles all revolved. +The long, straight, tapering main stem of the tendril of the Cobaea +bears alternate branches; and each branch is several times divided, +with the finer branches as thin as very thin bristles and extremely +flexible, so that they are blown about by a breath of air; yet they +are strong and highly elastic. The extremity of each branch is a +little flattened, and terminates in a minute double (though sometimes +single) hook, formed of a hard, translucent, woody substance, and as +sharp as the finest needle. On a tendril which was eleven inches +long I counted ninety-four of these beautifully constructed little +hooks. They readily catch soft wood, or gloves, or the skin of the +naked hand. With the exception of these hardened hooks, and of the +basal part of the central stem, every part of every branchlet is +highly sensitive on all sides to a slight touch, and bends in a few +minutes towards the touched side. By lightly rubbing several sub- +branches on opposite sides, the whole tendril rapidly assumed an +extraordinarily crooked shape. These movements from contact do not +interfere with the ordinary revolving movement. The branches, after +becoming greatly curved from being touched, straighten themselves at +a quicker rate than in almost any other tendril seen by me, namely, +in between half an hour and an hour. After the tendril has caught +any object, spiral contraction likewise begins after an unusually +short interval of time, namely, in about twelve hours. + +Before the tendril is mature, the terminal branchlets cohere, and the +hooks are curled closely inwards. At this period no part is +sensitive to a touch; but as soon as the branches diverge and the +hooks stand out, full sensitiveness is acquired. It is a singular +circumstance that immature tendrils revolve at their full velocity +before they become sensitive, but in a useless manner, as in this +state they can catch nothing. This want of perfect co-adaptation, +though only for a short time, between the structure and the functions +of a climbing-plant is a rare event. A tendril, as soon as it is +ready to act, stands, together with the supporting petiole, +vertically upwards. The leaflets borne by the petiole are at this +time quite small, and the extremity of the growing stem is bent to +one side so as to be out of the way of the revolving tendril, which +sweeps large circles directly over head. The tendrils thus revolve +in a position well adapted for catching objects standing above; and +by this means the ascent of the plant is favoured. If no object is +caught, the leaf with its tendril bends downwards and ultimately +assumes a horizontal position. An open space is thus left for the +next succeeding and younger tendril to stand vertically upwards and +to revolve freely. As soon as an old tendril bends downwards, it +loses all power of movement, and contracts spirally into an entangled +mass. Although the tendrils revolve with unusual rapidity, the +movement lasts for only a short time. In a plant placed in the hot- +house and growing vigorously, a tendril revolved for not longer than +36 hours, counting from the period when it first became sensitive; +but during this period it probably made at least 27 revolutions. + +When a revolving tendril strikes against a stick, the branches +quickly bend round and clasp it. The little hooks here play an +important part, as they prevent the branches from being dragged away +by the rapid revolving movement, before they have had time to clasp +the stick securely. This is especially the case when only the +extremity of a branch has caught hold of a support. As soon as a +tendril has bent a smooth stick or a thick rugged post, or has come +into contact with planed wood (for it can adhere temporarily even to +so smooth a surface as this), the same peculiar movements may be +observed as those described under Bignonia capreolata and +Eccremocarpus. The branches repeatedly lift themselves up and down; +those which have their hooks already directed downwards remaining in +this position and securing the tendril, whilst the others twist about +until they succeed in arranging themselves in conformity with every +irregularity of the surface, and in bringing their hooks into contact +with the wood. The use of the hooks was well shown by giving the +tendrils tubes and slips of glass to catch; for these, though +temporarily seized, were invariably lost, either during the re- +arrangement of the branches or ultimately when spiral contraction +ensued. + +The perfect manner in which the branches arranged themselves, +creeping like rootlets over every inequality of the surface and into +any deep crevice, is a pretty sight; for it is perhaps more +effectually performed by this than by any other species. The action +is certainly more conspicuous, as the upper surfaces of the main +stem, as well as of every branch to the extreme hooks, are angular +and green, whilst the lower surfaces are rounded and purple. I was +led to infer, as in former cases, that a less amount of light guided +these movements of the branches of the tendrils. I made many trials +with black and white cards and glass tubes to prove it, but failed +from various causes; yet these trials countenanced the belief. As a +tendril consists of a leaf split into numerous segments, there is +nothing surprising in all the segments turning their upper surfaces +towards the light, as soon as the tendril is caught and the revolving +movement is arrested. But this will not account for the whole +movement, for the segments actually bend or curve to the dark side +besides turning round on their axes so that their upper surfaces may +face the light. + +When the Cobaea grows in the open air, the wind must aid the +extremely flexible tendrils in seizing a support, for I found that a +mere breath sufficed to cause the extreme branches to catch hold by +their hooks of twigs, which they could not have reached by the +revolving movement. It might have been thought that a tendril, thus +hooked by the extremity of a single branch, could not have fairly +grasped its support. But several times I watched cases like the +following: tendril caught a thin stick by the hooks of one of its +two extreme branches; though thus held by the tip, it still tried to +revolve, bowing itself to all sides, and by this movement the other +extreme branch soon caught the stick. The first branch then loosed +itself, and, arranging its hooks, again caught hold. After a time, +from the continued movement of the tendril, the hooks of a third +branch caught hold. No other branches, as the tendril then stood, +could possibly have touched the stick. But before long the upper +part of the main stem began to contract into an open spire. It thus +dragged the shoot which bore the tendril towards the stick; and as +the tendril continually tried to revolve, a fourth branch was brought +into contact. And lastly, from the spiral contraction travelling +down both the main stem and the branches, all of them, one after +another, were ultimately brought into contact with the stick. They +then wound themselves round it and round one another, until the whole +tendril was tied together in an inextricable knot. The tendrils, +though at first quite flexible, after having clasped a support for a +time, become more rigid and stronger than they were at first. Thus +the plant is secured to its support in a perfect manner. + +LEGUMINOSAE.--Pisum sativum.--The common pea was the subject of a +valuable memoir by Dutrochet, {27} who discovered that the internodes +and tendrils revolve in ellipses. The ellipses are generally very +narrow, but sometimes approach to circles. I several times observed +that the longer axis slowly changed its direction, which is of +importance, as the tendril thus sweeps a wider space. Owing to this +change of direction, and likewise to the movement of the stem towards +the light, the successive irregular ellipses generally form an +irregular spire. I have thought it worth while to annex a tracing of +the course pursued by the upper internode (the movement of the +tendril being neglected) of a young plant from 8.40 A.M. to 9.15 P.M. +The course was traced on a hemispherical glass placed over the plant, +and the dots with figures give the hours of observation; each dot +being joined by a straight line. No doubt all the lines would have +been curvilinear if the course had been observed at much shorter +intervals. The extremity of the petiole, from which the young +tendril arose, was two inches from the glass, so that if a pencil two +inches in length could have been affixed to the petiole, it would +have traced the annexed figure on the under side of the glass; but it +must be remembered that the figure is reduced by one-half. +Neglecting the first great sweep towards the light from the figure 1 +to 2, the end of the petiole swept a space 4 inches across in one +direction, and 3 inches in another. As a full-grown tendril is +considerably above two inches in length, and as the tendril itself +bends and revolves in harmony with the internode, a considerably +wider space is swept than is here represented on a reduced scale. +Dutrochet observed the completion of an ellipse in 1 hr. 20 m.; and I +saw one completed in 1 hr. 30 m. The direction followed is variable, +either with or against the sun. + +Dutrochet asserts that the petioles of the leaves spontaneously +revolve, as well as the young internodes and tendrils; but he does +not say that he secured the internodes; when this was done, I could +never detect any movement in the petiole, except to and from the +light. + +The tendrils, on the other hand, when the internodes and petioles are +secured, describe irregular spires or regular ellipses, exactly like +those made by the internodes. A young tendril, only 1.125 of an inch +in length, revolved. Dutrochet has shown that when a plant is placed +in a room, so that the light enters laterally, the internodes travel +much quicker to the light than from it: on the other hand, he +asserts that the tendril itself moves from the light towards the dark +side of the room. With due deference to this great observer, I think +he was mistaken, owing to his not having secured the internodes. I +took a young plant with highly sensitive tendrils, and tied the +petiole so that the tendril alone could move; it completed a perfect +ellipse in 1 hr. 30 m.; I then turned the plant partly round, but +this made no change in the direction of the succeeding ellipse. The +next day I watched a plant similarly secured until the tendril (which +was highly sensitive) made an ellipse in a line exactly to and from +the light; the movement was so great that the tendril at the two ends +of its elliptical course bent itself a little beneath the horizon, +thus travelling more than 180 degrees; but the curvature was fully as +great towards the light as towards the dark side of the room. I +believe Dutrochet was misled by not having secured the internodes, +and by having observed a plant of which the internodes and tendrils +no longer curved in harmony together, owing to inequality of age. + +Dutrochet made no observations on the sensitiveness of the tendrils. +These, whilst young and about an inch in length with the leaflets on +the petiole only partially expanded, are highly sensitive; a single +light touch with a twig on the inferior or concave surface near the +tip caused them to bend quickly, as did occasionally a loop of thread +weighing one-seventh of a grain (9.25 mg.). The upper or convex +surface is barely or not at all sensitive. Tendrils, after bending +from a touch, straighten themselves in about two hours, and are then +ready to act again. As soon as they begin to grow old, the +extremities of their two or three pairs of branches become hooked, +and they then appear to form an excellent grappling instrument; but +this is not the case. For at this period they have generally quite +lost their sensitiveness; and when hooked on to twigs, some were not +at all affected, and others required from 18 hrs. to 24 hrs. before +clasping such twigs; nevertheless, they were able to utilise the last +vestige of irritability owing to their extremities being hooked. +Ultimately the lateral branches contract spirally, but not the middle +or main stem. + +Lathyrus aphaca.--This plant is destitute of leaves, except during a +very early age, these being replaced by tendrils, and the leaves +themselves by large stipules. It might therefore have been expected +that the tendrils would have been highly organized, but this is not +so. They are moderately long, thin, and unbranched, with their tips +slightly curved. Whilst young they are sensitive on all sides, but +chiefly on the concave side of the extremity. They have no +spontaneous revolving power, but are at first inclined upwards at an +angle of about 45 degrees, then move into a horizontal position, and +ultimately bend downwards. The young internodes, on the other hand, +revolve in ellipses, and carry with them the tendrils. Two ellipses +were completed, each in nearly 5 hrs.; their longer axes were +directed at about an angle of 45 degrees to the axis of the +previously made ellipse. + +Lathyrus grandiflorus.--The plants observed were young and not +growing vigorously, yet sufficiently so, I think, for my observations +to be trusted. If so, we have the rare case of neither internodes +nor tendrils revolving. The tendrils of vigorous plants are above 4 +inches in length, and are often twice divided into three branches; +the tips are curved and are sensitive on their concave sides; the +lower part of the central stem is hardly at all sensitive. Hence +this plant appears to climb simply by its tendrils being brought, +through the growth of the stem, or more efficiently by the wind, into +contact with surrounding objects, which they then clasp. I may add +that the tendrils, or the internodes, or both, of Vicia sativa +revolve. + +COMPOSITAE.--Mutisia clematis.--The immense family of the Compositae +is well known to include very few climbing plants. We have seen in +the Table in the first chapter that Mikania scandens is a regular +twiner, and F. Muller informs me that in S. Brazil there is another +species which is a leaf-climber. Mutisia is the only genus in the +family, as far as I can learn, which bears tendrils: it is therefore +interesting to find that these, though rather less metamorphosed from +their primordial foliar condition than are most other tendrils, yet +display all the ordinary characteristic movements, both those that +are spontaneous and those which are excited by contact. + +The long leaf bears seven or eight alternate leaflets, and terminates +in a tendril which, in a plant of considerable size, was 5 inches in +length. It consists generally of three branches; and these, although +much elongated, evidently represent the petioles and midribs of three +leaflets; for they closely resemble the same parts in an ordinary +leaf, in being rectangular on the upper surface, furrowed, and edged +with green. Moreover, the green edging of the tendrils of young +plants sometimes expands into a narrow lamina or blade. Each branch +is curved a little downwards, and is slightly hooked at the +extremity. + +A young upper internode revolved, judging from three revolutions, at +an average rate of 1 hr. 38 m.; it swept ellipses with the longer +axes directed at right angles to one another; but the plant, +apparently, cannot twine. The petioles and the tendrils are both in +constant movement. But their movement is slower and much less +regularly elliptical than that of the internodes. They appear to be +much affected by the light, for the whole leaf usually sinks down +during the night and rises during the day, moving, also, during the +day in a crooked course to the west. The tip of the tendril is +highly sensitive on the lower surface; and one which was just touched +with a twig became perceptibly curved in 3 m., and another in 5 m.; +the upper surface is not at all sensitive; the sides are moderately +sensitive, so that two branches which were rubbed on their inner +sides converged and crossed each other. The petiole of the leaf and +the lower parts of the tendril, halfway between the upper leaflet and +the lowest branch, are not sensitive. A tendril after curling from a +touch became straight again in about 6 hrs., and was ready to re-act; +but one that had been so roughly rubbed as to have coiled into a +helix did not become perfectly straight until after 13 hrs. The +tendrils retain their sensibility to an unusually late age; for one +borne by a leaf with five or six fully developed leaves above, was +still active. If a tendril catches nothing, after a considerable +interval of time the tips of the branches curl a little inwards; but +if it clasps some object, the whole contracts spirally. + +SMILACEAE.--Smilax aspera, var. maculata.--Aug. St.-Hilaire {28} +considers that the tendrils, which rise in pairs from the petiole, +are modified lateral leaflets; but Mohl (p. 41) ranks them as +modified stipules. These tendrils are from 1.5 to 1.75 inches in +length, are thin, and have slightly curved, pointed extremities. +They diverge a little from each other, and stand at first nearly +upright. When lightly rubbed on either side, they slowly bend to +that side, and subsequently become straight again. The back or +convex side when placed in contact with a stick became just +perceptibly curved in 1 hr. 20 m., but did not completely surround it +until 48 hrs. had elapsed; the concave side of another became +considerably curved in 2 hrs. and clasped a stick in 5 hrs. As the +pairs of tendrils grow old, one tendril diverges more and more from +the other, and both slowly bend backwards and downwards, so that +after a time they project on the opposite side of the stem to that +from which they arise. They then still retain their sensitiveness, +and can clasp a support placed BEHIND the stem. Owing to this power, +the plant is able to ascend a thin upright stick. Ultimately the two +tendrils belonging to the same petiole, if they do not come into +contact with any object, loosely cross each other behind the stem, as +at B, in fig. 7. This movement of the tendrils towards and round the +stem is, to a certain extent, guided by their avoidance of the light; +for when a plant stood so that one of the two tendrils was compelled +in thus slowly moving to travel towards the light, and the other from +the light, the latter always moved, as I repeatedly observed, more +quickly than its fellow. The tendrils do not contract spirally in +any case. Their chance of finding a support depends on the growth of +the plant, on the wind, and on their own slow backward and downward +movement, which, as we have just seen, is guided, to a certain +extent, by the avoidance of the light; for neither the internodes nor +the tendrils have any proper revolving movement. From this latter +circumstance, from the slow movements of the tendrils after contact +(though their sensitiveness is retained for an unusual length of +time), from their simple structure and shortness, this plant is a +less perfect climber than any other tendril-bearing species observed +by me. The plant whilst young and only a few inches in height, does +not produce any tendrils; and considering that it grows to only about +8 feet in height, that the stem is zigzag and is furnished, as well +as the petioles, with spines, it is surprising that it should be +provided with tendrils, comparatively inefficient though these are. +The plant might have been left, one would have thought, to climb by +the aid of its spines alone, like our brambles. As, however, it +belongs to a genus, some of the species of which are furnished with +much longer tendrils, we may suspect that it possesses these organs +solely from being descended from progenitors more highly organized in +this respect. + +FUMARIACEAE.--Corydalis claviculata.--According to Mohl (p. 43), the +extremities of the branched stem, as well as the leaves, are +converted into tendrils. In the specimens examined by me all the +tendrils were certainly foliar, and it is hardly credible that the +same plant should produce tendrils of a widely different homological +nature. Nevertheless, from this statement by Mohl, I have ranked +this species amongst the tendril-bearers; if classed exclusively by +its foliar tendrils, it would be doubtful whether it ought not to +have been placed amongst the leaf-climbers, with its allies, Fumaria +and Adlumia. A large majority of its so-called tendrils still bear +leaflets, though excessively reduced in size; but some few of them +may properly be designated as tendrils, for they are completely +destitute of laminae or blades. Consequently, we here behold a plant +in an actual state of transition from a leaf-climber to a tendril- +bearer. Whilst the plant is rather young, only the outer leaves, but +when full-grown all the leaves, have their extremities converted into +more or less perfect tendrils. I have examined specimens from one +locality alone, viz. Hampshire; and it is not improbable that plants +growing under different conditions might have their leaves a little +more or less changed into true tendrils. + +Whilst the plant is quite young, the first-formed leaves are not +modified in any way, but those next formed have their terminal +leaflets reduced in size, and soon all the leaves assume the +structure represented in the following drawing. This leaf bore nine +leaflets; the lower ones being much subdivided. The terminal portion +of the petiole, about 1.5 inch in length (above the leaflet f), is +thinner and more elongated than the lower part, and may be considered +as the tendril. The leaflets borne by this part are greatly reduced +in size, being, on an average, about the tenth of an inch in length +and very narrow; one small leaflet measured one-twelfth of an inch in +length and one-seventy-fifth in breadth (2.116 mm. and 0.339 mm.), so +that it was almost microscopically minute. All the reduced leaflets +have branching nerves, and terminate in little spines, like those of +the fully developed leaflets. Every gradation could be traced, until +we come to branchlets (as a and d in the figure) which show no +vestige of a lamina or blade. Occasionally all the terminal +branchlets of the petiole are in this condition, and we then have a +true tendril. + +The several terminal branches of the petiole bearing the much reduced +leaflets (a, b, c, d) are highly sensitive, for a loop of thread +weighing only the one-sixteenth of a grain (4.05 mg.) caused them to +become greatly curved in under 4 hrs. When the loop was removed, the +petioles straightened themselves in about the same time. The petiole +(e) was rather less sensitive; and in another specimen, in which the +corresponding petiole bore rather larger leaflets, a loop of thread +weighing one-eighth of a grain did not cause curvature until 18 hrs. +had elapsed. Loops of thread weighing one-fourth of a grain, left +suspended on the lower petioles (f to l) during several days, +produced no effect. Yet the three petioles f, g, and h were not +quite insensible, for when left in contact with a stick for a day or +two they slowly curled round it. Thus the sensibility of the petiole +gradually diminishes from the tendril-like extremity to the base. +The internodes of the stem are not at all sensitive, which makes +Mohl's statement that they are sometimes converted into tendrils the +more surprising, not to say improbable. + +The whole leaf, whilst young and sensitive, stands almost vertically +upwards, as we have seen to be the case with many tendrils. It is in +continual movement, and one that I observed swept at an average rate +of about 2 hrs. for each revolution, large, though irregular, +ellipses, which were sometimes narrow, sometimes broad, with their +longer axes directed to different points of the compass. The young +internodes, likewise revolved irregularly in ellipses or spires; so +that by these combined movements a considerable space was swept for a +support. If the terminal and attenuated portion of a petiole fails +to seize any object, it ultimately bends downwards and inwards, and +soon loses all irritability and power of movement. This bending down +differs much in nature from that which occurs with the extremities of +the young leaves in many species of Clematis; for these, when thus +bent downwards or hooked, first acquire their full degree of +sensitiveness. + +Dicentra thalictrifolia.--In this allied plant the metamorphosis of +the terminal leaflets is complete, and they are converted into +perfect tendrils. Whilst the plant is young, the tendrils appear +like modified branches, and a distinguished botanist thought that +they were of this nature; but in a full-grown plant there can be no +doubt, as I am assured by Dr. Hooker, that they are modified leaves. +When of full size, they are above 5 inches in length; they bifurcate +twice, thrice, or even four times; their extremities are hooked and +blunt. All the branches of the tendrils are sensitive on all sides, +but the basal portion of the main stem is only slightly so. The +terminal branches when lightly rubbed with a twig became curved in +the course of from 30 m. to 42 m., and straightened themselves in +between 10 hrs. and 20 hrs. A loop of thread weighing one-eighth of +a grain plainly caused the thinner branches to bend, as did +occasionally a loop weighing one-sixteenth of a grain; but this +latter weight, though left suspended, was not sufficient to cause a +permanent flexure. The whole leaf with its tendril, as well as the +young upper internodes, revolves vigorously and quickly, though +irregularly, and thus sweeps a wide space. The figure traced on a +bell-glass was either an irregular spire or a zigzag line. The +nearest approach to an ellipse was an elongated figure of 8, with one +end a little open, and this was completed in 1 hr. 53 m. During a +period of 6 hrs. 17 m. another shoot made a complex figure, +apparently representing three and a half ellipses. When the lower +part of the petiole bearing the leaflets was securely fastened, the +tendril itself described similar but much smaller figures. + +This species climbs well. The tendrils after clasping a stick become +thicker and more rigid; but the blunt hooks do not turn and adapt +themselves to the supporting surface, as is done in so perfect a +manner by some Bignoniaceae and Cobaea. The tendrils of young +plants, two or three feet in height, are only half the length of +those borne by the same plant when grown taller, and they do not +contract spirally after clasping a support, but only become slightly +flexuous. Full-sized tendrils, on the other hand, contract spirally, +with the exception of the thick basal portion. Tendrils which have +caught nothing simply bend downwards and inwards, like the +extremities of the leaves of the Corydalis claviculata. But in all +cases the petiole after a time is angularly and abruptly bent +downwards like that of Eccremocarpus. + + + +CHAPTER IV.--TENDRIL-BEARERS--(continued). + + + +CUCURBITACEAE.--Homologous nature of the tendrils--Echinocystis +lobata, remarkable movements of the tendrils to avoid seizing the +terminal shoot--Tendrils not excited by contact with another tendril +or by drops of water--Undulatory movement of the extremity of the +tendril--Hanburya, adherent discs--VITACAE--Gradation between the +flower-peduncles and tendrils of the vine--Tendrils of the Virginian +Creeper turn from the light, and, after contact, develop adhesive +discs--SAPINDACEAE--PASSIFLORACEAE--Passiflora gracilis--Rapid +revolving movement and sensitiveness of the tendrils--Not sensitive +to the contact of other tendrils or of drops of water--Spiral +contraction of tendrils--Summary on the nature and action of +tendrils. + +CUCURBITACEAE.--The tendrils in this family have been ranked by +competent judges as modified leaves, stipules, or branches; or as +partly a leaf and partly a branch. De Candolle believes that the +tendrils differ in their homological nature in two of the tribes. +{29} From facts recently adduced, Mr. Berkeley thinks that Payer's +view is the most probable, namely, that the tendril is "a separate +portion of the leaf itself;" but much may be said in favour of the +belief that it is a modified flower-peduncle. {30} + +Echinocystis lobata.--Numerous observations were made on this plant +(raised from seed sent me by Prof. Asa Gray), for the spontaneous +revolving movements of the internodes and tendrils were first +observed by me in this case, and greatly perplexed me. My +observations may now be much condensed. I observed thirty-five +revolutions of the internodes and tendrils; the slowest rate was 2 +hrs. and the average rate, with no great fluctuations, 1 hr. 40 m. +Sometimes I tied the internodes, so that the tendrils alone moved; at +other times I cut off the tendrils whilst very young, so that the +internodes revolved by themselves; but the rate was not thus +affected. The course generally pursued was with the sun, but often +in an opposite direction. Sometimes the movement during a short time +would either stop or be reversed; and this apparently was due to +interference from the light, as, for instance, when I placed a plant +close to a window. In one instance, an old tendril, which had nearly +ceased revolving, moved in one direction, whilst a young tendril +above moved in an opposite course. The two uppermost internodes +alone revolve; and as soon as the lower one grows old, only its upper +part continues to move. The ellipses or circles swept by the summits +of the internodes are about three inches in diameter; whilst those +swept by the tips of the tendrils, are from 15 to 16 inches in +diameter. During the revolving movement, the internodes become +successively curved to all points of the compass; in one part of +their course they are often inclined, together with the tendrils, at +about 45 degrees to the horizon, and in another part stand vertically +up. There was something in the appearance of the revolving +internodes which continually gave the false impression that their +movement was due to the weight of the long and spontaneously +revolving tendril; but, on cutting off the latter with sharp +scissors, the top of the shoot rose only a little, and went on +revolving. This false appearance is apparently due to the internodes +and tendrils all curving and moving harmoniously together. + +A revolving tendril, though inclined during the greater part of its +course at an angle of about 45 degrees (in one case of only 37 +degrees) above the horizon, stiffened and straightened itself from +tip to base in a certain part of its course, thus becoming nearly or +quite vertical. I witnessed this repeatedly; and it occurred both +when the supporting internodes were free and when they were tied up; +but was perhaps most conspicuous in the latter case, or when the +whole shoot happened to be much inclined. The tendril forms a very +acute angle with the projecting extremity of the stem or shoot; and +the stiffening always occurred as the tendril approached, and had to +pass over the shoot in its circular course. If it had not possessed +and exercised this curious power, it would infallibly have struck +against the extremity of the shoot and been arrested. As soon as the +tendril with its three branches begins to stiffen itself in this +manner and to rise from an inclined into a vertical position, the +revolving motion becomes more rapid; and as soon as the tendril has +succeeded in passing over the extremity of the shoot or point of +difficulty, its motion, coinciding with that from its weight, often +causes it to fall into its previously inclined position so quickly, +that the apex could be seen travelling like the minute hand of a +gigantic clock. + +The tendrils are thin, from 7 to 9 inches in length, with a pair of +short lateral branches rising not far from the base. The tip is +slightly and permanently curved, so as to act to a limited extent as +a hook. The concave side of the tip is highly sensitive to a touch; +but not so the convex side, as was likewise observed to be the case +with other species of the family by Mohl (p. 65). I repeatedly +proved this difference by lightly rubbing four or five times the +convex side of one tendril, and only once or twice the concave side +of another tendril, and the latter alone curled inwards. In a few +hours afterwards, when the tendrils which had been rubbed on the +concave side had straightened themselves, I reversed the process of +rubbing, and always with the same result. After touching the concave +side, the tip becomes sensibly curved in one or two minutes; and +subsequently, if the touch has been at all rough, it coils itself +into a helix. But the helix will, after a time, straighten itself, +and be again ready to act. A loop of thin thread only one-sixteenth +of a grain in weight caused a temporary flexure. The lower part was +repeatedly rubbed rather roughly, but no curvature ensued; yet this +part is sensitive to prolonged pressure, for when it came into +contact with a stick, it would slowly wind round it. + +One of my plants bore two shoots near together, and the tendrils were +repeatedly drawn across one another, but it is a singular fact that +they did not once catch each other. It would appear as if they had +become habituated to contact of this kind, for the pressure thus +caused must have been much greater than that caused by a loop of soft +thread weighing only the one-sixteenth of a grain. I have, however, +seen several tendrils of Bryonia dioica interlocked, but they +subsequently released one another. The tendrils of the Echinocystis +are also habituated to drops of water or to rain; for artificial rain +made by violently flirting a wet brush over them produced not the +least effect. + +The revolving movement of a tendril is not stopped by the curving of +its extremity after it has been touched. When one of the lateral +branches has firmly clasped an object, the middle branch continues to +revolve. When a stem is bent down and secured, so that the tendril +depends but is left free to move, its previous revolving movement is +nearly or quite stopped; but it soon begins to bend upwards, and as +soon as it has become horizontal the revolving movement recommences. +I tried this four times; the tendril generally rose to a horizontal +position in an hour or an hour and a half; but in one case, in which +a tendril depended at an angle of 45 degrees beneath the horizon, the +uprising took two hours; in half an hour afterwards it rose to 23 +degrees above the horizon and then recommenced revolving. This +upward movement is independent of the action of light, for it +occurred twice in the dark, and on another occasion the light came in +on one side alone. The movement no doubt is guided by opposition to +the force of gravity, as in the case of the ascent of the plumules of +germinating seeds. + +A tendril does not long retain its revolving power; and as soon as +this is lost, it bends downwards and contracts spirally. After the +revolving movement has ceased, the tip still retains for a short time +its sensitiveness to contact, but this can be of little or no use to +the plant. + +Though the tendril is highly flexible, and though the extremity +travels, under favourable circumstances, at about the rate of an inch +in two minutes and a quarter, yet its sensitiveness to contact is so +great that it hardly ever fails to seize a thin stick placed in its +path. The following case surprised me much: I placed a thin, +smooth, cylindrical stick (and I repeated the experiment seven times) +so far from a tendril, that its extremity could only curl half or +three-quarters round the stick; but I always found that the tip +managed in the course of a few hours to curl twice or even thrice +round the stick. I at first thought that this was due to rapid +growth on the outside; but by coloured points and measurements I +proved that there had been no sensible increase of length within the +time. When a stick, flat on one side, was similarly placed, the tip +of the tendril could not curl beyond the flat surface, but coiled +itself into a helix, which, turning to one side, lay flat on the +little flat surface of wood. In one instance a portion of tendril +three-quarters of an inch in length was thus dragged on to the flat +surface by the coiling in of the helix. But the tendril thus +acquires a very insecure hold, and generally after a time slips off. +In one case alone the helix subsequently uncoiled itself, and the tip +then passed round and clasped the stick. The formation of the helix +on the flat side of the stick apparently shows us that the continued +striving of the tip to curl itself closely inwards gives the force +which drags the tendril round a smooth cylindrical stick. In this +latter case, whilst the tendril was slowly and quite insensibly +crawling onwards, I observed several times through a lens that the +whole surface was not in close contact with the stick; and I can +understand the onward progress only by supposing that the movement is +slightly undulatory or vermicular, and that the tip alternately +straightens itself a little and then again curls inwards. It thus +drags itself onwards by an insensibly slow, alternate movement, which +may be compared to that of a strong man suspended by the ends of his +fingers to a horizontal pole, who works his fingers onwards until he +can grasp the pole with the palm of his hand. However this may be, +the fact is certain that a tendril which has caught a round stick +with its extreme point, can work itself onwards until it has passed +twice or even thrice round the stick, and has permanently grasped it. + +Hanburya Mexicana.--The young internodes and tendrils of this +anomalous member of the family, revolve in the same manner and at +about the same rate as those of the Echinocystis. The stem does not +twine, but can ascend an upright stick by the aid of its tendrils. +The concave tip of the tendril is very sensitive; after it had become +rapidly coiled into a ring owing to a single touch, it straightened +itself in 50 m. The tendril, when in full action, stands vertically +up, with the projecting extremity of the young stem thrown a little +on one side, so as to be out of the way; but the tendril bears on the +inner side, near its base, a short rigid branch, which projects out +at right angles like a spur, with the terminal half bowed a little +downwards. Hence, as the main vertical branch revolves, the spur, +from its position and rigidity, cannot pass over the extremity of the +shoot, in the same curious manner as do the three branches of the +tendril of the Echinocystis, namely, by stiffening themselves at the +proper point. The spur is therefore pressed laterally against the +young stem in one part of the revolving course, and thus the sweep of +the lower part of the main branch is much restricted. A nice case of +co-adaptation here comes into play: in all the other tendrils +observed by me, the several branches become sensitive at the same +period: had this been the case with the Hanburya, the inwardly +directed, spur-like branch, from being pressed, during the revolving +movement, against the projecting end of the shoot, would infallibly +have seized it in a useless or injurious manner. But the main branch +of the tendril, after revolving for a time in a vertical position, +spontaneously bends downwards; and in doing so, raises the spur-like +branch, which itself also curves upwards; so that by these combined +movements it rises above the projecting end of the shoot, and can now +move freely without touching the shoot; and now it first becomes +sensitive. + +The tips of both branches, when they come into contact with a stick, +grasp it like any ordinary tendril. But in the course of a few days, +the lower surface swells and becomes developed into a cellular layer, +which adapts itself closely to the wood, and firmly adheres to it. +This layer is analogous to the adhesive discs formed by the +extremities of the tendrils of some species of Bignonia and of +Ampelopsis; but in the Hanburya the layer is developed along the +terminal inner surface, sometimes for a length of 1.75 inches, and +not at the extreme tip. The layer is white, whilst the tendril is +green, and near the tip it is sometimes thicker than the tendril +itself; it generally spreads a little beyond the sides of the +tendril, and is fringed with free elongated cells, which have +enlarged globular or retort-shaped heads. This cellular layer +apparently secretes some resinous cement; for its adhesion to the +wood was not lessened by an immersion of 24 hrs. in alcohol or water, +but was quite loosened by a similar immersion in ether or turpentine. +After a tendril has once firmly coiled itself round a stick, it is +difficult to imagine of what use the adhesive cellular layer can be. +Owing to the spiral contraction which soon ensues, the tendrils were +never able to remain, excepting in one instance, in contact with a +thick post or a nearly flat surface; if they had quickly become +attached by means of the adhesive layer, this would evidently have +been of service to the plant. + +The tendrils of Bryonia dioica, Cucurbita ovifera, and Cucumis sativa +are sensitive and revolve. Whether the internodes likewise revolve I +did not observe. In Anguria Warscewiczii, the internodes, though +thick and stiff, revolve: in this plant the lower surface of the +tendril, some time after clasping a stick, produces a coarsely +cellular layer or cushion, which adapts itself closely to the wood, +like that formed by the tendril of the Hanburya; but it is not in the +least adhesive. In Zanonia Indica, which belongs to a different +tribe of the family, the forked tendrils and the internodes revolve +in periods between 2 hrs. 8 m. and 3 hrs. 35 m., moving against the +sun. + +VITACEAE.--In this family and in the two following, namely, the +Sapindaceae and Passifloraceae, the tendrils are modified flower- +peduncles; and are therefore axial in their nature. In this respect +they differ from all those previously described, with the exception, +perhaps, of the Cucurbitaceae. The homological nature, however, of a +tendril seems to make no difference in its action. + +Vitis vinifera.--The tendril is thick and of great length; one from a +vine growing out of doors and not vigorously, was 16 inches long. It +consists of a peduncle (A), bearing two branches which diverge +equally from it. One of the branches (B) has a scale at its base; it +is always, as far as I have seen, longer than the other and often +bifurcates. The branches when rubbed become curved, and subsequently +straighten themselves. After a tendril has clasped any object with +its extremity, it contracts spirally; but this does not occur (Palm, +p. 56) when no object has been seized. The tendrils move +spontaneously from side to side; and on a very hot day, one made two +elliptical revolutions, at an average rate of 2 hrs. 15 m. During +these movements a coloured line, painted along the convex surface, +appeared after a time on one side, then on the concave side, then on +the opposite side, and lastly again on the convex side. The two +branches of the same tendril have independent movements. After a +tendril has spontaneously revolved for a time, it bends from the +light towards the dark: I do not state this on my own authority, but +on that of Mohl and Dutrochet. Mohl (p. 77) says that in a vine +planted against a wall the tendrils point towards it, and in a +vineyard generally more or less to the north. + +The young internodes revolve spontaneously; but the movement is +unusually slight. A shoot faced a window, and I traced its course on +the glass during two perfectly calm and hot days. On one of these +days it described, in the course of ten hours, a spire, representing +two and a half ellipses. I also placed a bell-glass over a young +Muscat grape in the hot-house, and it made each day three or four +very small oval revolutions; the shoot moving less than half an inch +from side to side. Had it not made at least three revolutions whilst +the sky was uniformly overcast, I should have attributed this slight +degree of movement to the varying action of the light. The extremity +of the stem is more or less bent downwards, but it never reverses its +curvature, as so generally occurs with twining plants. + +Various authors (Palm, p. 55; Mohl, p. 45; Lindley, &c.) believe that +the tendrils of the vine are modified flower-peduncles. I here give +a drawing (fig. 10) of the ordinary state of a young flower-stalk: +it consists of the "common peduncle" (A); of the "flower-tendril" +(B), which is represented as having caught a twig; and of the "sub- +peduncle" (C) bearing the flower-buds. The whole moves +spontaneously, like a true tendril, but in a less degree; the +movement, however, is greater when the sub-peduncle (C) does not bear +many flower-buds. The common peduncle (A) has not the power of +clasping a support, nor has the corresponding part of a true tendril. +The flower-tendril (B) is always longer than the sub-peduncle (C) and +has a scale at its base; it sometimes bifurcates, and therefore +corresponds in every detail with the longer scale-bearing branch (B, +fig. 9) of the true tendril. It is, however, inclined backwards +from the sub-peduncle (C), or stands at right angles with it, and is +thus adapted to aid in carrying the future bunch of grapes. When +rubbed, it curves and subsequently straightens itself; and it can, as +is shown in the drawing, securely clasp a support. I have seen an +object as soft as a young vine-leaf caught by one. + +The lower and naked part of the sub-peduncle (C) is likewise slightly +sensitive to a rub, and I have seen it bent round a stick and even +partly round a leaf with which it had come into contact. That the +sub-peduncle has the same nature as the corresponding branch of an +ordinary tendril, is well shown when it bears only a few flowers; for +in this case it becomes less branched, increases in length, and gains +both in sensitiveness and in the power of spontaneous movement. I +have twice seen sub-peduncles which bore from thirty to forty flower- +buds, and which had become considerably elongated and were completely +wound round sticks, exactly like true tendrils. The whole length of +another sub-peduncle, bearing only eleven flower-buds, quickly became +curved when slightly rubbed; but even this scanty number of flowers +rendered the stalk less sensitive than the other branch, that is, the +flower-tendril; for the latter after a lighter rub became curved more +quickly and in a greater degree. I have seen a sub-peduncle thickly +covered with flower-buds, with one of its higher lateral branchlets +bearing from some cause only two buds; and this one branchlet had +become much elongated and had spontaneously caught hold of an +adjoining twig; in fact, it formed a little sub-tendril. The +increasing length of the sub-peduncle (C) with the decreasing number +of the flower-buds is a good instance of the law of compensation. In +accordance with this same principle, the true tendril as a whole is +always longer than the flower-stalk; for instance, on the same plant, +the longest flower-stalk (measured from the base of the common +peduncle to the tip of the flower-tendril) was 8.5 inches in length, +whilst the longest tendril was nearly double this length, namely 16 +inches. + +The gradations from the ordinary state of a flower-stalk, as +represented in the drawing (fig. 10), to that of a true tendril (fig. +9) are complete. We have seen that the sub-peduncle (C), whilst +still bearing from thirty to forty flower-buds, sometimes becomes a +little elongated and partially assumes all the characters of the +corresponding branch of a true tendril. From this state we can trace +every stage till we come to a full-sized perfect tendril, bearing on +the branch which corresponds with the sub-peduncle one single flower- +bud! Hence there can be no doubt that the tendril is a modified +flower-peduncle. + +Another kind of gradation well deserves notice. Flower-tendrils (B, +fig. 10) sometimes produce a few flower-buds. For instance, on a +vine growing against my house, there were thirteen and twenty-two +flower-buds respectively on two flower-tendrils, which still retained +their characteristic qualities of sensitiveness and spontaneous +movement, but in a somewhat lessened degree. On vines in hothouses, +so many flowers are occasionally produced on the flower-tendrils that +a double bunch of grapes is the result; and this is technically +called by gardeners a "cluster." In this state the whole bunch of +flowers presents scarcely any resemblance to a tendril; and, judging +from the facts already given, it would probably possess little power +of clasping a support, or of spontaneous movement. Such flower- +stalks closely resemble in structure those borne by Cissus. This +genus, belonging to the same family of the Vitaceae, produces well- +developed tendrils and ordinary bunches of flowers; but there are no +gradations between the two states. If the genus Vitis had been +unknown, the boldest believer in the modification of species would +never have surmised that the same individual plant, at the same +period of growth, would have yielded every possible gradation between +ordinary flower-stalks for the support of the flowers and fruit, and +tendrils used exclusively for climbing. But the vine clearly gives +us such a case; and it seems to me as striking and curious an +instance of transition as can well be conceived. + +Cissus discolor.--The young shoots show no more movement than can be +accounted for by daily variations in the action of the light. The +tendrils, however, revolve with much regularity, following the sun; +and, in the plants observed by me, swept circles of about 5 inches in +diameter. Five circles were completed in the following times:- 4 +hrs. 45 m., 4 hrs. 50 m., 4 hrs. 45 m., 4 hrs. 30 m., and 5 hrs. The +same tendril continues to revolve during three or four days. The +tendrils are from 3.5 to 5 inches in length. They are formed of a +long foot-stalk, bearing two short branches, which in old plants +again bifurcate. The two branches are not of quite equal length; and +as with the vine, the longer one has a scale at its base. The +tendril stands vertically upwards; the extremity of the shoot being +bent abruptly downwards, and this position is probably of service to +the plant by allowing the tendril to revolve freely and vertically. + +Both branches of the tendril, whilst young, are highly sensitive. A +touch with a pencil, so gentle as only just to move a tendril borne +at the end of a long flexible shoot, sufficed to cause it to become +perceptibly curved in four or five minutes. It became straight again +in rather above one hour. A loop of soft thread weighing one-seventh +of a grain (9.25 mg.) was thrice tried, and each time caused the +tendril to become curved in 30 or 40 m. Half this weight produced no +effect. The long foot-stalk is much less sensitive, for a slight +rubbing produced no effect, although prolonged contact with a stick +caused it to bend. The two branches are sensitive on all sides, so +that they converge if touched on their inner sides, and diverge if +touched on their outer sides. If a branch be touched at the same +time with equal force on opposite sides, both sides are equally +stimulated and there is no movement. Before examining this plant, I +had observed only tendrils which are sensitive on one side alone, and +these when lightly pressed between the finger and thumb become +curved; but on thus pinching many times the tendrils of the Cissus no +curvature ensued, and I falsely inferred at first that they were not +at all sensitive. + +Cissus antarcticus.--The tendrils on a young plant were thick and +straight, with the tips a little curved. When their concave surfaces +were rubbed, and it was necessary to do this with some force, they +very slowly became curved, and subsequently straight again. They are +therefore much less sensitive than those of the last species; but +they made two revolutions, following the sun, rather more rapidly, +viz., in 3 hrs. 30 m. and 4 hrs. The internodes do not revolve. + +Ampelopsis hederacea (Virginian Creeper).--The internodes apparently +do not move more than can be accounted for by the varying action of +the light. The tendrils are from 4 to 5 inches in length, with the +main stem sending off several lateral branches, which have their tips +curved, as may be seen in the upper figure (fig. 11). They exhibit +no true spontaneous revolving movement, but turn, as was long ago +observed by Andrew Knight, {31} from the light to the dark. I have +seen several tendrils move in less than 24 hours, through an angle of +180 degrees to the dark side of a case in which a plant was placed, +but the movement is sometimes much slower. The several lateral +branches often move independently of one another, and sometimes +irregularly, without any apparent cause. These tendrils are less +sensitive to a touch than any others observed by me. By gentle but +repeated rubbing with a twig, the lateral branches, but not the main +stem, became in the course of three or four hours slightly curved; +but they seemed to have hardly any power of again straightening +themselves. The tendrils of a plant which had crawled over a large +box-tree clasped several of the branches; but I have repeatedly seen +that they will withdraw themselves after seizing a stick. When they +meet with a flat surface of wood or a wall (and this is evidently +what they are adapted for), they turn all their branches towards it, +and, spreading them widely apart, bring their hooked tips laterally +into contact with it. In effecting this, the several branches, after +touching the surface, often rise up, place themselves in a new +position, and again come down into contact with it. + +In the course of about two days after a tendril has arranged its +branches so as to press on any surface, the curved tips swell, become +bright red, and form on their under-sides the well-known little discs +or cushions with which they adhere firmly. In one case the tips were +slightly swollen in 38 hrs. after coming into contact with a brick; +in another case they were considerably swollen in 48 hrs., and in an +additional 24 hrs. were firmly attached to a smooth board; and +lastly, the tips of a younger tendril not only swelled but became +attached to a stuccoed wall in 42 hrs. These adhesive discs +resemble, except in colour and in being larger, those of Bignonia +capreolata. When they were developed in contact with a ball of tow, +the fibres were separately enveloped, but not in so effective a +manner as by B. capreolata. Discs are never developed, as far as I +have seen, without the stimulus of at least temporary contact with +some object. {32} They are generally first formed on one side of the +curved tip, the whole of which often becomes so much changed in +appearance, that a line of the original green tissue can be traced +only along the concave surface. When, however, a tendril has clasped +a cylindrical stick, an irregular rim or disc is sometimes formed +along the inner surface at some little distance from the curved tip; +this was also observed (p. 71) by Mohl. The discs consist of +enlarged cells, with smooth projecting hemispherical surfaces, +coloured red; they are at first gorged with fluid (see section given +by Mohl, p. 70), but ultimately become woody. + +As the discs soon adhere firmly to such smooth surfaces as planed or +painted wood, or to the polished leaf of the ivy, this alone renders +it probable that some cement is secreted, as has been asserted to be +the case (quoted by Mohl, p. 71) by Malpighi. I removed a number of +discs formed during the previous year from a stuccoed wall, and left +them during many hours, in warm water, diluted acetic acid and +alcohol; but the attached grains of silex were not loosened. +Immersion in sulphuric ether for 24 hrs. loosened them much, but +warmed essential oils (I tried oil of thyme and peppermint) +completely released every particle of stone in the course of a few +hours. This seems to prove that some resinous cement is secreted. +The quantity, however, must be small; for when a plant ascended a +thinly whitewashed wall, the discs adhered firmly to the whitewash; +but as the cement never penetrated the thin layer, they were easily +withdrawn, together with little scales of the whitewash. It must not +be supposed that the attachment is effected exclusively by the +cement; for the cellular outgrowth completely envelopes every minute +and irregular projection, and insinuates itself into every crevice. + +A tendril which has not become attached to any body, does not +contract spirally; and in course of a week or two shrinks into the +finest thread, withers and drops off. An attached tendril, on the +other hand, contracts spirally, and thus becomes highly elastic, so +that when the main foot-stalk is pulled the strain is distributed +equally between all the attached discs. For a few days after the +attachment of the discs, the tendril remains weak and brittle, but it +rapidly increases in thickness and acquires great strength. During +the following winter it ceases to live, but adheres firmly in a dead +state both to its own stem and to the surface of attachment. In the +accompanying diagram (fig. 11.) we see the difference between a +tendril (B) some weeks after its attachment to a wall, with one (A) +from the same plant fully grown but unattached. That the change in +the nature of the tissues, as well as the spiral contraction, are +consequent on the formation of the discs, is well shown by any +lateral branches which have not become attached; for these in a week +or two wither and drop off, in the same manner as does the whole +tendril if unattached. The gain in strength and durability in a +tendril after its attachment is something wonderful. There are +tendrils now adhering to my house which are still strong, and have +been exposed to the weather in a dead state for fourteen or fifteen +years. One single lateral branchlet of a tendril, estimated to be at +least ten years old, was still elastic and supported a weight of +exactly two pounds. The whole tendril had five disc-bearing branches +of equal thickness and apparently of equal strength; so that after +having been exposed during ten years to the weather, it would +probably have resisted a strain of ten pounds! + +SAPINDACEAE.--Cardiospermum halicacabum.--In this family, as in the +last, the tendrils are modified flower-peduncles. In the present +plant the two lateral branches of the main flower-peduncle have been +converted into a pair of tendrils, corresponding with the single +"flower-tendril" of the common vine. The main peduncle is thin, +stiff, and from 3 to 4.5 inches in length. Near the summit, above +two little bracts, it divides into three branches. The middle one +divides and re-divides, and bears the flowers; ultimately it grows +half as long again as the two other modified branches. These latter +are the tendrils; they are at first thicker and longer than the +middle branch, but never become more than an inch in length. They +taper to a point and are flattened, with the lower clasping surface +destitute of hairs. At first they project straight up; but soon +diverging, spontaneously curl downwards so as to become symmetrically +and elegantly hooked, as represented in the diagram. They are now, +whilst the flower-buds are still small, ready for action. + +The two or three upper internodes, whilst young, steadily revolve; +those on one plant made two circles, against the course of the sun, +in 3 hrs. 12 m.; in a second plant the same course was followed, and +the two circles were completed in 3 hrs. 41 m.; in a third plant, the +internodes followed the sun and made two circles in 3 hrs. 47 m. The +average rate of these six revolutions was 1 hr. 46 m. The stem shows +no tendency to twine spirally round a support; but the allied +tendril-bearing genus Paullinia is said (Mohl, p. 4) to be a twiner. +The flower-peduncles, which stand up above the end of the shoot, are +carried round and round by the revolving movement of the internodes; +and when the stem is securely tied, the long and thin flower- +peduncles themselves are seen to be in continued and sometimes rapid +movement from side to side. They sweep a wide space, but only +occasionally revolve in a regular elliptical course. By the combined +movements of the internodes and peduncles, one of the two short +hooked tendrils, sooner or later, catches hold of some twig or +branch, and then it curls round and securely grasps it. These +tendrils are, however, but slightly sensitive; for by rubbing their +under surface only a slight movement is slowly produced. I hooked a +tendril on to a twig; and in 1 hr. 45 m. it was curved considerably +inwards; in 2 hrs. 30 m. it formed a ring; and in from 5 to 6 hours +from being first hooked, it closely grasped the stick. A second +tendril acted at nearly the same rate; but I observed one that took +24 hours before it curled twice round a thin twig. Tendrils which +have caught nothing, spontaneously curl up to a close helix after the +interval of several days. Those which have curled round some object, +soon become a little thicker and tougher. The long and thin main +peduncle, though spontaneously moving, is not sensitive and never +clasps a support. Nor does it ever contract spirally, {33} although +a contraction of this kind apparently would have been of service to +the plant in climbing. Nevertheless it climbs pretty well without +this aid. The seed-capsules though light, are of enormous size +(hence its English name of balloon-vine), and as two or three are +carried on the same peduncle, the tendrils rising close to them may +be of service in preventing their being dashed to pieces by the wind. +In the hothouse the tendrils served simply for climbing. + +The position of the tendrils alone suffices to show their homological +nature. In two instances one of two tendrils produced a flower at +its tip; this, however, did not prevent its acting properly and +curling round a twig. In a third case both lateral branches which +ought to have been modified into tendrils, produced flowers like the +central branch, and had quite lost their tendril-structure. + +I have seen, but was not enabled carefully to observe, only one other +climbing Sapindaceous plant, namely, Paullinia. It was not in +flower, yet bore long forked tendrils. So that, Paullinia, with +respect to its tendrils, appears to bear the same relation to +Cardiospermum that Cissus does to Vitis. + +PASSIFLORACEAE.--After reading the discussion and facts given by Mohl +(p. 47) on the nature of the tendrils in this family, no one can +doubt that they are modified flower-peduncles. The tendrils and the +flower-peduncles rise close side by side; and my son, William E. +Darwin, made sketches for me of their earliest state of development +in the hybrid P. floribunda. The two organs appear at first as a +single papilla which gradually divides; so that the tendril appears +to be a modified branch of the flower-peduncle. My son found one +very young tendril surmounted by traces of floral organs, exactly +like those on the summit of the true flower-peduncle at the same +early age. + +Passiflora gracilis.--This well-named, elegant, annual species +differs from the other members of the group observed by me, in the +young internodes having the power of revolving. It exceeds all the +other climbing plants which I have examined, in the rapidity of its +movements, and all tendril-bearers in the sensitiveness of the +tendrils. The internode which carries the upper active tendril and +which likewise carries one or two younger immature internodes, made +three revolutions, following the sun, at an average rate of 1 hr. 4 +m.; it then made, the day becoming very hot, three other revolutions +at an average rate of between 57 and 58 m.; so that the average of +all six revolutions was 1 hr. 1 m. The apex of the tendril describes +elongated ellipses, sometimes narrow and sometimes broad, with their +longer axes inclined in slightly different directions. The plant can +ascend a thin upright stick by the aid of its tendrils; but the stem +is too stiff for it to twine spirally round it, even when not +interfered with by the tendrils, these having been successively +pinched off at an early age. + +When the stem is secured, the tendrils are seen to revolve in nearly +the same manner and at the same rate as the internodes. {34} The +tendrils are very thin, delicate, and straight, with the exception of +the tips, which are a little curved; they are from 7 to 9 inches in +length. A half-grown tendril is not sensitive; but when nearly full- +grown they are extremely sensitive. A single delicate touch on the +concave surface of the tip soon caused one to curve; and in 2 minutes +it formed an open helix. A loop of soft thread weighing one thirty- +second of a grain (2.02 mg.) placed most gently on the tip, thrice +caused distinct curvature. A bent bit of thin platina wire weighing +only fiftieth of a grain (1.23 mg.) twice produced the same effect; +but this latter weight, when left suspended, did not suffice to cause +a permanent curvature. These trials were made under a bell-glass, so +that the loops of thread and wire were not agitated by the wind. The +movement after a touch is very rapid: I took hold of the lower part +of several tendrils, and then touched their concave tips with a thin +twig and watched them carefully through a lens; the tips evidently +began to bend after the following intervals--31, 25, 32, 31, 28, 39, +31, and 30 seconds; so that the movement was generally perceptible in +half a minute after a touch; but on one occasion it was distinctly +visible in 25 seconds. One of the tendrils which thus became bent in +31 seconds, had been touched two hours previously and had coiled into +a helix; so that in this interval it had straightened itself and had +perfectly recovered its irritability. + +To ascertain how often the same tendril would become curved when +touched, I kept a plant in my study, which from being cooler than the +hot-house was not very favourable for the experiment. The extremity +was gently rubbed four or five times with a thin stick, and this was +done as often as it was observed to have become nearly straight again +after having been in action; and in the course of 54 hrs. it answered +to the stimulus 21 times, becoming each time hooked or spiral. On +the last occasion, however, the movement was very slight, and soon +afterwards permanent spiral contraction commenced. No trials were +made during the night, so that the tendril would perhaps have +answered a greater number of times to the stimulus; though, on the +other hand, from having no rest it might have become exhausted from +so many quickly repeated efforts. + +I repeated the experiment made on the Echinocystis, and placed +several plants of this Passiflora so close together, that their +tendrils were repeatedly dragged over each other; but no curvature +ensued. I likewise repeatedly flirted small drops of water from a +brush on many tendrils, and syringed others so violently that the +whole tendril was dashed about, but they never became curved. The +impact from the drops of water was felt far more distinctly on my +hand than that from the loops of thread (weighing one thirty-second +of a grain) when allowed to fall on it from a height, and these +loops, which caused the tendrils to become curved, had been placed +most gently on them. Hence it is clear, that the tendrils either +have become habituated to the touch of other tendrils and drops of +rain, or that they were from the first rendered sensitive only to +prolonged though excessively slight pressure of solid objects, with +the exclusion of that from other tendrils. To show the difference in +the kind of sensitiveness in different plants and likewise to show +the force of the syringe used, I may add that the lightest jet from +it instantly caused the leaves of a Mimosa to close; whereas the loop +of thread weighing one thirty-second of a grain, when rolled into a +ball and placed gently on the glands at the bases of the leaflets of +the Mimosa, caused no action. + +Passiflora punctata.--The internodes do not move, but the tendrils +revolve regularly. A half-grown and very sensitive tendril made +three revolutions, opposed to the course of the sun, in 3 hrs. 5 m., +2 hrs. 40 m. and 2 hrs. 50 m.; perhaps it might have travelled more +quickly when nearly full-grown. A plant was placed in front of a +window, and, as with twining stems, the light accelerated the +movement of the tendril in one direction and retarded it in the +other; the semicircle towards the light being performed in one +instance in 15 m. less time and in a second instance in 20 m. less +time than that required by the semicircle towards the dark end of the +room. Considering the extreme tenuity of these tendrils, the action +of the light on them is remarkable. The tendrils are long, and, as +just stated, very thin, with the tip slightly curved or hooked. The +concave side is extremely sensitive to a touch--even a single touch +causing it to curl inwards; it subsequently straightened itself, and +was again ready to act. A loop of soft thread weighing one +fourteenth of a grain (4.625 mg.) caused the extreme tip to bend; +another time I tried to hang the same little loop on an inclined +tendril, but three times it slid off; yet this extraordinarily slight +degree of friction sufficed to make the tip curl. The tendril, +though so sensitive, does not move very quickly after a touch, no +conspicuous movement being observable until 5 or 10 m. had elapsed. +The convex side of the tip is not sensitive to a touch or to a +suspended loop of thread. On one occasion I observed a tendril +revolving with the convex side of the tip forwards, and in +consequence it was not able to clasp a stick, against which it +scraped; whereas tendrils revolving with the concave side forward, +promptly seize any object in their path. + +Passiflora quadrangularis.--This is a very distinct species. The +tendrils are thick, long, and stiff; they are sensitive to a touch +only on the concave surface towards the extremity. When a stick was +placed so that the middle of the tendril came into contact with it, +no curvature ensued. In the hothouse a tendril made two revolutions, +each in 2 hrs. 22 m.; in a cool room one was completed in 3 hrs., and +a second in 4 hrs. The internodes do not revolve; nor do those of +the hybrid P. floribunda. + +Tacsonia manicata.--Here again the internodes do not revolve. The +tendrils are moderately thin and long; one made a narrow ellipse in 5 +hrs. 20 m., and the next day a broad ellipse in 5 hrs. 7 m. The +extremity being lightly rubbed on the concave surface, became just +perceptibly curved in 7 m., distinctly in 10 m., and hooked in 20 m. + +We have seen that the tendrils in the last three families, namely, +the Vitaceae, Sapindaceae and Passifloraceae, are modified flower- +peduncles. This is likewise the case, according to De Candolle (as +quoted by Mohl), with the tendrils of Brunnichia, one of the +Polygonaceae. In two or three species of Modecca, one of the +Papayaceae, the tendrils, as I hear from Prof. Oliver, occasionally +bear flowers and fruit; so that they are axial in their nature. + + +The Spiral Contraction of Tendrils. + + +This movement, which shortens the tendrils and renders them elastic, +commences in half a day, or in a day or two after their extremities +have caught some object. There is no such movement in any leaf- +climber, with the exception of an occasional trace of it in the +petioles of Tropaeolum tricolorum. On the other hand, the tendrils +of all tendril-bearing plants, contract spirally after they have +caught an object with the following exceptions. Firstly, Corydalis +claviculata, but then this plant might be called a leaf-climber. +Secondly and thirdly, Bignonia unguis with its close allies, and +Cardiospermum; but their tendrils are so short that their contraction +could hardly occur, and would be quite superfluous. Fourthly, Smilax +aspera offers a more marked exception, as its tendrils are moderately +long. The tendrils of Dicentra, whilst the plant is young, are short +and after attachment only become slightly flexuous; in older plants +they are longer and then they contract spirally. I have seen no +other exceptions to the rule that tendrils, after clasping with their +extremities a support, undergo spiral contraction. When, however, +the tendril of a plant of which the stem is immovably fixed, catches +some fixed object, it does not contract, simply because it cannot; +this, however, rarely occurs. In the common Pea the lateral branches +alone contract, and not the central stem; and with most plants, such +as the Vine, Passiflora, Bryony, the basal portion never forms a +spire. + +I have said that in Corydalis claviculata the end of the leaf or +tendril (for this part may be indifferently so called) does not +contract into a spire. The branchlets, however, after they have +wound round thin twigs, become deeply sinuous or zigzag. Moreover +the whole end of the petiole or tendril, if it seizes nothing, bends +after a time abruptly downwards and inwards, showing that its outer +surface has gone on growing after the inner surface has ceased to +grow. That growth is the chief cause of the spiral contraction of +tendrils may be safely admitted, as shown by the recent researches of +H. de Vries. I will, however, add one little fact in support of this +conclusion. + +If the short, nearly straight portion of an attached tendril of +Passiflora gracilis, (and, as I believe, of other tendrils,) between +the opposed spires, be examined, it will be found to be transversely +wrinkled in a conspicuous manner on the outside; and this would +naturally follow if the outer side had grown more than the inner +side, this part being at the same time forcibly prevented from +becoming curved. So again the whole outer surface of a spirally +wound tendril becomes wrinkled if it be pulled straight. +Nevertheless, as the contraction travels from the extremity of a +tendril, after it has been stimulated by contact with a support, down +to the base, I cannot avoid doubting, from reasons presently to be +given, whether the whole effect ought to be attributed to growth. An +unattached tendril rolls itself up into a flat helix, as in the case +of Cardiospermum, if the contraction commences at the extremity and +is quite regular; but if the continued growth of the outer surface is +a little lateral, or if the process begins near the base, the +terminal portion cannot be rolled up within the basal portion, and +the tendril then forms a more or less open spire. A similar result +follows if the extremity has caught some object, and is thus held +fast. + +The tendrils of many kinds of plants, if they catch nothing, contract +after an interval of several days or weeks into a spire; but in these +cases the movement takes place after the tendril has lost its +revolving power and hangs down; it has also then partly or wholly +lost its sensibility; so that this movement can be of no use. The +spiral contraction of unattached tendrils is a much slower process +than that of attached ones. Young tendrils which have caught a +support and are spirally contracted, may constantly be seen on the +same stem with the much older unattached and uncontracted tendrils. +In the Echinocystis I have seen a tendril with the two lateral +branches encircling twigs and contracted into beautiful spires, +whilst the main branch which had caught nothing remained for many +days straight. In this plant I once observed a main branch after it +had caught a stick become spirally flexuous in 7 hrs., and spirally +contracted in 18 hrs. Generally the tendrils of the Echinocystis +begin to contract in from 12 hrs. to 24 hrs. after catching some +object; whilst unattached tendrils do not begin to contract until two +or three or even more days after all revolving movement has ceased. +A full-grown tendril of Passiflora quadrangularis which had caught a +stick began in 8 hrs. to contract, and in 24 hrs. formed several +spires; a younger tendril, only two-thirds grown, showed the first +trace of contraction in two days after clasping a stick, and in two +more days formed several spires. It appears, therefore, that the +contraction does not begin until the tendril is grown to nearly its +full length. Another young tendril of about the same age and length +as the last did not catch any object; it acquired its full length in +four days; in six additional days it first became flexuous, and in +two more days formed one complete spire. This first spire was formed +towards the basal end, and the contraction steadily but slowly +progressed towards the apex; but the whole was not closely wound up +into a spire until 21 days had elapsed from the first observation, +that is, until 17 days after the tendril had grown to its full +length. + +The spiral contraction of tendrils is quite independent of their +power of spontaneously revolving, for it occurs in tendrils, such as +those of Lathyrus grandiflorus and Ampelopsis hederacea, which do not +revolve. It is not necessarily related to the curling of the tips +round a support, as we see with the Ampelopsis and Bignonia +capreolata, in which the development of adherent discs suffices to +cause spiral contraction. Yet in some cases this contraction seems +connected with the curling or clasping movement, due to contact with +a support; for not only does it soon follow this act, but the +contraction generally begins close to the curled extremity, and +travels downwards to the base. If, however, a tendril be very slack, +the whole length almost simultaneously becomes at first flexuous and +then spiral. Again, the tendrils of some few plants never contract +spirally unless they have first seized hold of some object; if they +catch nothing they hang down, remaining straight, until they wither +and drop off: this is the case with the tendrils of Bignonia, which +consist of modified leaves, and with those of three genera of the +Vitaceae, which are modified flower-peduncles. But in the great +majority of cases, tendrils which have never come in contact with any +object, after a time contract spirally. All these facts taken +together, show that the act of clasping a support and the spiral +contraction of the whole length of the tendril, are phenomena not +necessarily connected. + +The spiral contraction which ensues after a tendril has caught a +support is of high service to the plant; hence its almost universal +occurrence with species belonging to widely different orders. When a +shoot is inclined and its tendril has caught an object above, the +spiral contraction drags up the shoot. When the shoot is upright, +the growth of the stem, after the tendrils have seized some object +above, would leave it slack, were it not for the spiral contraction +which draws up the stem as it increases in length. Thus there is no +waste of growth, and the stretched stem ascends by the shortest +course. When a terminal branchlet of the tendril of Cobaea catches a +stick, we have seen how well the spiral contraction successively +brings the other branchlets, one after the other, into contact with +the stick, until the whole tendril grasps it in an inextricable knot. +When a tendril has caught a yielding object, this is sometimes +enveloped and still further secured by the spiral folds, as I have +seen with Passiflora quadrangularis; but this action is of little +importance. + +A far more important service rendered by the spiral contraction of +the tendrils is that they are thus made highly elastic. As before +remarked under Ampelopsis, the strain is thus distributed equally +between the several attached branches; and this renders the whole far +stronger than it otherwise would be, as the branches cannot break +separately. It is this elasticity which protects both branched and +simple tendrils from being torn away from their supports during +stormy weather. I have more than once gone on purpose during a gale +to watch a Bryony growing in an exposed hedge, with its tendrils +attached to the surrounding bushes; and as the thick and thin +branches were tossed to and fro by the wind, the tendrils, had they +not been excessively elastic, would instantly have been torn off and +the plant thrown prostrate. But as it was, the Bryony safely rode +out the gale, like a ship with two anchors down, and with a long +range of cable ahead to serve as a spring as she surges to the storm. + +When an unattached tendril contracts spirally, the spire always runs +in the same direction from tip to base. A tendril, on the other +hand, which has caught a support by its extremity, although the same +side is concave from end to end, invariably becomes twisted in one +part in one direction, and in another part in the opposite direction; +the oppositely turned spires being separated by a short straight +portion. This curious and symmetrical structure has been noticed by +several botanists, but has not been sufficiently explained. {35} It +occurs without exception with all tendrils which after catching an +object contract spirally, but is of course most conspicuous in the +longer tendrils. It never occurs with uncaught tendrils; and when +this appears to have occurred, it will be found that the tendril had +originally seized some object and had afterwards been torn free. +Commonly, all the spires at one end of an attached tendril run in one +direction, and all those at the other end in the opposite direction, +with a single short straight portion in the middle; but I have seen a +tendril with the spires alternately turning five times in opposite +directions, with straight pieces between them; and M. Leon has seen +seven or eight such alternations. Whether the spires turn once or +more than once in opposite directions, there are as many turns in the +one direction as in the other. For instance, I gathered ten attached +tendrils of the Bryony, the longest with 33, and the shortest with +only 8 spiral turns; and the number of turns in the one direction was +in every case the same (within one) as in the opposite direction. + +The explanation of this curious little fact is not difficult. I will +not attempt any geometrical reasoning, but will give only a practical +illustration. In doing this, I shall first have to allude to a point +which was almost passed over when treating of Twining-plants. If we +hold in our left hand a bundle of parallel strings, we can with our +right hand turn these round and round, thus imitating the revolving +movement of a twining plant, and the strings do not become twisted. +But if we hold at the same time a stick in our left hand, in such a +position that the strings become spirally turned round it, they will +inevitably become twisted. Hence a straight coloured line, painted +along the internodes of a twining plant before it has wound round a +support, becomes twisted or spiral after it has wound round. I +painted a red line on the straight internodes of a Humulus, Mikania, +Ceropegia, Convolvulus, and Phaseolus, and saw it become twisted as +the plant wound round a stick. It is possible that the stems of some +plants by spontaneously turning on their own axes, at the proper rate +and in the proper direction, might avoid becoming twisted; but I have +seen no such case. + +In the above illustration, the parallel strings were wound round a +stick; but this is by no means necessary, for if wound into a hollow +coil (as can be done with a narrow slip of elastic paper) there is +the same inevitable twisting of the axis. When, therefore, a free +tendril coils itself into a spire, it must either become twisted +along its whole length (and this never occurs), or the free extremity +must turn round as many times as there are spires formed. It was +hardly necessary to observe this fact; but I did so by affixing +little paper vanes to the extreme points of the tendrils of +Echinocystis and Passiflora quadrangularis; and as the tendril +contracted itself into successive spires, the vane slowly revolved. + +We can now understand the meaning of the spires being invariably +turned in opposite directions, in tendrils which from having caught +some object are fixed at both ends. Let us suppose a caught tendril +to make thirty spiral turns all in the same direction; the inevitable +result would be that it would become twisted thirty times on its own +axis. This twisting would not only require considerable force, but, +as I know by trial, would burst the tendril before the thirty turns +were completed. Such cases never really occur; for, as already +stated, when a tendril has caught a support and is spirally +contracted, there are always as many turns in one direction as in the +other; so that the twisting of the axis in the one direction is +exactly compensated by the twisting in the opposite direction. We +can further see how the tendency is given to make the later formed +coils opposite to those, whether turned to the right or to the left, +which are first made. Take a piece of string, and let it hang down +with the lower end fixed to the floor; then wind the upper end +(holding the string quite loosely) spirally round a perpendicular +pencil, and this will twist the lower part of the string; and after +it has been sufficiently twisted, it will be seen to curve itself +into an open spire, with the curves running in an opposite direction +to those round the pencil, and consequently with a straight piece of +string between the opposed spires. In short, we have given to the +string the regular spiral arrangement of a tendril caught at both +ends. The spiral contraction generally begins at the extremity which +has clasped a support; and these first-formed spires give a twist to +the axis of the tendril, which necessarily inclines the basal part +into an opposite spiral curvature. I cannot resist giving one other +illustration, though superfluous: when a haberdasher winds up ribbon +for a customer, he does not wind it into a single coil; for, if he +did, the ribbon would twist itself as many times as there were coils; +but he winds it into a figure of eight on his thumb and little +finger, so that he alternately takes turns in opposite directions, +and thus the ribbon is not twisted. So it is with tendrils, with +this sole difference, that they take several consecutive turns in one +direction and then the same number in an opposite direction; but in +both cases the self-twisting is avoided. + + +Summary on the Nature and Action of Tendrils. + + +With the majority of tendril-bearing plants the young internodes +revolve in more or less broad ellipses, like those made by twining +plants; but the figures described, when carefully traced, generally +form irregular ellipsoidal spires. The rate of revolution varies +from one to five hours in different species, and consequently is in +some cases more rapid than with any twining plant, and is never so +slow as with those many twiners which take more than five hours for +each revolution. The direction is variable even in the same +individual plant. In Passiflora, the internodes of only one species +have the power of revolving. The Vine is the weakest revolver +observed by me, apparently exhibiting only a trace of a former power. +In the Eccremocarpus the movement is interrupted by many long pauses. +Very few tendril-bearing plants can spirally twine up an upright +stick. Although the power of twining has generally been lost, either +from the stiffness or shortness of the internodes, from the size of +the leaves, or from some other unknown cause, the revolving movement +of the stem serves to bring the tendrils into contact with +surrounding objects. + +The tendrils themselves also spontaneously revolve. The movement +begins whilst the tendril is young, and is at first slow. The mature +tendrils of Bignonia littoralis move much slower than the internodes. +Generally, the internodes and tendrils revolve together at the same +rate; in Cissus, Cobaea, and most Passiflorae, the tendrils alone +revolve; in other cases, as with Lathyrus aphaca, only the internodes +move, carrying with them the motionless tendrils; and, lastly (and +this is the fourth possible case), neither internodes nor tendrils +spontaneously revolve, as with Lathyrus grandiflorus and Ampelopsis. +In most Bignonias, Eccremocarpus Mutisia, and the Fumariaceae, the +internodes, petioles and tendrils all move harmoniously together. In +every case the conditions of life must be favourable in order that +the different parts should act in a perfect manner. + +Tendrils revolve by the curvature of their whole length, excepting +the sensitive extremity and the base, which parts do not move, or +move but little. The movement is of the same nature as that of the +revolving internodes, and, from the observations of Sachs and H. de +Vries, no doubt is due to the same cause, namely, the rapid growth of +a longitudinal band, which travels round the tendril and successively +bows each part to the opposite side. Hence, if a line be painted +along that surface which happens at the time to be convex, the line +becomes first lateral, then concave, then lateral, and ultimately +again convex. This experiment can be tried only on the thicker +tendrils, which are not affected by a thin crust of dried paint. The +extremities are often slightly curved or hooked, and the curvature of +this part is never reversed; in this respect they differ from the +extremities of twining shoots, which not only reverse their +curvature, or at least become periodically straight, but curve +themselves in a greater degree than the lower part. In most other +respects a tendril acts as if it were one of several revolving +internodes, which all move together by successively bending to each +point of the compass. There is, however, in many cases this +unimportant difference, that the curving tendril is separated from +the curving internode by a rigid petiole. With most tendril-bearers +the summit of the stem or shoot projects above the point from which +the tendril arises; and it is generally bent to one side, so as to be +out of the way of the revolutions swept by the tendril. In those +plants in which the terminal shoot is not sufficiently out of the +way, as we have seen with the Echinocystis, as soon as the tendril +comes in its revolving course to this point, it stiffens and +straightens itself, and thus rising vertically up passes over the +obstacle in an admirable manner. + +All tendrils are sensitive, but in various degrees, to contact with +an object, and curve towards the touched side. With several plants a +single touch, so slight as only just to move the highly flexible +tendril, is enough to induce curvature. Passiflora gracilis +possesses the most sensitive tendrils which I have observed: a bit +of platina wire 0.02 of a grain (1.23 mg.) in weight, gently placed +on the concave point, caused a tendril to become hooked, as did a +loop of soft, thin cotton thread weighing one thirty-second of a +grain (2.02 mg.) With the tendrils of several other plants, loops +weighing one sixteenth of a grain (4.05 mg.) sufficed. The point of +a tendril of Passiflora gracilis began to move distinctly in 25 +seconds after a touch, and in many cases after 30 seconds. Asa Gray +also saw movement in the tendrils of the Cucurbitaceous genus, +Sicyos, in 30 seconds. The tendrils of some other plants, when +lightly rubbed, moved in a few minutes; with Dicentra in half-an- +hour; with Smilax in an hour and a quarter or half; and with +Ampelopsis still more slowly. The curling movement consequent on a +single touch continues to increase for a considerable time, then +ceases; after a few hours the tendril uncurls itself, and is again +ready to act. When the tendrils of several kinds of plants were +caused to bend by extremely light weights suspended on them, they +seemed to grow accustomed to so slight a stimulus, and straightened +themselves, as if the loops had been removed. It makes no difference +what sort of object a tendril touches, with the remarkable exception +of other tendrils and drops of water, as was observed with the +extremely sensitive-tendrils of Passiflora gracilis and of the +Echinocystis. I have, however, seen tendrils of the Bryony which had +temporarily caught other tendrils, and often in the case of the vine. + +Tendrils of which the extremities are permanently and slightly +curved, are sensitive only on the concave surface; other tendrils, +such as those of the Cobaea (though furnished with horny hooks +directed to one side) and those of Cissus discolor, are sensitive on +all sides. Hence the tendrils of this latter plant, when stimulated +by a touch of equal force on opposite sides, did not bend. The +inferior and lateral surfaces of the tendrils of Mutisia are +sensitive, but not the upper surface. With branched tendrils, the +several branches act alike; but in the Hanburya the lateral spur-like +branch does not acquire (for excellent reasons which have been +explained) its sensitiveness nearly so soon as the main branch. With +most tendrils the lower or basal part is either not at all sensitive, +or sensitive only to prolonged contact. We thus see that the +sensitiveness of tendrils is a special and localized capacity. It is +quite independent of the power of spontaneously revolving; for the +curling of the terminal portion from touch does not in the least +interrupt the former movement. In Bignonia unguis and its close +allies, the petioles of the leaves, as well as the tendrils, are +sensitive to a touch. + +Twining plants when they come into contact with a stick, curl round +it invariably in the direction of their revolving movement; but +tendrils curl indifferently to either side, in accordance with the +position of the stick and the side which is first touched. The +clasping movement of the extremity is apparently not steady, but +undulatory or vermicular in its nature, as may be inferred from the +curious manner in which the tendrils of the Echinocystis slowly +crawled round a smooth stick. + +As with a few exceptions tendrils spontaneously revolve, it may be +asked,--why have they been endowed with sensitiveness?--why, when +they come into contact with a stick, do they not, like twining +plants, spirally wind round it? One reason may be that they are in +most cases so flexible and thin, that when brought into contact with +any object, they would almost certainly yield and be dragged onwards +by the revolving movement. Moreover, the sensitive extremities have +no revolving power as far as I have observed, and could not by this +means curl round a support. With twining plants, on the other hand, +the extremity spontaneously bends more than any other part; and this +is of high importance for the ascent of the plant, as may be seen on +a windy day. It is, however, possible that the slow movement of the +basal and stiffer parts of certain tendrils, which wind round sticks +placed in their path, may be analogous to that of twining plants. +But I hardly attended sufficiently to this point, and it would have +been difficult to distinguish between a movement due to extremely +dull irritability, from the arrestment of the lower part, whilst the +upper part continued to move onwards. + +Tendrils which are only three-fourths grown, and perhaps even at an +earlier age, but not whilst extremely young, have the power of +revolving and of grasping any object which they touch. These two +capacities are generally acquired at about the same period, and both +fail when the tendril is full grown. But in Cobaea and Passiflora +punctata the tendrils begin to revolve in a useless manner, before +they have become sensitive. In the Echinocystis they retain their +sensitiveness for some time after they have ceased to revolve and +after they have sunk downwards; in this position, even if they were +able to seize an object, such power would be of no service in +supporting the stem. It is a rare circumstance thus to detect any +superfluity or imperfection in the action of tendrils--organs which +are so excellently adapted for the functions which they have to +perform; but we see that they are not always perfect, and it would be +rash to assume that any existing tendril has reached the utmost limit +of perfection. + +Some tendrils have their revolving motion accelerated or retarded, in +moving to or from the light; others, as with the Pea, seem +indifferent to its action; others move steadily from the light to the +dark, and this aids them in an important manner in finding a support. +For instance, the tendrils of Bignonia capreolata bend from the light +to the dark as truly as a wind-vane from the wind. In the +Eccremocarpus the extremities alone twist and turn about so as to +bring their finer branches and hooks into close contact with any dark +surface, or into crevices and holes. + +A short time after a tendril has caught a support, it contracts with +some rare exceptions into a spire; but the manner of contraction and +the several important advantages thus gained have been discussed so +lately, that nothing need here be repeated on the subject. Tendrils +soon after catching a support grow much stronger and thicker, and +sometimes more durable to a wonderful degree; and this shows how much +their internal tissues must be changed. Occasionally it is the part +which is wound round a support which chiefly becomes thicker and +stronger; I have seen, for instance, this part of a tendril of +Bignonia aequinoctialis twice as thick and rigid as the free basal +part. Tendrils which have caught nothing soon shrink and wither; but +in some species of Bignonia they disarticulate and fall off like +leaves in autumn. + + +Any one who had not closely observed tendrils of many kinds would +probably infer that their action was uniform. This is the case with +the simpler kinds, which simply curl round an object of moderate +thickness, whatever its nature may be. {36} But the genus Bignonia +shows us what diversity of action there may be between the tendrils +of closely allied species. In all the nine species observed by me, +the young internodes revolve vigorously; the tendrils also revolve, +but in some of the species in a very feeble manner; and lastly the +petioles of nearly all revolve, though with unequal power. The +petioles of three of the species, and the tendrils of all are +sensitive to contact. In the first-described species, the tendrils +resemble in shape a bird's foot, and they are of no service to the +stem in spirally ascending a thin upright stick, but they can seize +firm hold of a twig or branch. When the stem twines round a somewhat +thick stick, a slight degree of sensitiveness possessed by the +petioles is brought into play, and the whole leaf together with the +tendril winds round it. In B. unguis the petioles are more +sensitive, and have greater power of movement than those of the last +species; they are able, together with the tendrils, to wind +inextricably round a thin upright stick; but the stem does not twine +so well. B. Tweedyana has similar powers, but in addition, emits +aerial roots which adhere to the wood. In B. venusta the tendrils +are converted into elongated three-pronged grapnels, which move +spontaneously in a conspicuous manner; the petioles, however, have +lost their sensitiveness. The stem of this species can twine round +an upright stick, and is aided in its ascent by the tendrils seizing +the stick alternately some way above and then contracting spirally. +In B. littoralis the tendrils, petioles, and internodes, all revolve +spontaneously. The stem, however, cannot twine, but ascends an +upright stick by seizing it above with both tendrils together, which +then contract into a spire. The tips of these tendrils become +developed into adhesive discs. B. speciosa possesses similar powers +of movement as the last species, but it cannot twine round a stick, +though it can ascend by clasping the stick horizontally with one or +both of its unbranched tendrils. These tendrils continually insert +their pointed ends into minute crevices or holes, but as they are +always withdrawn by the subsequent spiral contraction, the habit +seems to us in our ignorance useless. Lastly, the stem of B. +capreolata twines imperfectly; the much-branched tendrils revolve in +a capricious manner, and bend from the light to the dark; their +hooked extremities, even whilst immature, crawl into crevices, and, +when mature, seize any thin projecting point; in either case they +develop adhesive discs, and these have the power of enveloping the +finest fibres. + +In the allied Eccremocarpus the internodes, petioles, and much- +branched tendrils all spontaneously revolve together. The tendrils +do not as a whole turn from the light; but their bluntly-hooked +extremities arrange themselves neatly on any surface with which they +come into contact, apparently so as to avoid the light. They act +best when each branch seizes a few thin stems, like the culms of a +grass, which they afterwards draw together into a solid bundle by the +spiral contraction of all the branches. In Cobaea the finely- +branched tendrils alone revolve; the branches terminate in sharp, +hard, double, little hooks, with both points directed to the same +side; and these turn by well-adapted movements to any object with +which they come into contact. The tips of the branches also crawl +into dark crevices or holes. The tendrils and internodes of +Ampelopsis have little or no power of revolving; the tendrils are but +little sensitive to contact; their hooked extremities cannot seize +thin objects; they will not even clasp a stick, unless in extreme +need of a support; but they turn from the light to the dark, and, +spreading out their branches in contact with any nearly flat surface, +develop discs. These adhere by the secretion of some cement to a +wall, or even to a polished surface; and this is more than the discs +of the Bignonia capreolata can effect. + +The rapid development of these adherent discs is one of the most +remarkable peculiarities possessed by any tendrils. We have seen +that such discs are formed by two species of Bignonia, by Ampelopsis, +and, according to Naudin, {37} by the Cucurbitaceous genus Peponopsis +adhaerens. In Anguria the lower surface of the tendril, after it has +wound round a stick, forms a coarsely cellular layer, which closely +fits the wood, but is not adherent; whilst in Hanburya a similar +layer is adherent. The growth of these cellular out-growths depends, +(except in the case of the Haplolophium and of one species of +Ampelopsis,) on the stimulus from contact. It is a singular fact +that three families, so widely distinct as the Bignoniaceae, +Vitaceae, and Cucurbitaceae, should possess species with tendrils +having this remarkable power. + + +Sachs attributes all the movements of tendrils to rapid growth on the +side opposite to that which becomes concave. These movements consist +of revolving nutation, the bending to and from the light, and in +opposition to gravity, those caused by a touch, and spiral +contraction. It is rash to differ from so great an authority, but I +cannot believe that one at least of these movements--curvature from a +touch--is thus caused. {38} In the first place it may be remarked +that the movement of nutation differs from that due to a touch, in so +far that in some cases the two powers are acquired by the same +tendril at different periods of growth; and the sensitive part of the +tendril does not seem capable of nutation. One of my chief reasons +for doubting whether the curvature from a touch is the result of +growth, is the extraordinary rapidity of the movement. I have seen +the extremity of a tendril of Passiflora gracilis, after being +touched, distinctly bent in 25 seconds, and often in 30 seconds; and +so it is with the thicker tendril of Sicyos. It appears hardly +credible that their outer surfaces could have actually grown in +length, which implies a permanent modification of structure, in so +short a time. The growth, moreover, on this view must be +considerable, for if the touch has been at all rough the extremity is +coiled in two or three minutes into a spire of several turns. + +When the extreme tip of the tendril of Echinocystis caught hold of a +smooth stick, it coiled itself in a few hours (as described at p. +132) twice or thrice round the stick, apparently by an undulatory +movement. At first I attributed this movement to the growth of the +outside; black marks were therefore made, and the interspaces +measured, but I could not thus detect any increase in length. Hence +it seems probable in this case and in others, that the curvature of +the tendril from a touch depends on the contraction of the cells +along the concave side. Sachs himself admits {39} that "if the +growth which takes place in the entire tendril at the time of contact +with a support is small, a considerable acceleration occurs on the +convex surface, but in general there is no elongation on the concave +surface, or there may even be a contraction; in the case of a tendril +of Cucurbita this contraction amounted to nearly one-third of the +original length." In a subsequent passage Sachs seems to feel some +difficulty in accounting for this kind of contraction. It must not +however be supposed from the foregoing remarks that I entertain any +doubt, after reading De Vries' observations, about the outer and +stretched surfaces of attached tendrils afterwards increasing in +length by growth. Such increase seems to me quite compatible with +the first movement being independent of growth. Why a delicate touch +should cause one side of a tendril to contract we know as little as +why, on the view held by Sachs, it should lead to extraordinarily +rapid growth of the opposite side. The chief or sole reason for the +belief that the curvature of a tendril when touched is due to rapid +growth, seems to be that tendrils lose their sensitiveness and power +of movement after they have grown to their full length; but this fact +is intelligible, if we bear in mind that all the functions of a +tendril are adapted to drag up the terminal growing shoot towards the +light. Of what use would it be, if an old and full-grown tendril, +arising from the lower part of a shoot, were to retain its power of +clasping a support? This would be of no use; and we have seen with +tendrils so many instances of close adaptation and of the economy of +means, that we may feel assured that they would acquire irritability +and the power of clasping a support at the proper age--namely, youth- +-and would not uselessly retain such power beyond the proper age. + + + +CHAPTER V.--HOOK AND ROOT-CLIMBERS.--CONCLUDING REMARKS. + + + +Plants climbing by the aid of hooks, or merely scrambling over other +plants--Root-climbers, adhesive matter secreted by the rootlets-- +General conclusions with respect to climbing plants, and the stages +of their development. + +Hook-Climbers.--In my introductory remarks, I stated that, besides +the two first great classes of climbing plants, namely, those which +twine round a support, and those endowed with irritability enabling +them to seize hold of objects by means of their petioles or tendrils, +there are two other classes, hook-climbers and root-climbers. Many +plants, moreover, as Fritz Muller has remarked, {40} climb or +scramble up thickets in a still more simple fashion, without any +special aid, excepting that their leading shoots are generally long +and flexible. It may, however, be suspected from what follows, that +these shoots in some cases tend to avoid the light. The few hook- +climbers which I have observed, namely, Galium aparine, Rubus +australis, and some climbing Roses, exhibit no spontaneous revolving +movement. If they had possessed this power, and had been capable of +twining, they would have been placed in the class of Twiners; for +some twiners are furnished with spines or hooks, which aid them in +their ascent. For instance, the Hop, which is a twiner, has reflexed +hooks as large as those of the Galium; some other twiners have stiff +reflexed hairs; and Dipladenia has a circle of blunt spines at the +bases of its leaves. I have seen only one tendril-bearing plant, +namely, Smilax aspera, which is furnished with reflexed spines; but +this is the case with several branch-climbers in South Brazil and +Ceylon; and their branches graduate into true tendrils. Some few +plants apparently depend solely on their hooks for climbing, and yet +do so efficiently, as certain palms in the New and Old Worlds. Even +some climbing Roses will ascend the walls of a tall house, if covered +with a trellis. How this is effected I know not; for the young +shoots of one such Rose, when placed in a pot in a window, bent +irregularly towards the light during the day and from the light +during the night, like the shoots of any common plant; so that it is +not easy to understand how they could have got under a trellis close +to the wall. {41} + +Root-climbers.--A good many plants come under this class, and are +excellent climbers. One of the most remarkable is the Marcgravia +umbellata, the stem of which in the tropical forests of South +America, as I hear from Mr. Spruce, grows in a curiously flattened +manner against the trunks of trees; here and there it puts forth +claspers (roots), which adhere to the trunk, and, if the latter be +slender, completely embrace it. When this plant has climbed to the +light, it produces free branches with rounded stems, clad with sharp- +pointed leaves, wonderfully different in appearance from those borne +by the stem as long as it remains adherent. This surprising +difference in the leaves, I have also observed in a plant of +Marcgravia dubia in my hothouse. Root-climbers, as far as I have +seen, namely, the Ivy (Hedera helix), Ficus repens, and F. barbatus, +have no power of movement, not even from the light to the dark. As +previously stated, the Hoya carnosa (Asclepiadaceae) is a spiral +twiner, and likewise adheres by rootlets even to a flat wall. The +tendril-bearing Bignonia Tweedyana emits roots, which curve half +round and adhere to thin sticks. The Tecoma radicans (Bignoniaceae), +which is closely allied to many spontaneously revolving species, +climbs by rootlets; nevertheless, its young shoots apparently move +about more than can be accounted for by the varying action of the +light. + +I have not closely observed many root-climbers, but can give one +curious fact. Ficus repens climbs up a wall just like Ivy; and when +the young rootlets are made to press lightly on slips of glass, they +emit after about a week's interval, as I observed several times, +minute drops of clear fluid, not in the least milky like that exuded +from a wound. This fluid is slightly viscid, but cannot be drawn out +into threads. It has the remarkable property of not soon drying; a +drop, about the size of half a pin's head, was slightly spread out on +glass, and I scattered on it some minute grains of sand. The glass +was left exposed in a drawer during hot and dry weather, and if the +fluid had been water, it would certainly have dried in a few minutes; +but it remained fluid, closely surrounding each grain of sand, during +128 days: how much longer it would have remained I cannot say. Some +other rootlets were left in contact with the glass for about ten days +or a fortnight, and the drops of secreted fluid were now rather +larger, and so viscid that they could be drawn out into threads. +Some other rootlets were left in contact during twenty-three days, +and these were firmly cemented to the glass. Hence we may conclude +that the rootlets first secrete a slightly viscid fluid, subsequently +absorb the watery parts, (for we have seen that the fluid will not +dry by itself,) and ultimately leave a cement. When the rootlets +were torn from the glass, atoms of yellowish matter were left on it, +which were partly dissolved by a drop of bisulphide of carbon; and +this extremely volatile fluid was rendered very much less volatile by +what it had dissolved. + +As the bisulphide of carbon has a strong power of softening indurated +caoutchouc, I soaked in it during a short time several rootlets of a +plant which had grown up a plaistered wall; and I then found many +extremely thin threads of transparent, not viscid, excessively +elastic matter, precisely like caoutchouc, attached to two sets of +rootlets on the same branch. These threads proceeded from the bark +of the rootlet at one end, and at the other end were firmly attached +to particles of silex or mortar from the wall. There could be no +mistake in this observation, as I played with the threads for a long +time under the microscope, drawing them out with my dissecting- +needles and letting them spring back again. Yet I looked repeatedly +at other rootlets similarly treated, and could never again discover +these elastic threads. I therefore infer that the branch in question +must have been slightly moved from the wall at some critical period, +whilst the secretion was in the act of drying, through the absorption +of its watery parts. The genus Ficus abounds with caoutchouc, and we +may conclude from the facts just given that this substance, at first +in solution and ultimately modified into an unelastic cement, {42} is +used by the Ficus repens to cement its rootlets to any surface which +it ascends. Whether other plants, which climb by their rootlets, +emit any cement I do not know; but the rootlets of the Ivy, placed +against glass, barely adhered to it, yet secreted a little yellowish +matter. I may add, that the rootlets of the Marcgravia dubia can +adhere firmly to smooth painted wood. + +Vanilla aromatica emits aerial roots a foot in length, which point +straight down to the ground. According to Mohl (p. 49), these crawl +into crevices, and when they meet with a thin support, wind round it, +as do tendrils. A plant which I kept was young, and did not form +long roots; but on placing thin sticks in contact with them, they +certainly bent a little to that side, in the course of about a day, +and adhered by their rootlets to the wood; but they did not bend +quite round the sticks, and afterwards they re-pursued their downward +course. It is probable that these slight movements of the roots are +due to the quicker growth of the side exposed to the light, in +comparison with the other side, and not because the roots are +sensitive to contact in the same manner as true tendrils. According +to Mohl, the rootlets of certain species of Lycopodium act as +tendrils. {43} + + +Concluding Remarks on Climbing Plants. + + +Plants become climbers, in order, as it may be presumed, to reach the +light, and to expose a large surface of their leaves to its action +and to that of the free air. This is effected by climbers with +wonderfully little expenditure of organized matter, in comparison +with trees, which have to support a load of heavy branches by a +massive trunk. Hence, no doubt, it arises that there are so many +climbing plants in all quarters of the world, belonging to so many +different orders. These plants have been arranged under four +classes, disregarding those which merely scramble over bushes without +any special aid. Hook-climbers are the least efficient of all, at +least in our temperate countries, and can climb only in the midst of +an entangled mass of vegetation. Root-climbers are excellently +adapted to ascend naked faces of rock or trunks of trees; when, +however, they climb trunks they are compelled to keep much in the +shade; they cannot pass from branch to branch and thus cover the +whole summit of a tree, for their rootlets require long-continued and +close contact with a steady surface in order to adhere. The two +great classes of twiners and of plants with sensitive organs, namely, +leaf-climbers and tendril-bearers taken together, far exceed in +number and in the perfection of their mechanism the climbers of the +two first classes. Those which have the power of spontaneously +revolving and of grasping objects with which they come in contact, +easily pass from branch to branch, and securely ramble over a wide, +sun-lit surface. + +The divisions containing twining plants, leaf-climbers, and tendril- +bearers graduate to a certain extent into one another, and nearly all +have the same remarkable power of spontaneously revolving. Does this +gradation, it may be asked, indicate that plants belonging to one +subdivision have actually passed during the lapse of ages, or can +pass, from one state to the other? Has, for instance, any tendril- +bearing plant assumed its present structure without having previously +existed as a leaf-climber or a twiner? If we consider leaf-climbers +alone, the idea that they were primordially twiners is forcibly +suggested. The internodes of all, without exception, revolve in +exactly the same manner as twiners; some few can still twine well, +and many others in an imperfect manner. Several leaf-climbing genera +are closely allied to other genera which are simple twiners. It +should also be observed, that the possession of leaves with sensitive +petioles, and with the consequent power of clasping an object, would +be of comparatively little use to a plant, unless associated with +revolving internodes, by which the leaves are brought into contact +with a support; although no doubt a scrambling plant would be apt, as +Professor Jaeger has remarked, to rest on other plants by its leaves. +On the other hand, revolving internodes, without any other aid, +suffice to give the power of climbing; so that it seems probable that +leaf-climbers were in most cases at first twiners, and subsequently +became capable of grasping a support; and this, as we shall presently +see, is a great additional advantage. + +From analogous reasons, it is probable that all tendril-bearers were +primordially twiners, that is, are the descendants of plants having +this power and habit. For the internodes of the majority revolve; +and, in a few species, the flexible stem still retains the capacity +of spirally twining round an upright stick. Tendril-bearers have +undergone much more modification than leaf-climbers; hence it is not +surprising that their supposed primordial habits of revolving and +twining have been more frequently lost or modified than in the case +of leaf-climbers. The three great tendril-bearing families in which +this loss has occurred in the most marked manner, are the +Cucurbitaceae, Passifloraceae, and Vitaceae. In the first, the +internodes revolve; but I have heard of no twining form, with the +exception (according to Palm, p. 29. 52) of Momordica balsamina, and +this is only an imperfect twiner. In the two other families I can +hear of no twiners; and the internodes rarely have the power of +revolving, this power being confined to the tendrils. The +internodes, however, of Passiflora gracilis have the power in a +perfect manner, and those of the common Vine in an imperfect degree: +so that at least a trace of the supposed primordial habit has been +retained by some members of all the larger tendril-bearing groups. + +On the view here given, it may be asked, Why have the species which +were aboriginally twiners been converted in so many groups into leaf- +climbers or tendril-bearers? Of what advantage has this been to +them? Why did they not remain simple twiners? We can see several +reasons. It might be an advantage to a plant to acquire a thicker +stem, with short internodes bearing many or large leaves; and such +stems are ill fitted for twining. Any one who will look during windy +weather at twining plants will see that they are easily blown from +their support; not so with tendril-bearers or leaf-climbers, for they +quickly and firmly grasp their support by a much more efficient kind +of movement. In those plants which still twine, but at the same time +possess tendrils or sensitive petioles, as some species of Bignonia, +Clematis, and Tropaeolum, it can readily be observed how incomparably +better they grasp an upright stick than do simple twiners. Tendrils, +from possessing this power of grasping an object, can be made long +and thin; so that little organic matter is expended in their +development, and yet they sweep a wide circle in search of a support. +Tendril-bearers can, from their first growth, ascend along the outer +branches of any neighbouring bush, and they are thus always fully +exposed to the light; twiners, on the contrary, are best fitted to +ascend bare stems, and generally have to start in the shade. Within +tall and dense tropical forests, twining plants would probably +succeed better than most kinds of tendril-bearers; but the majority +of twiners, at least in our temperate regions, from the nature of +their revolving movement, cannot ascend thick trunks, whereas this +can be affected by tendril-bearers if the trunks are branched or bear +twigs, and by some species if the bark is rugged. + +The advantage gained by climbing is to reach the light and free air +with as little expenditure of organic matter as possible; now, with +twining plants, the stem is much longer than is absolutely necessary; +for instance, I measured the stem of a kidney-bean, which had +ascended exactly two feet in height, and it was three feet in length: +the stem of a pea, on the other hand, which had ascended to the same +height by the aid of its tendrils, was but little longer than the +height reached. That this saving of the stem is really an advantage +to climbing plants, I infer from the species that still twine but are +aided by clasping petioles or tendrils, generally making more open +spires than those made by simple twiners. Moreover, the plants thus +aided, after taking one or two turns in one direction, generally +ascend for a space straight, and then reverse the direction of their +spire. By this means they ascend to a considerably greater height, +with the same length of stem, than would otherwise have been +possible; and they do this with safety, as they secure themselves at +intervals by their clasping petioles or tendrils. + +We have seen that tendrils consist of various organs in a modified +state, namely, leaves, flower-peduncles, branches, and perhaps +stipules. With respect to leaves, the evidence of their modification +is ample. In young plants of Bignonia the lower leaves often remain +quite unchanged, whilst the upper ones have their terminal leaflets +converted into perfect tendrils; in Eccremocarpus I have seen a +single lateral branch of a tendril replaced by a perfect leaflet; in +Vicia sativa, on the other hand, leaflets are sometimes replaced by +tendril-branches; and many other such cases could be given. But he +who believes in the slow modification of species will not be content +simply to ascertain the homological nature of different kinds of +tendrils; he will wish to learn, as far as is possible, by what +actual steps leaves, flower-peduncles, &c., have had their functions +wholly changed, and have come to serve merely as prehensile organs. + +In the whole group of leaf-climbers abundant evidence has been given +that an organ, still subserving the functions of a leaf, may become +sensitive to a touch, and thus grasp an adjoining object. With +several leaf-climbers the true leaves spontaneously revolve; and +their petioles, after clasping a support grow thicker and stronger. +We thus see that leaves may acquire all the leading and +characteristic qualities of tendrils, namely, sensitiveness, +spontaneous movement, and subsequently increased strength. If their +blades or laminae were to abort, they would form true tendrils. And +of this process of abortion we can follow every step, until no trace +of the original nature of the tendril is left. In Mutisia clematis, +the tendril, in shape and colour, closely resembles the petiole of +one of the ordinary leaves, together with the midribs of the +leaflets, but vestiges of the laminae are still occasionally +retained. In four genera of the Fumariaceae we can follow the whole +process of transformation. The terminal leaflets of the leaf- +climbing Fumaria officinalis are not smaller than the other leaflets; +those of the leaf-climbing Adlumia cirrhosa are greatly reduced; +those of Corydalis claviculata (a plant which may indifferently be +called a leaf-climber or a tendril-bearer) are either reduced to +microscopical dimensions or have their blades wholly aborted, so that +this plant is actually in a state of transition; and, finally, in the +Dicentra the tendrils are perfectly characterized. If, therefore, we +could behold at the same time all the progenitors of Dicentra, we +should almost certainly see a series like that now exhibited by the +above-named three genera. In Tropaeolum tricolorum we have another +kind of passage; for the leaves which are first formed on the young +stems are entirely destitute of laminae, and must be called tendrils, +whilst the later formed leaves have well-developed laminae. In all +cases the acquirement of sensitiveness by the mid-ribs of the leaves +appears to stand in some close relation with the abortion of their +laminae or blades. + +On the view here given, leaf-climbers were primordially twiners, and +tendril-bearers (when formed of modified leaves) were primordially +leaf-climbers. The latter, therefore, are intermediate in nature +between twiners and tendril-bearers, and ought to be related to both. +This is the case: thus the several leaf-climbing species of the +Antirrhineae, of Solanum, Cocculus, and Gloriosa, have within the +same family and even within the same genus, relatives which are +twiners. In the genus Mikania, there are leaf-climbing and twining +species. The leaf-climbing species of Clematis are very closely +allied to the tendril-bearing Naravelia. The Fumariaceae include +closely allied genera which are leaf-climbers and tendril-bearers. +Lastly, a species of Bignonia is at the same time both a leaf-climber +and a tendril-bearer; and other closely allied species are twiners. + +Tendrils of another kind consist of modified flower-peduncles. In +this case we likewise have many interesting transitional states. The +common Vine (not to mention the Cardiospermum) gives us every +possible gradation between a perfectly developed tendril and a +flower-peduncle covered with flowers, yet furnished with a branch, +forming the flower-tendril. When the latter itself bears a few +flowers, as we know sometimes is the case, and still retains the +power of clasping a support, we see an early condition of all those +tendrils which have been formed by the modification of flower- +peduncles. + +According to Mohl and others, some tendrils consist of modified +branches: I have not observed any such cases, and know nothing of +their transitional states, but these have been fully described by +Fritz Muller. The genus Lophospermum also shows us how such a +transition is possible; for its branches spontaneously revolve and +are sensitive to contact. Hence, if the leaves on some of the +branches of the Lophospermum were to abort, these branches would be +converted into true tendrils. Nor is there anything improbable in +certain branches alone being thus modified, whilst others remained +unaltered; for we have seen with certain varieties of Phaseolus, that +some of the branches are thin, flexible, and twine, whilst other +branches on the same plant are stiff and have no such power. + +If we inquire how a petiole, a branch or flower-peduncle first became +sensitive to a touch, and acquired the power of bending towards the +touched side, we get no certain answer. Nevertheless an observation +by Hofmeister {44} well deserves attention, namely, that the shoots +and leaves of all plants, whilst young, move after being shaken. +Kerner also finds, as we have seen, that the flower-peduncles of a +large number of plants, if shaken or gently rubbed bend to this side. +And it is young petioles and tendrils, whatever their homological +nature may be, which move on being touched. It thus appears that +climbing plants have utilized and perfected a widely distributed and +incipient capacity, which capacity, as far as we can see, is of no +service to ordinary plants. If we further inquire how the stems, +petioles, tendrils, and flower-peduncles of climbing plants first +acquired their power of spontaneously revolving, or, to speak more +accurately, of successively bending to all points of the compass, we +are again silenced, or at most can only remark that the power of +moving, both spontaneously and from various stimulants, is far more +common with plants, than is generally supposed to be the case by +those who have not attended to the subject. I have given one +remarkable instance, namely that of the Maurandia semperflorens, the +young flower-peduncles of which spontaneously revolve in very small +circles, and bend when gently rubbed to the touched side; yet this +plant certainly does not profit by these two feebly developed powers. +A rigorous examination of other young plants would probably show +slight spontaneous movements in their stems, petioles or peduncles, +as well as sensitiveness to a touch. {45} We see at least that the +Maurandia might, by a little augmentation of the powers which it +already possesses, come first to grasp a support by its flower- +peduncles, and then, by the abortion of some of its flowers (as with +Vitis or Cardiospermum), acquire perfect tendrils. + +There is one other interesting point which deserves notice. We have +seen that some tendrils owe their origin to modified leaves, and +others to modified flower-peduncles; so that some are foliar and +others axial in their nature. It might therefore have been expected +that they would have presented some difference in function. This is +not the case. On the contrary, they present the most complete +identity in their several characteristic powers. Tendrils of both +kinds spontaneously revolve at about the same rate. Both, when +touched, bend quickly to the touched side, and afterwards recover +themselves and are able to act again. In both the sensitiveness is +either confined to one side or extends all round the tendril. Both +are either attracted or repelled by the light. The latter property +is seen in the foliar tendrils of Bignonia capreolata and in the +axial tendrils of Ampelopsis. The tips of the tendrils in these two +plants become, after contact, enlarged into discs, which are at first +adhesive by the secretion of some cement. Tendrils of both kinds, +soon after grasping a support, contract spirally; they then increase +greatly in thickness and strength. When we add to these several +points of identity the fact that the petiole of Solanum jasminoides, +after it has clasped a support, assumes one of the most +characteristic features of the axis, namely, a closed ring of woody +vessels, we can hardly avoid asking, whether the difference between +foliar and axial organs can be of so fundamental a nature as is +generally supposed? {46} + +We have attempted to trace some of the stages in the genesis of +climbing plants. But, during the endless fluctuations of the +conditions of life to which all organic beings have been exposed, it +might be expected that some climbing plants would have lost the habit +of climbing. In the cases given of certain South African plants +belonging to great twining families, which in their native country +never twine, but reassume this habit when cultivated in England, we +have a case in point. In the leaf-climbing Clematis flammula, and in +the tendril-bearing Vine, we see no loss in the power of climbing, +but only a remnant of the revolving power which is indispensable to +all twiners, and is so common as well as so advantageous to most +climbers. In Tecoma radicans, one of the Bignoniaceae, we see a last +and doubtful trace of the power of revolving. + +With respect to the abortion of tendrils, certain cultivated +varieties of Cucurbita pepo have, according to Naudin, {47} either +quite lost these organs or bear semi-monstrous representatives of +them. In my limited experience, I have met with only one apparent +instance of their natural suppression, namely, in the common bean. +All the other species of Vicia, I believe, bear tendrils; but the +bean is stiff enough to support its own stem, and in this species, at +the end of the petiole, where, according to analogy, a tendril ought +to have existed, a small pointed filament projects, about a third of +an inch in length, and which is probably the rudiment of a tendril. +This may be the more safely inferred, as in young and unhealthy +specimens of other tendril-bearing plants similar rudiments may +occasionally be observed. In the bean these filaments are variable +in shape, as is so frequently the case with rudimentary organs; they +are either cylindrical, or foliaceous, or are deeply furrowed on the +upper surface. They have not retained any vestige of the power of +revolving. It is a curious fact, that many of these filaments, when +foliaceous, have on their lower surfaces, dark-coloured glands like +those on the stipules, which excrete a sweet fluid; so that these +rudiments have been feebly utilized. + +One other analogous case, though hypothetical, is worth giving. +Nearly all the species of Lathyrus possesses tendrils; but L. +nissolia is destitute of them. This plant has leaves, which must +have struck everyone with surprise who has noticed them, for they are +quite unlike those of all common papilionaceous plants, and resemble +those of a grass. In another species, L. aphaca, the tendril, which +is not highly developed (for it is unbranched, and has no spontaneous +revolving-power), replaces the leaves, the latter being replaced in +function by large stipules. Now if we suppose the tendrils of L. +aphaca to become flattened and foliaceous, like the little +rudimentary tendrils of the bean, and the large stipules to become at +the same time reduced in size, from not being any longer wanted, we +should have the exact counterpart of L. nissolia, and its curious +leaves are at once rendered intelligible to us. + +It may be added, as serving to sum up the foregoing views on the +origin of tendril-bearing plants, that L. nissolia is probably +descended from a plant which was primordially a twiner; this then +became a leaf-climber, the leaves being afterwards converted by +degrees into tendrils, with the stipules greatly increased in size +through the law of compensation. {48} After a time the tendrils lost +their branches and became simple; they then lost their revolving- +power (in which state they would have resembled the tendrils of the +existing L. aphaca), and afterwards losing their prehensile power and +becoming foliaceous would no longer be thus designated. In this last +stage (that of the existing L. nissolia) the former tendrils would +reassume their original function of leaves, and the stipules which +were recently much developed being no longer wanted, would decrease +in size. If species become modified in the course of ages, as almost +all naturalists now admit, we may conclude that L. nissolia has +passed through a series of changes, in some degree like those here +indicated. + +The most interesting point in the natural history of climbing plants +is the various kinds of movement which they display in manifest +relation to their wants. The most different organs--stems, branches, +flower-peduncles, petioles, mid-ribs of the leaf and leaflets, and +apparently aerial roots--all possess this power. + +The first action of a tendril is to place itself in a proper +position. For instance, the tendril of Cobaea first rises vertically +up, with its branches divergent and with the terminal hooks turned +outwards; the young shoot at the extremity of the stem is at the same +time bent to one side, so as to be out of the way. The young leaves +of Clematis, on the other hand, prepare for action by temporarily +curving themselves downwards, so as to serve as grapnels. + +Secondly, if a twining plant or a tendril gets by any accident into +an inclined position, it soon bends upwards, though secluded from the +light. The guiding stimulus no doubt is the attraction of gravity, +as Andrew Knight showed to be the case with germinating plants. If a +shoot of any ordinary plant be placed in an inclined position in a +glass of water in the dark, the extremity will, in a few hours, bend +upwards; and if the position of the shoot be then reversed, the +downward-bent shoot reverses its curvature; but if the stolen of a +strawberry, which has no tendency to grow upwards, be thus treated, +it will curve downwards in the direction of, instead of in opposition +to, the force of gravity. As with the strawberry, so it is generally +with the twining shoots of the Hibbertia dentata, which climbs +laterally from bush to bush; for these shoots, if placed in a +position inclined downwards, show little and sometimes no tendency to +curve upwards. + +Thirdly, climbing plants, like other plants, bend towards the light +by a movement closely analogous to the incurvation which causes them +to revolve, so that their revolving movement is often accelerated or +retarded in travelling to or from the light. On the other hand, in a +few instances tendrils bend towards the dark. + +Fourthly, we have the spontaneous revolving movement which is +independent of any outward stimulus, but is contingent on the youth +of the part, and on vigorous health; and this again of course depends +on a proper temperature and other favourable conditions of life. + +Fifthly, tendrils, whatever their homological nature may be, and the +petioles or tips of the leaves of leaf-climbers, and apparently +certain roots, all have the power of movement when touched, and bend +quickly towards the touched side. Extremely slight pressure often +suffices. If the pressure be not permanent, the part in question +straightens itself and is again ready to bend on being touched. + +Sixthly, and lastly, tendrils, soon after clasping a support, but not +after a mere temporary curvature, contract spirally. If they have +not come into contact with any object, they ultimately contract +spirally, after ceasing to revolve; but in this case the movement is +useless, and occurs only after a considerable lapse of time. + +With respect to the means by which these various movements are +effected, there can be little doubt from the researches of Sachs and +H. de Vries, that they are due to unequal growth; but from the +reasons already assigned, I cannot believe that this explanation +applies to the rapid movements from a delicate touch. + +Finally, climbing plants are sufficiently numerous to form a +conspicuous feature in the vegetable kingdom, more especially in +tropical forests. America, which so abounds with arboreal animals, +as Mr. Bates remarks, likewise abounds according to Mohl and Palm +with climbing plants; and of the tendril-bearing plants examined by +me, the highest developed kinds are natives of this grand continent, +namely, the several species of Bignonia, Eccremocarpus, Cobaea, and +Ampelopsis. But even in the thickets of our temperate regions the +number of climbing species and individuals is considerable, as will +be found by counting them. They belong to many and widely different +orders. To gain some rude idea of their distribution in the +vegetable series, I marked, from the lists given by Mohl and Palm +(adding a few myself, and a competent botanist, no doubt, could have +added many more), all those families in Lindley's 'Vegetable Kingdom' +which include twiners, leaf-climbers, or tendril-bearers. Lindley +divides Phanerogamic plants into fifty-nine Alliances; of these, no +less than thirty-five include climbing plants of the above kinds, +hook and root-climbers being excluded. To these a few Cryptogamic +plants must be added. When we reflect on the wide separation of +these plants in the series, and when we know that in some of the +largest, well-defined orders, such as the Compositae, Rubiaceae, +Scrophulariaceae, Liliaceae, &c., species in only two or three genera +have the power of climbing, the conclusion is forced on our minds +that the capacity of revolving, on which most climbers depend, is +inherent, though undeveloped, in almost every plant in the vegetable +kingdom. + +It has often been vaguely asserted that plants are distinguished from +animals by not having the power of movement. It should rather be +said that plants acquire and display this power only when it is of +some advantage to them; this being of comparatively rare occurrence, +as they are affixed to the ground, and food is brought to them by the +air and rain. We see how high in the scale of organization a plant +may rise, when we look at one of the more perfect tendril-bearers. +It first places its tendrils ready for action, as a polypus places +its tentacula. If the tendril be displaced, it is acted on by the +force of gravity and rights it self. It is acted on by the light, +and bends towards or from it, or disregards it, whichever may be most +advantageous. During several days the tendrils or internodes, or +both, spontaneously revolve with a steady motion. The tendril +strikes some object, and quickly curls round and firmly grasps it. +In the course of some hours it contracts into a spire, dragging up +the stem, and forming an excellent spring. All movements now cease. +By growth the tissues soon become wonderfully strong and durable. +The tendril has done its work, and has done it in an admirable +manner. + + + +Footnotes: + +{1} An English translation of the 'Lehrbuch der Botanik' by +Professor Sachs, has recently (1875), appeared under the title of +'Text-Book of Botany,' and this is a great boon to all lovers of +natural science in England. + +{2} 'Proc. Amer. Acad. of Arts and Sciences,' vol. iv. Aug. 12, +1858, p. 98. + +{3} Ludwig H. Palm, 'Ueber das Winden der Pflanzen;' Hugo von Mohl, +'Ueber den Bau und des Winden der Ranken und Schlingpflanzen,' 1827. +Palm's Treatise was published only a few weeks before Mohl's. See +also 'The Vegetable Cell' (translated by Henfrey), by H. von Mohl, p. +147 to end. + +{4} "Des Mouvements revolutife Respontanes," &c., 'Comptes Rendus,' +tom. xvii. (1843) p. 989; "Recherches sur la Volubilite des Tiges," +&c., tom. xix. (1844) p. 295. + +{5} 'Bull. Bot Soc. de France,' tom. v. 1858, p. 356. + +{6} This whole subject has been ably discussed and explained by H. +de Vries, 'Arbeiten des Bot. Instituts in Wurzburg,' Heft iii. pp. +331, 336. See also Sachs ('Text-Book of Botany,' English +translation, 1875, p. 770), who concludes "that torsion is the result +of growth continuing in the outer layers after it has ceased or begun +to cease in the inner layers." + +{7} Professor Asa Gray has remarked to me, in a letter, that in +Thuja occidentalis the twisting of the bark is very conspicuous. The +twist is generally to the right of the observer; but, in noticing +about a hundred trunks, four or five were observed to be twisted in +an opposite direction. The Spanish chestnut is often much twisted: +there is an interesting article on this subject in the 'Scottish +Farmer,' 1865, p. 833. + +{8} It is well known that the stems of many plants occasionally +become spirally twisted in a monstrous manner; and after my paper was +read before the Linnean Society, Dr. Maxwell Masters remarked to me +in a letter that "some of these cases, if not all, are dependent upon +some obstacle or resistance to their upward growth." This conclusion +agrees with what I have said about the twisting of stems, which have +twined round rugged supports; but does not preclude the twisting +being of service to the plant by giving greater rigidity to the stem. + +{9} The view that the revolving movement or nutation of the stems of +twining plants is due to growth is that advanced by Sachs and H. de +Vries; and the truth of this view is proved by their excellent +observations. + +{10} The mechanism by which the end of the shoot remains hooked +appears to be a difficult and complex problem, discussed by Dr. H. de +Vries (ibid. p. 337): he concludes that "it depends on the relation +between the rapidity of torsion and the rapidity of nutation." + +{11} Dr. H. de Vries also has shown (ibid. p. 321 and 325) by a +better method than that employed by me, that the stems of twining +plants are not irritable, and that the cause of their winding up a +support is exactly what I have described. + +{12} Dr. H. de Vries states (ibid. p. 322) that the stem of Cuscuta +is irritable like a tendril. + +{13} See Dr. H. de Vries (ibid. p. 324) on this subject. + +{14} Comptes Rendus, 1844, tom. xix. p. 295, and Annales des Sc. Nat +3rd series, Bot., tom. ii. p. 163. + +{15} I am much indebted to Dr. Hooker for having sent me many plants +from Kew; and to Mr. Veitch, of the Royal Exotic Nursery, for having +generously given me a collection of fine specimens of climbing +plants. Professor Asa Gray, Prof. Oliver, and Dr. Hooker have +afforded me, as on many previous occasions, much information and many +references. + +{16} Journal of the Linn. Soc. (Bot.) vol. ix. p. 344. I shall have +occasion often to quote this interesting paper, in which he corrects +or confirms various statements made by me. + +{17} I raised nine plants of the hybrid Loasa Herbertii, and six of +these also reversed their spire in ascending a support. + +{18} In another genus, namely Davilla, belonging to the same family +with Hibbertia, Fritz Muller says (ibid. p. 349) that "the stem +twines indifferently from left to right, or from right to left; and I +once saw a shoot which ascended a tree about five inches in diameter, +reverse its course in the same manner as so frequently occurs with +Loasa." + +{19} Fritz Muller states (ibid. p. 349) that he saw on one occasion +in the forests of South Brazil a trunk about five feet in +circumference spirally ascended by a plant, apparently belonging to +the Menispermaceae. He adds in his letter to me that most of the +climbing plants which there ascend thick trees, are root-climbers; +some being tendril-bearers. + +{20} Fritz Muller has published some interesting facts and views on +the structure of the wood of climbing plants in 'Bot. Zeitung,' 1866, +pp. 57, 66. + +{21} It appears from A. Kerner's interesting observations, that the +flower-peduncles of a large number of plants are irritable, and bend +when they are rubbed or shaken: Die Schutzmittel des Pollens, 1873, +p. 34. + +{22} I have already referred to the case of the twining stem of +Cuscuta, which, according to H. de Vries (ibid. p. 322) is sensitive +to a touch like a tendril. + +{23} Dr. Maxwell Masters informs me that in almost all petioles +which are cylindrical, such as those bearing peltate leaves, the +woody vessels form a closed ring; semilunar bands of vessels being +confined to petioles which are channelled along their upper surfaces. +In accordance with this statement, it may be observed that the +enlarged and clasped petiole of the Solanum, with its closed ring of +woody vessels, has become more cylindrical than it was in its +original unclasped condition. + +{24} Never having had the opportunity of examining tendrils produced +by the modification of branches, I spoke doubtfully about them in +this essay when originally published. But since then Fritz Muller +has described (Journal of Linn. Soc. vol. ix. p. 344) many striking +cases in South Brazil. In speaking of plants which climb by the aid +of their branches, more or less modified, he states that the +following stages of development can be traced: (1.) Plants +supporting themselves simply by their branches stretched out at right +angles--for example, Chiococca. (2.) Plants clasping a support with +their unmodified branches, as with Securidaca. (3.) Plants climbing +by the extremities of their branches which appear like tendrils, as +is the case according to Endlicher with Helinus. (4.) Plants with +their branches much modified and temporarily converted into tendrils, +but which may be again transformed into branches, as with certain +Papilionaceous plants. (5.) Plants with their branches forming true +tendrils, and used exclusively for climbing--as with Strychnos and +Caulotretus. Even the unmodified branches become much thickened when +they wind round a support. I may add that Mr. Thwaites sent me from +Ceylon a specimen of an Acacia which had climbed up the trunk of a +rather large tree, by the aid of tendril-like, curved or convoluted +branchlets, arrested in their growth and furnished with sharp +recurved hooks. + +{25} As far as I can make out, the history of our knowledge of +tendrils is as follows:- We have seen that Palm and von Mohl observed +about the same time the singular phenomenon of the spontaneous +revolving movement of twining-plants. Palm (p. 58), I presume, +observed likewise the revolving movement of tendrils; but I do not +feel sure of this, for he says very little on the subject. Dutrochet +fully described this movement of the tendril in the common pea. Mohl +first discovered that tendrils are sensitive to contact; but from +some cause, probably from observing too old tendrils, he was not +aware how sensitive they were, and thought that prolonged pressure +was necessary to excite their movement. Professor Asa Gray, in a +paper already quoted, first noticed the extreme sensitiveness and +rapidity of the movements of the tendrils of certain Cucurbitaceous +plants. + +{26} Fritz Muller states (ibid. p. 348) that in South Brazil the +trifid tendrils of Haplolophium, (one of the Bignoniaceae) without +having come into contact with any object, terminate in smooth shining +discs. These, however, after adhering to any object, sometimes +become considerably enlarged. + +{27} Comptes Rendus, tom. xvii. 1843, p. 989. + +{28} 'Lecons de Botanique,' &c., 1841, p. 170. + +{29} I am indebted to Prof. Oliver for information on this head. In +the Bulletin de la Societe Botanique de France, 1857, there are +numerous discussions on the nature of the tendrils in this family. + +{30} 'Gardeners' Chronicle,' 1864, p. 721. From the affinity of the +Cucurbitaceae to the Passifloraceae, it might be argued that the +tendrils of the former are modified flower-peduncles, as is certainly +the case with those of Passion flowers. Mr. R. Holland (Hardwicke's +'Science-Gossip,' 1865, p. 105) states that "a cucumber grew, a few +years ago in my own garden, where one of the short prickles upon the +fruit had grown out into a long, curled tendril." + +{31} Trans. Phil. Soc. 1812, p. 314. + +{32} Dr. M'Nab remarks (Trans. Bot. Soc. Edinburgh, vol xi. p. 292) +that the tendrils of Amp. Veitchii bear small globular discs before +they have came into contact with any object; and I have since +observed the same fact. These discs, however, increase greatly in +size, if they press against and adhere to any surface. The tendrils, +therefore, of one species of Ampelopsis require the stimulus of +contact for the first development of their discs, whilst those of +another species do not need any such stimulus. We have seen an +exactly parallel case with two species of Bignoniaceae. + +{33} Fritz Muller remarks (ibid. p. 348) that a related genus, +Serjania, differs from Cardiospermum in bearing only a single +tendril; and that the common peduncle contracts spirally, when, as +frequently happens, the tendril has clasped the plant's own stem. + +{34} Prof. Asa Gray informs me that the tendrils of P. sicyoides +revolve even at a quicker rate than those of P. gracilis; four +revolutions were completed (the temperature varying from 88 degrees- +92 degrees Fahr.) in the following times, 40 m., 45 m., 38.5 m., and +46 m. One half-revolution was performed in 15 m. + +{35} See M. Isid. Leon in Bull. Soc. Bot. de France, tom. v. 1858, +p. 650. Dr. H. de Vries points out (p. 306) that I have overlooked, +in the first edition of this essay, the following sentence by Mohl: +"After a tendril has caught a support, it begins in some days to wind +into a spire, which, since the tendril is made fast at both +extremities, must of necessity be in some places to the right, in +others to the left." But I am not surprised that this brief +sentence, without any further explanation did not attract my +attention. + +{36} Sachs, however ('Text-Book of Botany,' Eng. Translation, 1875, +p. 280), has shown that which I overlooked, namely, that the tendrils +of different species are adapted to clasp supports of different +thicknesses. He further shows that after a tendril has clasped a +support it subsequently tightens its hold. + +{37} Annales des Sc. Nat. Bot. 4th series, tom. xii. p. 89. + +{38} It occurred to me that the movement of notation and that from a +touch might be differently affected by anaesthetics, in the same +manner as Paul Bert has shown to be the case with the sleep-movements +of Mimosa and those from a touch. I tried the common pea and +Passiflora gracilis, but I succeeded only in observing that both +movements were unaffected by exposure for 1.5 hrs. to a rather large +dose of sulphuric ether. In this respect they present a wonderful +contrast with Drosera, owing no doubt to the presence of absorbent +glands in the latter plant. + +{39} Text-Book of Botany, 1875, p. 779. + +{40} Journal of Linn. Soc. vol. ix. p. 348. Professor G. Jaeger has +well remarked ('In Sachen Darwin's, insbesondere contra Wigand,' +1874, p. 106) that it is highly characteristic of climbing plants to +produce thin, elongated, and flexible stems. He further remarks that +plants growing beneath other and taller species or trees, are +naturally those which would be developed into climbers; anti such +plants, from stretching towards the light, and from not being much +agitated by the wind, tend to produce long, thin and flexible shoots. + +{41} Professor Asa Gray has explained, as it would appear, this +difficulty in his review (American Journal of Science, vol. xl. Sept. +1865, p. 282) of the present work. He has observed that the strong +summer shoots of the Michigan rose (Rosa setigera) are strongly +disposed to push into dark crevices and away from the light, so that +they would be almost sure to place themselves under a trellis. He +adds that the lateral shoots, made on the following spring emerged +from the trellis as they sought the light. + +{42} Mr. Spiller has recently shown (Chemical Society, Feb. 16, +1865), in a paper on the oxidation of india-rubber or caoutchouc, +that this substance, when exposed in a fine state of division to the +air, gradually becomes converted into brittle, resinous matter, very +similar to shell-lac. + +{43} Fritz Muller informs me that he saw in the forests of South +Brazil numerous black strings, from some lines to nearly an inch in +diameter, winding spirally round the trunks of gigantic trees. At +first sight he thought that they were the stems of twining plants +which were thus ascending the trees: but he afterwards found that +they were the aerial roots of a Philodendron which grew on the +branches above. These roots therefore seem to be true twiners, +though they use their powers to descend, instead of to ascend like +twining plants. The aerial roots of some other species of +Philodendron hang vertically downwards, sometimes for a length of +more than fifty feet. + +{44} Quoted by Cohn, in his remarkable memoir, "Contractile Gewebe +im Pflanzenreiche," 'Abhandl. der Schlesischen Gesell. 1861, Heft i. +s. 35. + +{45} Such slight spontaneous movements, I now find, have been for +some time known to occur, for instance with the flower-stems of +Brassica napus and with the leaves of many plants: Sachs' 'Text-Book +of Botany' 1875, pp. 766, 785. Fritz Muller also has shown in +relation to our present subject ('Jenaischen Zeitschrift,' Bd. V. +Heft 2, p. 133) that the stems, whilst young, of an Alisma and of a +Linum are continually performing slight movements to all points of +the compass, like those of climbing plants. + +{46} Mr. Herbert Spencer has recently argued ('Principles of +Biology,' 1865, p. 37 et seq.) with much force that there is no +fundamental distinction between the foliar and axial organs of +plants. + +{47} Annales des Sc. Nat. 4th series, Bot. tom. vi. 1856, p. 31. + +{48} Moquin-Tandon (Elements de Teratologie. 1841, p. 156) gives the +case of a monstrous bean, in which a case of compensation of this +nature was suddenly effected; for the leaves completely disappeared +and the stipules grew to an enormous size. + + + + + +End of The Project Gutenberg Etext of Climbing Plants by Charles Darwin + |