On the movements and habits of climbing plants/Part 2
It has long been observed that several plants climb by the aid of their leaves, either by the petiole or by the produced midrib; but beyond this simple fact nothing is known of them. Palm and Mohl class these plants with those which bear tendrils; but as a leaf is generally a defined object, the present classification has, at least, some plain advantages. There are other advantages, as leaf-climbers are intermediate in many respects between twiners and certain tendril-bearing plants. I have observed eight species of Clematis and seven of Tropæolum in order to discover what amount of difference there may be within the same genus; and the differences, as we shall see, 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 h. 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 spire and wound two turns in an opposite course. This was rendered possible by the straight piece between the opposed spires having become rigid. The simple, broad, ovate leaves of this tropical species, so unlike those of most of the other species of the genus, with their short thick petioles, seem but ill-fitted for any movement. 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 it becomes straight again. The under side seemed to be the most sensitive; but the sensitiveness or irritability is but slight compared to that which we shall meet
Clematis glandulosa, with two young leaves clasping twigs, with tho clasping portions thickened. with in some of the following species; for a loop of string, weighing 1.64 grain, 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 two twigs on each side of the stem. A forked twig placed so as to lightly press on the under side of a young footstalk caused it, in 12 h., to bend greatly, and ultimately to bent 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 proper position.
The young leaves change their position in a rather odd manner: when first developed the petioles are upturned, 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. If they come into contact with no object, they retain this position for a considerable time, and then bending upwards they reassume their original upturned position, which is retained ever afterwards. The young leaves, being hooked, are thus enabled to catch twigs when brought into contact with them by the revolving movement of the internodes. The petioles which have clasped any object soon become much thickened and strengthened, as may be seen in the diagram.
Clematis montana.—The long and 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 caused them to bend; a loop weighing only one-eighth of a grain sometimes acted and sometimes did not act. The sensitiveness extends to the angle between the stem and leaf-stalk. I may here state that I ascertained the weights of the string and thread used in all cases by carefully weighing 50 inches in a chemical balance, and then cutting off measured lengths[1]. The main petiole carries three leaflets; but the short petioles of these leaflets 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 h. 20 m., but the next day, being very cold, the time was 5 h. 10 m. A stick placed near the revolving stem was soon struck by the petioles which stand out at right angles, and the revolving movement was arrested. The petiole then began, being excited by the contact, to slowly wind round the stick. When the stick was thin, the petiole sometimes wound twice round it. The opposite leaf was in no way affected. The attitude assumed by the stem after the petiole has clasped a stick, is that of a man standing by a column, who throws his whole 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 h. 11 m. The power of twining is like that of the last species. Its leaves are nearly similar, except that the 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 it took between two and three days to produce any effect. The leaves have the remarkable habit and power 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 h. 30 m., and another in 6 h. 12 m.: they followed the course of the sun; but in all the species of the genus the course followed, if observed long enough, would no doubt be found to differ. This is a rather better twiner than the two last species: the stem, when a thin upright stick free from twigs was placed near, sometimes made two spiral turns round it; then, being arrested by the clasping of the petioles, it would run up for a space straight and then generally reversed its course and took one or two spiral 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-fitted 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 becomes flat. I gently rubbed with a thin twig the lower surfaces of two young petioles; and in 2 h. 30 m. they were slightly curved downwards; in 5 h., after being rubbed, the end of one was bent completely back parallel to the basal portion; and in 4 h. subsequently it became nearly straight again. To show how sensitive the young petioles are, I may mention that I put, in order to mark them, short streaks of water-colour on their under sides; an infinitely thin crust was thus formed, but it sufficed in 24 h. to cause both to bend downwards. Whilst the plant is young, each leaf consists of three divided leaflets, which have barely distinct petioles, and these are not then sensitive; but when the plant is well grown, the two lateral and terminal leaflets have long petioles, and these now become sensitive and are capable of clasping in any direction any object.
When the petiole has clasped a twig, it undergoes some remarkable changes, which occur with the several other species, but in a less strongly marked manner, and will be here 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 leaf-stalk which has clasped nothing. When thin transverse slices of the two are placed under the microscope their difference is conspicuous: the side of the footstalk which has been in contact with the support is formed of a layer of colourless cells with their longer axes directed from the centre of the petiole, and very much larger than any cells found 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 externally visible: the petiole of the unclasped leaf is flexible, and can be easily snapped, whereas the clasped footstalk acquires an extraordinary 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 plain, 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 h. 51 m.; so that this species moves more quickly than any other 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 general shape, and act in the same manner like a hook, as will be described under C. viticella; but the leaflets are more divided, as in C. calycina, and each segment whilst young terminates in a hardish point, and is much curved downwards and inwards; so that the whole leaf readily catches and becomes entangled with any neighbouring object. The petioles of the young terminal leaflets are acted on by loops of thread weighing 1⁄8th and 1⁄16th of a grain: the basal portion of the main petiole is much less sensitive, but will clasp a stick against which it presses.
The whole leaf, whilst young, is in continual, spontaneous, slow movement. The stem was secured close to the base of the leaves, and, a bell-glass being placed over the shoot, 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 traced, clearly represented three and a half irregular ellipses, the most perfect one of which was completed in 2 h. 35 m. The two opposite leaves moved quite independently of each other. This movement would aid that of the internodes in bringing the petioles into contact with surrounding objects. I discovered this spontaneous movement too late to be enabled to observe the leaves in all 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 light, at an average rate of 2 h. 40 m.; when placed so that the movement was to and from the light, the rate was greatly accelerated and retarded, as in the case of twining plants. The ellipses were small; the longer diameter, described by the apex of a shoot bearing a pair of not expanded leaves, being only 4⅝ inches, and that by the apex of the penultimate internode only 1⅛ 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 with the spontaneous movements in bringing the petioles into contact with surrounding objects.
The leaves are of large size. There are three pairs of lateral leaflets and a terminal one, all borne by rather long petioles. The main petiole bends a little angularly downwards at each point where a pair of leaflets arises, 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, with the lateral petioles 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. Both the medial and lateral petioles are sensitive; and the three branches, into which the basi-lateral petioles are generally subdivided, likewise are sensitive. The basal portion of the main petiole between the stem and the first pair of leaflets is less sensitive than the remainder, but it will clasp a stick when in contact. On the other hand, the
A young leaf of Clematis viticella.
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 distant supports. To show the difference in sensibility, I gently placed loops of string of the same weight (in one instance weighing .82 of a grain) on the several lateral and on the terminal sub-petioles; in a few hours the latter were bent, but after 24 h. no effect was produced on any of the lateral petioles. Again, a terminal sub-petiole placed in contact with a thin stick became sensibly curved in 45 m., and in 1 h. 10 m. had moved through ninety degrees, whereas a lateral petiole did not become sensibly curved until 3 h. 30 m. had elapsed. In this latter case, and in all other such cases, if the sticks be taken away, the petioles continue to move during many hours afterwards; so they do after a slight rubbing; but ultimately, if the flexure has not been very great or long-continued, they become, after about a day's interval, straight again.
The gradation in the extension of the sensitiveness in the petioles of the several 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 has spread to the three sub-petioles. In C. viticella it has spread to the petioles of the seven leaflets, and to the subdivisions of the basi-lateral sub-petioles. In this latter species the sensitiveness has diminished in the basal part of the main petiole, in which alone it resided in C. montana, and has accumulated in the abruptly bent terminal portion.
Clematis flammula.—The shoots, which are rather thick, straight, and stiff, whilst growing vigorously in the spring, made small oval revolutions, following the sun in their course. Four were made at an average rate of 3 h. 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 13⁄8, and in another case 16⁄8 inch; so that the young leaves are 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 clays moved to and fro only about the eighth of an inch. Consequently the revolving power is here much enfeebled, and under unfavourable circumstances is completely lost. This species must depend on the probable, though not ascertained, spontaneous movements of its leaves, on the rapid growth of its shoots, and on movements from the wind, for coming into contact with surrounding objects: hence, perhaps, it is that the petioles have acquired, as we shall see, in compensation a high degree of sensitiveness.
The petioles are bowed downwards, and have the same general hook-like form as in C. viticella. The medial petiole and 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 each other, are not sensitive; when the lamina of a leaflet has grown to 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 much more fully developed proportionally than the laminæ of the leaves. Full-grown petioles are not in the least sensitive. A thin stick placed so as to press lightly against a petiole, bearing a leaflet a quarter of an inch in length, caused the petiole to bend in 3 h. 15 m.; in another case a petiole curled completely round a stick in 12 h. These petioles were left curled for 24 h., and then the sticks were removed; but they never straightened themselves. I took a twig, thinner than the petiole itself, and lightly rubbed with it several petioles four times up and down; these in 1 h. 45 m. became slightly curled; the curvature increased during some hours and then began to decrease, but after 25 h. from the time of rubbing a vestige of the curvature remained. Some other petioles similarly rubbed once up and down became perceptibly curved in about 2 h. 30 m., a terminal sub-petiole moving more than a lateral sub-petiole; they became quite straight again in between 12 h. and 14 h. Lastly, a length of about one-eighth of an inch of a sub-petiole, lightly rubbed with the same twig only once down, became slightly curved in 3 h., and remained so during 11 h., but the next morning was quite straight.
The following observations are more precise. After finding that heavier pieces of string and thread acted, I placed a loop of string, weighing 1.04 gr., on a terminal petiole: in 6 h. 40 m. a curvature could be seen; in 24 h. the petiole formed an open ring round the string; in 48 h. the ring had almost closed on the string, and in 72 h. it had firmly seized the fine twine so that it required some force to withdraw it. A loop weighing .52 of a grain caused a lateral sub-petiole just perceptibly to curve in 14 h., but after 24 h. it had moved through ninety degrees. These observations were made during the summer: the following were made in the spring, when the petioles are apparently more sensitive:—A loop of thread, weighing one-eighth of a grain, produced no effect on the lateral sub-petioles, but placed on a terminal one caused, after 24 h., a moderate curvature in it; the curvature, though the loop remained suspended, was after 48 h. diminished, but never disappeared, showing that the petiole had become partially accustomed to the insufficient stimulus. This experiment was twice repeated with nearly similar results. Lastly, a loop of thread, weighing only one-sixteenth of a grain (nearly equal to four milligrammes), 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 had 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 is, these facts are remarkable. But I have reason to believe that even a less weight causes a curvature when acting over a broader surface than can be affected by thin thread. Having noticed that the tail of a suspended string, which accidentally touched a petiole, had 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 nature, after being stretched, would permit; I then quietly placed their ends so as just to rest on two petioles with their tips hanging about the tenth of an inch beneath; both these petioles certainly became curved in 36 h. One of the ends of string, which just touched the angle between a terminal and lateral sub-petiole, was in 48 h. caught as by a forceps between them. 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.—My plants in pots were not healthy; so that I dare not trust my observations, which indicated much similarity in habits with C.flammula. I mention this species only because I saw many proofs that the petioles of plants growing naturally are excited to movement by very slight pressure. For instance, I found petioles which had clasped thin withered blades of grass, the soft young leaves of a maple, and the lateral flower-peduncles of the quaking-grass or Briza: the latter are only about as thick as a hair from a man's beard, but they were completely surrounded and clasped. The petioles of a leaf, so young that none of the leaflets had expanded, had partially seized on a twig. The petioles of almost every old leaf, 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 the several above-described species, cultivated in pots and thus carefully observed, there never was any bending of the petioles without the stimulus of contact. When winter comes on, the blades of the leaves of C. vitalba drop off; but the petioles (as was also observed by Mohl) remain, sometimes during two seasons, attached to the branches; and, being convoluted, they curiously resemble true tendrils, such as those occurring in the allied genus Naravelia. The petioles which have clasped an object become much more woody, stiff, hard, and polished than those which have failed in this their proper purpose.
Tropæolum.—I observed T. tricolorum, T. azureum, T. pentaphyllum, T. peregrinum, T. elegans, T. tuberosum, and a dwarf variety of, as I believe, T. minus.
Tropæolum tricolorum, var. grandiflorum.—The flexible shoot, which first rises from the tuber, is as thin as thin twine. One such shoot revolved in a course opposed to the sun, at an average rate, judging from three revolutions, of 1 h. 23 m.; but no doubt the direction of the revolving movement is variable. When the plant had grown tall and much branched, all the many lateral shoots continued to revolve. The stem, whilst young, twined regularly round a thin vertical stick; in one case I counted eight spiral turns: 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 has grown to a height of two or three feet, about a month after the first shoot has appeared above ground, no true leaves, but in their place little filaments, coloured like the stem, are produced. 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; and soon other segments appear, until a perfect leaf is formed with seven deep segments. So that on the same plant we may see every step from tendril-like filaments to perfect leaves. Hence this plant, whilst young, might be classed with tendril-bearers. After the plant has grown to a considerable height, and is secured to its support by the clasping 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, as well as the petioles of the perfect leaves, whilst young, are highly sensitive on all sides to a touch. The slightest rub causes them to curve towards the rubbed side in about three minutes: one bent itself into a ring in six minutes; they subsequently became straight again: if, however, they have once completely clasped a stick, when this is removed, they do not recover themselves. The most remarkable fact, and which I have observed in no other species of the genus, is that the filaments and petioles of the young leaves, if they catch no object, after standing in their original position for some days spontaneously and slowly move, oscillating a little from side to side, towards the stem of the plant. Hence all the petioles and filaments, though arising on different sides of the axis, ultimately bend towards and clasp either their own stem or the supporting stick. The petioles and filaments often become, after a time, in some degree spirally contracted. In these spontaneous movements, and in the abortion of their laminæ, the sensitive filaments present a much nearer approach to the condition of tendrils than do the petioles of any other leaf-climber observed by me.
Tropæolum azureum.—An upper internode made four revolutions, following the sun, at an average rate of 1 h. 47 m. The stem twined spirally in the same irregular manner as in the last species; it produced no filaments or rudimentary leaves. The petioles of the young leaves are very sensitive: a single very light rub with a twig caused one to move perceptibly in 5 m., and another in 6 m.; the former petiole became bent at right angles in 15 m., and became straight again in between 5 h. and 6 h. A loop of thread weighing ⅛th of a grain caused a petiole to curve.
Tropæolum pentaphyllum.—The plant observed by me had not the power of spirally twining, which seemed due, not to the want of flexibility in the stem, but rather to continual interference from the clasping petioles. An upper internode made three revolutions, following the sun, at an average rate of 1 h. 46 m. The main purpose of the revolving movement in all the species is manifestly 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.; but others generally in from 8 m. to 10 m.: the curvature usually increased greatly in from 15 m. to 20 m. The petioles became straight again in between 5 h. and 6 h., and on one occasion in 3 h. When a petiole had fairly clasped a stick, it could not on the removal of the stick recover itself; but the free upper part of a petiole, which had already clasped a stick by its basal part, still had the power of movement. A loop of thread weighing ⅛th of a grain certainly 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 that the angle between the stem and petiole of Clematis montana is sensitive.
Tropæolum peregrinum.—In a very young plant the internodes did not revolve, resembling in this respect a young twining plant. The four upper internodes in an older plant made three irregular revolutions, in a course opposed to the sun, at an average rate of 1 h. 48 m. It is remarkable how nearly the same the average rate of revolution (taken, however, but from few observations) is in this and the two last species, namely, 1 h. 47 m., 1 h. 46 m, and 1 h. 48 m. The present species cannot spirally twine, which seems mainly due to the rigidity of its 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., but others in 20 m.; the petioles became straight again in from 5 h. 45 m. to 8 h. Petioles which have naturally come into contact with a stick, sometimes take two turns round it. When clasped round a support, they become rigid and hard. The petioles are less sensitive to a weight than in the previous species; for loops of string weighing .82 of a grain did not cause any curvature, whilst a loop of double this weight (1.64 gr.) did act.
Tropæolum elegans.—I did not make many observations on this species. The short and stiff internodes revolve irregularly, and describe extremely small oval figures; one was completed in 3 h. A young petiole, when rubbed, became slightly curved in 17 m.; then much more so; and was nearly straight again in 8 h.
Tropæolum tuberosum.—The internodes on a plant nine inches high did not move at all; but on an older plant they moved irregularly, and made very 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 h. The movement of the apex of the shoot, from extreme point to point of the oval, was only about one inch or one and a half; yet this slight movement brought the petioles into contact with closely surrounding twigs, which were then clasped. With the lessened power of spontaneously revolving compared with the previous species, the sensitiveness of the petioles is likewise 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 the petioles sometimes required 24 h. to become straight again. The petioles of very young leaves can act perfectly; one with the lamina only .15 of an inch in diameter, that is, about a twentieth of the full size, firmly clasped a thin twig: but leaves grown to one quarter of their full size can likewise act.
Tropæolum minus (?).—The internodes of a variety named "dwarf crimson Nasturtium" had no power of revolving; but they moved during the day to the light, and from it at night, in a rather irregular course. The petioles, when well rubbed, showed no power of curving; nor could I see that they ever clasped any neighbouring support. We have seen in this genus a gradation from species such as T. tricolorum, which have exquisitely sensitive petioles, and internodes which have rapid revolving powers and can 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 this species and in T. elegans, and probably in others, the flower-peduncles, as soon as the seed-capsule begins to swell, spontaneously bend abruptly downwards and become somewhat convoluted: when a stick lies in the path, it is to a certain extent clasped; but, as far as I have been able to observe, the movement of the peduncle is quite independent of the stimulus from contact.
Antirrhineæ.—In this tribe (Lindley) of the Scrophulariaceæ, 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 h. 17 m.; this same shoot, the day before, revolved in an opposite direction. The shoots do not spirally twine, but climb excellently by the aid of the young sensitive petioles. These petioles, when lightly rubbed, move after a considerable interval of time, and subsequently become straight again; a loop of thread weighing 1⁄8th of a grain caused them to bend.
Maurandia semperflorens.—This freely growing species climbs exactly like the last, by its sensitive petioles. A young internode made two circles, each in 1 h. 46 m.; so that it moves almost twice as rapidly as the last species. But I should not have noticed the present species, had it not been for the following unique case. Mohl says (S. 45) that "the flower-peduncles, as well as the petioles, are wound into tendrils;" and he adds nothing more about the genus. But it must be observed that Mohl classes as tendrils even such objects as the spiral flower-stalks of the Vallisneria. Nevertheless this remark, and the well-known fact that the flower-peduncles of this Maurandia are flexuous, led me carefully to examine them. They never act as tendrils: I repeatedly placed thin sticks in contact with young and old peduncles, and I allowed nine vigorous plants to grow over an entangled mass of branches; but in no one instance did a peduncle bend round any object. It is indeed in the highest degree improbable that this should occur, for the flower-peduncles are generally developed on branches which have already securely clasped a support by their petioles; and when borne on free depending branches, they are not produced by the terminal portion of the internode which alone has the power of revolving; so that they can only accidentally and rarely be brought into contact with any surrounding object. Nevertheless (and this is the remarkable fact) these flower-peduncles, whilst young, exhibit feeble revolving powers, and are slightly sensitive to a touch. I selected some stems which had firmly clasped a stick by their petioles, and, placing a bell-glass over them, traced the movements of the young flower-peduncles. Some days these moved over a short and extremely irregular line, making little loops in their course. One day a young peduncle 1½ inch in extreme length was carefully observed, and it made four and a half narrow, vertical, irregular, and very short ellipses—each at an average rate of about 2 h. 25 m.; an adjoining peduncle described during the same time similar, but fewer, ellipses. As the plant had for some time occupied exactly the same position, these movements could not be attributed to the varying action of the light. Peduncles, old enough for the coloured petals to be just visible, do not move. With respect to irritability, I rubbed a few times very lightly with a thin twig two young peduncles (1½ inch in length), one on the upper side and the other on the lower side, and they became in between 4 h. and 5 h. plainly bowed towards the rubbed sides; in 24 h. 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 bowed towards each other, that the arcs of the bows stood at nearly right angles to their previous positions; 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 l½ inch in length required to be rubbed two or three times, and then became only just perceptibly curved. Loops of thread suspended on the peduncles produced no effect; but loops of string weighing .82 and 1.64 grain acted capriciously, sometimes causing a slight curvature; but they were never clasped, like the far lighter loops of thread by the petioles.
In the nine vigorous plants which I observed, it is certain that neither the slight spontaneous movements nor the slight sensitiveness of the flower-peduncles were of any service to the plants in climbing. If any member of the Scrophulariaceæ had been known to have flower-peduncles used for climbing, or had tendrils produced by their modification, I should have thought that this Maurandia still retained a useless or rudimentary vestige of a former habit; but this view cannot be maintained. We are almost compelled to believe that by some correlation of growth the power of movement has been transferred from the young internodes to the young peduncles, and in the same manner sensitiveness from the young petioles to the young peduncles; but this latter supposition is the more improbable, as I could detect no sensitiveness in the young internodes of the Maurandia, though in a closely allied genus, Lophospermum, the young internodes, as we shall see, are sensitive. By whatever means the peduncles of this Maurandia have acquired their power of spontaneous movement and their sensitiveness, the case is interesting for us; for we can see that if these now useless capacities were a little perfected, the flower-peduncles could be made as useful for climbing as are the flower-peduncles of Vitis and Cardiospermum, as will hereafter be described.
Rhodochiton volubile.—A long flexible shoot swept a large circle, following the sun, in 5 h. 30 m.; and, as the day became warmer, a second circle in 4 h. 10 m. The shoots sometimes make a whole or half spire round a vertical stick, then run up for a space straight, and afterwards make spiral turns in an opposite direction. The petioles of very young leaves, about one-tenth of their full size are highly sensitive, and bend towards any side which has been touched; but they do not move quickly: one, after being lightly rubbed, was perceptibly curved in 1 h. 10 m., and became considerably arched in 5 h. 40 m. after the rubbing; some other petioles, after being rubbed, were scarcely curved in 5 h. 30 m., but in 6 h. 30 m. were distinctly curved. A curvature was perceptible in a petiole in between 4 h. 30 m. and 5 h., after the suspension of a little loop of string. A loop of fine cotton thread, weighing one-sixteenth of a grain, not only slowly caused a petiole to bend, but was ultimately firmly clasped by it, so 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 turn round it; ultimately they increase much in thickness. Leaves arising on the side of the stem opposite to the light move towards it; and, in doing so, the petioles are sometimes brought into contact with the stem, and consequently clasp it; but the petioles have no true spontaneous movement.
Lophospermum scandens, var. purpureum.—Some long, moderately thin internodes made four revolutions at an average rate of 3 h. 15 m. The course pursued was very irregular—sometimes an extremely narrow ellipse, sometimes a large circle, sometimes an irregular spire or zigzag line, and sometimes the apex stood still. The young petioles, when brought by the revolving movement into contact with a stick, clasp it, and soon increase considerably in thickness; but they are not quite so sensitive to a light weight as those of the Rhodochiton, for loops of thread weighing one-eighth of a grain did not invariably cause them to bend.
This plant presents a case not observed in any other leaf-climber or twiner or tendril-bearer, or in any other plant as far as I know, namely, that the young internodes are sensitive to a touch. When a petiole clasps a stick, it draws the base of the internode against it; and then the internode itself bends towards the stick, which is thus caught between the stem and the petiole as by a pair of pincers. The internode straightens itself again, excepting the part in 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 lightly rubbing two or three times with a thin twig several internodes; and in about 2 h., but in one case in 3 h., all became bent: they became straight again in about 4 h., subsequently. An internode, which was rubbed as much as six or seven times with a twig, became just perceptibly curved in 1 h. 15 m., and subsequently in 3 h. the curvature increased much; the internode became straight again in the course of the night. I rubbed some internodes one day on one side, and the next day on the opposite side or at right angles; and the curvature was always towards the rubbed side.
According to Palm (S. 63), the petioles of Limaria cirrhosa and, to a limited degree, those of L. elatine have the power of clasping a support.
Solanum.—S. jasminoides.—Some of the species of this large genus are twiners; but this 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 h. 26 m. The shoots, however, sometimes stand 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 h. In the greenhouse a
Solanum jasminoides, with one of its leaves clasping a stick.
petiole was not affected by a loop of string, suspended during several days and weighing 2½ grains; in the hothouse one was made to curve by a loop weighing 1.64 (and, on the removal of the string, became straight again), but was not at all affected by another loop weighing .82 of a grain. We have seen that the petioles of some other leaf-climbing plants were affected by one-thirteenth of this latter weight. In this plant, and in no other leaf-climber seen by me, a leaf grown to its full size was capable of clasping a stick; but the movement was so extraordinarily slow that in the greenhouse the act required several weeks; but on each succeeding week it was clear that the petiole became more and more curved, until finally it firmly clasped the stick.
When the flexible petiole of a half- or a quarter-grown leaf has clasped any object, in three or four days it increases much in thickness, and after several weeks becomes wonderfully hard and rigid; so that I could hardly remove one from its support. On comparing a thin transverse slice of this petiole with one from the next or older leaf 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
A. Section of a petiole.
B. Section of a petiole some weeks after it had clasped a stick, as shown in fig. 3.
semilunar hand of cellular tissue slightly different from that outside it, and including three closely approximate groups of dark vessels. Near the upper surface of the petiole, beneath two ridges, there are two other small circular groups of vessels. In the section of the petiole (B) which had during several weeks clasped a stick, the two upper ridges have become much less prominent, and the two groups of woody vessels beneath them much increased in diameter. The semilunar band is converted into a complete ring of very hard, white, woody tissue, with lines radiating from the centre. The three groups of vessels, which, though closely approximate, were before distinct, are now completely blended together. The upper part of the new ring of woody vessels, formed by the prolongation of the horns of the original semilunar band, is thinner than the lower part, and is slightly different in appearance from being less compact. This clasped petiole had actually become thicker than the stem close beneath; and this was chiefly due to the greater thickness of the ring of wood, which presented, both in transverse and longitudinal sections, a closely similar structure in the petiole and axis. The assumption by a petiole of this structure is a singular morphological fact; but 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[2].
Fumariaceæ.—Fumaria officinalis.—It could not have been anticipated that so lowly a plant would have been a climber. This it effects by the aid of the main and lateral petioles of its compound leaves; 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 became rather thicker and more cylindrical. On lightly rubbing with a twig several petioles, they became perceptibly curved in 1 h. 15 m., and subsequently they straightened themselves. A stick gently placed in the angle between two sub-petioles caused movement in 7 h., and was almost clasped in 9 h. A loop of thread, weighing one-eighth of a grain, caused, after 12 h. and before 20 h. had elapsed, a considerable curvature; but the petiole never fairly clasped the thread. The young internodes arc in continual movement; the movement is considerable, but very irregular in course; a zigzag line, or a spire crossing itself, or a figure of 8 is formed; the course during 12 h., being traced on a bell-glass, apparently represented about four ellipses. The leaves themselves also move spontaneously, the main petiole curving itself in accordance with the movement of the internodes; so that when the latter move to one side the petiole is curved to that side, then, becoming straight, is curved to the opposite side. Thus a wider space is swept for a support to be clasped. The movement, however, is small, as could be seen when the shoot was securely tied to a stick and the leaf alone allowed to move. The leaf in this case followed an irregular course, like that made by the young internodes.
Adlumia cirrhosa.—I raised some plants late in the summer; they formed magnificent leaves, but threw up no central stem. The first-formed leaves were not sensitive; but some of the later leaves were sensitive, but only towards their extremities, and were able 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 be when it had grown tall enough to climb. The tip of one of these ground leaves, whilst young, described in 1 h. 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 2½ inches in length, and was completed in 2 h. 2 m. From analogy with Fumaria and Corydalis, I have no doubt that the internodes 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 is classed amongst tendril-bearers.
Besides the plants already described, Bignonia unguis and its close allies, though aided by tendrils, as will hereafter be described, have clasping petioles. According to Mohl (S. 40), Cocculus Japonicus (one of the Menispermaceæ) and a fern, the Ophioglossum Japonicum (S. 39), climb by their leaf-stalks.
We now come to a small section of plants which climb by the aid of the produced midribs or tips of their leaves.
Gloriosa.—G. Plantii (Liliaceæ).—The stem of a half-grown plant continually moved, generally describing an irregular spire, but sometimes ovals, with the longer axes running in different directions. It either followed the sun, or moved in an opposite course, and sometimes stood still before reversing its course. One oval was completed in 3 h. 40 m.; of two horseshoe-shaped figures, one was completed in 4 h. 35 m. and the other in 3 h. The tip of the shoot, in its 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 has got into an inclined position, the end has bent itself downwards into a well-formed hook; and this is now strong and rigid enough to catch any object, and, when caught, to anchor the plant and stop the revolving movement. This hook is sensitive on its inner surface, but not in nearly so high a degree as with 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 h. to 3 h. to have curled a little inwards; and, under favourable circumstances, in from 8 h. to 10 h. it finally curls round and seizes the object, which it never again looses. The hook when first formed, before the leaf has become inclined, is less sensitive. The hook, if it catches hold of nothing, remains for a long period open and sensitive; ultimately the tip 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 inwards so much that at last only a very thin twig could have been inserted. As soon as the curling-in of the tip has closed the hook and converted it into a ring, its sensibility, both within and without, is lost; but as long as the hook remains open its sensibility is retained.
When the plant had grown from the bulb to the height of only about six inches, the leaves, four or five in number, were broader than those subsequently produced, and their soft and but little-attenuated tips did not form hooks, and were not sensitive; nor did the stem revolve. At this early period of growth, the plant can support itself; its climbing apparatus is not required, and therefore is not acquired. On the other hand, a full-grown plant which was flowering, and which would not have grown any taller, had leaves on the summit, which were not sensitive, and could not clasp a stick.
Flagellaria Indica (Commelynaceæ).—From dried specimens it is manifest that this plant climbs exactly like Gloriosa. A young plant, 12 inches in height, and bearing fifteen leaves, had not one leaf as yet produced into a hook or tendril-like filament; nor did the stem revolve. Hence this plant acquires its climbing power later in life than the Gloriosa lily. According to Mohl (S. 41), Uvularia (Melanthaceæ) climbs like Gloriosa.
These three last-named genera are all Monocotyledons; but there is one Dicotyledon, namely Nepenthes, which is ranked by Mohl (S. 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 twisting round any support. The twisted part becomes thicker; but I observed at Mr. Veitch's that the stalk often takes a turn when not in contact with any object, and that this twisted part likewise becomes thickened. Two vigorous young plants of N. lævis and N. distillatoria, in my hothouse, whilst less than a foot in height, showed no sensitiveness in their leaves or power of movement or of climbing. But when N. lævis had grown to a height of 16 inches, there were signs of these powers. Each young leaf when first formed stands upright, but soon becomes inclined; at this period of growth it terminates in a stalk or filament, with the pitcher at the extremity so little developed that this part is not thicker than any other part. The leaf in this state certainly exhibited slight spontaneous movements; and when the stalk came into contact with a stick, it very slowly bent round and firmly seized it. But the leaf by its subsequent growth became quite slack, though the terminal stalk remained 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. With all the plants observed by me, the young internodes revolved more or less regularly, in some cases as regularly as does any twining plant, and at various rates, but generally rather rapidly. Some few can ascend by twining spirally round a support. Differently from most twiners, there is a strong tendency in the same shoot to revolve first in one and then in the opposite direction. The object gained by the revolving movement, as could be plainly seen, was to bring the petioles or the tips of the leaves into contact with surrounding objects; without this aid there would be a poor chance of success. 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 in very different degrees in different parts of the same petiole. The hooked tips of the leaves of the Gloriosa are sensitive only on their inner or inferior surface. 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; and there is reason to believe that the rather thick and stiff petioles of Clematis flammula are sensitive to even a less weight when spread over a wider surface. The petioles always bend towards the touched or pressed side, at different rates in different plants, 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 habituated to the stimulus, and either bends no more or becomes straight again, the weight still remaining suspended. Petioles which have elapsed any 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 or on one side alone; they subsequently become, sometimes in a wonderful degree, stronger and more woody; and in some cases they acquire an internal structure like that of the stem or axis.
The young internodes of the Lophospermum are sensitive as well as the petioles, and by their combined movement seize any object. The flower-peduncles of the Maurandia semperflorens revolve spontaneously, and are sensitive to a touch, yet are certainly useless for climbing. The leaves of at least two and probably of most of the species of Clematis, and of Fumaria and Adlumia, spontaneously curve from side to side, like the internodes, and are thus better adapted to seize any distant object. The petioles of the perfect leaves, as well as the rudimentary or tendril-like leaves of Tropæolum tricolorum move spontaneously and slowly towards their own stem or the supporting stick, which they then clasp; these petioles 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. These several facts are interesting, as we shall see, in relation to true tendrils.
It was observed in some cases that, as with twining plants, so with leaf-climbers, the first internodes which rise from the ground do not spontaneously revolve; nor are the petioles or tips of the first-formed leaves sensitive. In certain species of Clematis the high development and spontaneous movements of the leaves, with their highly sensitive petioles, apparently have rendered almost superfluous the spontaneous movements of the internodes, which have consequently become enfeebled. In certain species of Tropæolum it would appear as if both the spontaneous movements of the internodes and the sensitiveness of the petioles have become enfeebled; and in one species they have been completely lost.
- ↑ Our English grain equals nearly 65 milligrammes.
- ↑ Dr. Maxwell Masters informs me that in most, or all, petioles which are cylindrical, such as those bearing peltate leaves, the woody vessels form a closed ring, and that the semilunar band of vessels is 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 much more cylindrical than it was in its original unclasped condition.