The Movements and Habits of Climbing Plants Part 4

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-gla.s.s 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 a.s.serts (p. 45) that "the flower-peduncles, as well as the petioles, wind like tendrils;" but he cla.s.ses 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 ma.s.s 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- gla.s.s 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 s.p.a.ce, 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 antic.i.p.ated 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 gra.s.s 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-gla.s.s, 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 s.p.a.ce, 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 a.n.a.logy 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 a.s.signed, it has been cla.s.sed 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 j.a.ponicus (one of the Menispermaceae) and a fern, the Ophioglossum j.a.ponic.u.m (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 circ.u.mstances, 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 b.u.t.ton-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, cla.s.ses them according to their h.o.m.ological 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 h.o.m.ological nature of certain tendrils.

Consequently I will describe tendril-bearing plants by natural families, following Lindley's cla.s.sification; 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, Pa.s.sifloraceae. {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 pa.s.s 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 pa.s.sing 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 a.n.a.logy 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.