Whilst the plant is quite young, the first-formed leaves are not modified in any way, but those next formed have their terminal leaflets reduced in size, and soon all the leaves assume the structure represented in the following drawing. This leaf bore nine leaflets; the lower ones being much subdivided. The terminal portion of the petiole, about 1.5 inch in length (above the leaflet f), is thinner and more elongated than the lower part, and may be considered as the tendril. The leaflets borne by this part are greatly reduced in size, being, on an average, about the tenth of an inch in length and very narrow; one small leaflet measured one-twelfth of an inch in length and one-seventy-fifth in breadth (2.116 mm. and 0.339 mm.), so that it was almost microscopically minute. All the reduced leaflets have branching nerves, and terminate in little spines, like those of the fully developed leaflets. Every gradation could be traced, until we come to branchlets (as a and d in the figure) which show no vestige of a lamina or blade. Occasionally all the terminal branchlets of the petiole are in this condition, and we then have a true tendril.

The several terminal branches of the petiole bearing the much reduced leaflets (a, b, c, d) are highly sensitive, for a loop of thread weighing only the one-sixteenth of a grain (4.05 mg.) caused them to become greatly curved in under 4 hrs. When the loop was removed, the petioles straightened themselves in about the same time. The petiole (e) was rather less sensitive; and in another specimen, in which the corresponding petiole bore rather larger leaflets, a loop of thread weighing one-eighth of a grain did not cause curvature until 18 hrs. had elapsed. Loops of thread weighing one-fourth of a grain, left suspended on the lower petioles (f to l) during several days, produced no effect. Yet the three petioles f, g, and h were not quite insensible, for when left in contact with a stick for a day or two they slowly curled round it. Thus the sensibility of the petiole gradually diminishes from the tendril-like extremity to the base. The internodes of the stem are not at all sensitive, which makes Mohl's statement that they are sometimes converted into tendrils the more surprising, not to say improbable.

The whole leaf, whilst young and sensitive, stands almost vertically upwards, as we have seen to be the case with many tendrils. It is in continual movement, and one that I observed swept at an average rate of about 2 hrs. for each revolution, large, though irregular, ellipses, which were sometimes narrow, sometimes broad, with their longer axes directed to different points of the compass. The young internodes, likewise revolved irregularly in ellipses or spires; so that by these combined movements a considerable space was swept for a support. If the terminal and attenuated portion of a petiole fails to seize any object, it ultimately bends downwards and inwards, and soon loses all irritability and power of movement. This bending down differs much in nature from that which occurs with the extremities of the young leaves in many species of Clematis; for these, when thus bent downwards or hooked, first acquire their full degree of sensitiveness.

Dicentra thalictrifolia.--In this allied plant the metamorphosis of the terminal leaflets is complete, and they are converted into perfect tendrils. Whilst the plant is young, the tendrils appear like modified branches, and a distinguished botanist thought that they were of this nature; but in a full-grown plant there can be no doubt, as I am assured by Dr. Hooker, that they are modified leaves. When of full size, they are above 5 inches in length; they bifurcate twice, thrice, or even four times; their extremities are hooked and blunt. All the branches of the tendrils are sensitive on all sides, but the basal portion of the main stem is only slightly so. The terminal branches when lightly rubbed with a twig became curved in the course of from 30 m. to 42 m., and straightened themselves in between 10 hrs. and 20 hrs. A loop of thread weighing one-eighth of a grain plainly caused the thinner branches to bend, as did occasionally a loop weighing one-sixteenth of a grain; but this latter weight, though left suspended, was not sufficient to cause a permanent flexure. The whole leaf with its tendril, as well as the young upper internodes, revolves vigorously and quickly, though irregularly, and thus sweeps a wide space. The figure traced on a bell-glass was either an irregular spire or a zigzag line. The nearest approach to an ellipse was an elongated figure of 8, with one end a little open, and this was completed in 1 hr. 53 m. During a period of 6 hrs. 17 m. another shoot made a complex figure, apparently representing three and a half ellipses. When the lower part of the petiole bearing the leaflets was securely fastened, the tendril itself described similar but much smaller figures.

This species climbs well. The tendrils after clasping a stick become thicker and more rigid; but the blunt hooks do not turn and adapt themselves to the supporting surface, as is done in so perfect a manner by some Bignoniaceae and Cobaea. The tendrils of young plants, two or three feet in height, are only half the length of those borne by the same plant when grown taller, and they do not contract spirally after clasping a support, but only become slightly flexuous. Full-sized tendrils, on the other hand, contract spirally, with the exception of the thick basal portion. Tendrils which have caught nothing simply bend downwards and inwards, like the extremities of the leaves of the Corydalis claviculata. But in all cases the petiole after a time is angularly and abruptly bent downwards like that of Eccremocarpus.

The Movements and Habits of Climbing Plants Page 43

Charles Darwin

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Charles Darwin

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