CLIMBING PLANTS
Clematis is a genus of climbers admired since antiquity, the very name derived from one of the Ancient Greek words for vine: κλήμα, klema. There are some 300 Clematis species worldwide, and a multitude of cultivars grown for their showy flowers. Darwin’s interest had more to do with their leaves, however—specifically, the petioles, the thin stalks that attach a leaf to a stem. Writing to his friend Joseph Hooker in early 1864 to wheedle more specimens from Kew for study, Darwin explained what he had found so far: “when by growth the [petioles] of leaves are brought into contact with any object they bend and catch hold. The slightest stimulus suffices, even a bit of cotton thread a few inches long; but the stimulus must be applied during 6 or 12 hours, and when the [petioles] once bend, though touching object be removed, they never get straight again.”36 Hooker dutifully sent more species. Darwin was able to show that most Clematis vines are premier “leaf climbers,” relying on their petioles to twist around and clasp onto whatever they can for support to reach heights. The petiole’s touch-sensitivity could even betray the identity of rather un-Clematis-like members of the genus; in another letter to Hooker later that year Darwin remarked how, when he received as a gift a flowerless specimen of Clematis glandulosa (now smilacifolia), his gardener was initially dubious that it was a Clematis. “So,” Darwin said, “I put a little twig to the [petiole] and the next day my gardener said ‘you see it is a Clem[atis] for it feels.’… That’s the way we make out plants at Down,”37 he quipped.
Considered by Darwin to be intermediate between twiners and tendril bearers, leaf climbers are represented in various families, with Clematis species being especially responsive. He enjoyed observing their growth patterns, even as he himself suffered from various ailments. Working with eight species, he tested the reactions of the petioles under various circumstances, remarking on the “nervous system” that responded to touch by bending, holding, and expanding, growing thicker and stronger. Noting the minutes and hours it took the petioles to twine, he also measured and weighed bits of string that they clasped on to, emphasizing the sensitive nature of the petioles.
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 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 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.
C. glandulosa.—The thin upper internodes revolve, moving against the course of the sun, precisely like those of a true twiner, at an average rate, judging from three revolutions, of 3 hrs. 48 m. The leading shoot immediately twined round a stick placed near it; but, after making an open spire of only one turn and a half, it ascended for a short space straight, and then reversed its course and wound two turns in an opposite direction. This was rendered possible by the straight piece between the opposed spires having become rigid. The simple, broad, ovate leaves of this tropical species, with their short thick petioles, seem but ill-fitted for any movement; and whilst twining up a vertical stick, no use is made of them. Nevertheless, if the footstalk of a young leaf be rubbed with a thin twig a few times on any side, it will in the course of a few hours bend to that side; afterwards becoming straight again. The under side seemed to be the most sensitive; but the sensitiveness or irritability is slight compared to that which we shall meet with in some of the following species; thus, a loop of string, weighing 1.64 grain (106.2 mg.) and hanging for some days on a young footstalk, produced a scarcely perceptible effect. A sketch is here given of two young leaves which had naturally caught hold of two thin branches. A forked twig placed so as to press lightly on the under side of a young footstalk caused it, in 12 hrs., to bend greatly, and ultimately to such an extent that the leaf passed to the opposite side of the stem; the forked stick having been removed, the leaf slowly recovered its former position.
The young leaves spontaneously and gradually change their position: when first developed the petioles are upturned and parallel to the stem; they then slowly bend downwards, remaining for a short time at right angles to the stem, and then become so much arched downwards that the blade of the leaf points to the ground with its tip curled inwards, so that the whole petiole and lead together form a hook. They are thus enabled to catch hold of any twig with which they may be brought into contact by the revolving movement of the internodes. If this does not happen, they retain their hooked shape for a considerable time, and then bending upwards reassume their original upturned position, which is preserved ever afterwards. The petioles which have clasped any object soon become much thickened and strengthened, as may be seen in the drawing.
Clematis glandulosa. With two young leaves clasping two twigs, with the clasping portions thickened.
Clematis montana.—The long, thin petioles of the leaves, whilst young, are sensitive, and when lightly rubbed bend to the rubbed side, subsequently becoming straight. They are far more sensitive than the petioles of C. glandulosa; for a loop of thread weighing a quarter of a grain (16.2 mg.) caused them to bend; a loop weighing only one-eighth of a grain (8.1 mg.) sometimes acted and sometimes did not act. The sensitiveness extends from the blade of the leaf to the stem. I may here state that I ascertained in all cases the weights of the string and thread used by carefully weighing 50 inches in a chemical balance, and then cutting off measured lengths. The main petiole carries three leaflets; but their short, sub-petioles are not sensitive. A young, inclined shoot (the plant being in the greenhouse) made a large circle opposed to the course of the sun in 4 hrs. 20 m., but the next day, being very cold, the time was 5 hrs. 10m. A stick placed near a revolving stem was soon struck by the petioles which stand out at right angles, and the revolving movement was thus arrested. The petioles then began, being excited by the contact, to slowly wind round the stick. When the stick was thin, a petiole sometimes wound twice round it. The opposite leaf was in no way affected. The attitude assumed by the stem after the petiole had clasped the stick, was that of a man standing by a column, who throws his arm horizontally round it.
Cobaea scandens. Hand-colored engraving drawn by Sydenham Edwards, from The Botanical Magazine 22: 851.