CLIMBING PLANTS
Bignoniaceae is a large and mainly tropical family of trees, shrubs, and lianas (woody vines). The climbers of the family most interested Darwin, and in distinguishing between types of climbing, those endowed with what he called “irritable organs” (modified leaves, branches, or flower peduncles sensitive to touch) especially impressed him. The New World type-genus Bignonia, the name of which honors the Frenchman Abbé Jean-Paul Bignon (1662–1743), checked all these boxes for him, sporting “irritable” internodes, petioles, and tendrils. Many members of this genus of vigorous woody vines are prized horticulturally for their large trumpet-shaped flowers. He studied nearly a dozen species,1 taking what he could get from his contacts at Kew Gardens and Veitch’s Royal Exotic Nursery in London.
Bignonia capreolata, known as crossvine, is native to the southern and central United States and is one of the most remarkable species Darwin grew. This species was well known in English horticultural circles as one of the earliest botanical imports from the American colonies. Darwin noticed that they tend to grow away from light, which seemed counterintuitive—shouldn’t vines always reach for the sun? In one experiment, he placed a potted crossvine with six tendrils into a box open on one side and positioned the open side obliquely to a light source. Checking on the plant two days later, he found that the tendrils were all reaching toward the darkest corner of the box. “Six wind-vanes could not have more truly shown the direction of the wind,” he declared, “than did these branched tendrils the course of the stream of light which entered the box.”25 This vine’s shade-seeking behavior—apheliotropism, as the tendency to move away from light is called—is thought to help young earthbound vines on the forest floor locate a tree to climb, a surer path to getting upwards to the needed light rather than responding to light at ground level.
Through trial-and-error, Darwin also found that this species seems to prefer climbing a rough woolly-textured surface. They refused, so to speak, smooth sticks but readily climbed when Darwin wrapped the sticks in flax, moss, or wool. Darwin surmised that this preference was related to the plant’s native environment and put the question to his botanist friend Asa Gray at Harvard in the United States. “Have you travelled south, and can you tell me, whether the trees, which Bignonia capreolata climbs, are covered with moss, or filamentous lichen or Tillandsia; I ask because its tendrils abhor a simple stick, do not much relish rough bark, but delight in wool or moss,” he wrote, anthropomorphizing to evoke the “animal” in the plant, as he often did. Gray confirmed that the vines did indeed climb trees “well furnished with Lichens and Mosses” in the wet, shady forests of the southern United States.26 Darwin found that several other Bignonia species, as well as some species in the allied genera Tecoma and Eccremocarpus, also grow away from sunlight to attach themselves for support to walls or trees.
Note how, near the end of Darwin’s account of his experiments with B. capreolata, excerpted here, he steps back to marvel at the significance of his findings: insofar as tendrils are highly modified leaves, adapted to seek light, it’s amazing that here they should evolve into a structure that moves away from light, seeking, root-like, nooks and crannies to grab hold of.
Bignonia unguis.— … 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 figure. … It is curiously like the leg and foot of a small bird, with the hind toe cut off. The straight leg or tarsus is longer than the three toes, which are of equal length, and diverging, lie in the same plane. The toes terminate in sharp, hard claws, much curved downwards, like those on a bird’s foot. The petiole of the leaf is sensitive to contact; even a small loop of thread suspended for two days caused it to bend upwards; but the sub-petioles of the two lateral leaflets are not sensitive. The whole tendril, namely, the tarsus and the three toes, are likewise sensitive to contact, especially on their under surfaces. When a shoot grows in the midst of thin branches, the tendrils are soon brought by the revolving movement of the internodes into contact with them; and then one toe of the tendril or more, commonly all three, bend, and after several hours seize fast hold of the twigs, like a bird when perched. If the tarsus of the tendril comes into contact with a twig, it goes on slowly bending, until the whole foot is carried quite round, and the toes pass on each side of the tarsus and seize it. In like manner, if the petiole comes into contact with a twig, it bends round, carrying the tendril, which then seizes its own petiole or that of the opposite leaf. The petioles move spontaneously, and thus, when a shoot attempts to twine round an upright stick, those on both sides after a time come into contact with it and are excited to bend. Ultimately the two petioles clasp the stick in opposite directions, and the foot-like tendrils, seizing on each other or on their own petioles, fasten the stem to the support with surprising security. … 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. …
Bignonia Unnamed species from Kew
Bignonia capreolata.—We now come to a species having tendrils of a different type; but first for the internodes. A young shoot made three large revolutions, following the sun, at an average rate of 2 hrs. 23 m. The stem is thin and flexible, and I have seen one make four regular spiral turns round a thin upright stick, ascending of course from right to left, and therefore in a reversed direction compared with the before described species. Afterwards, from the interference of the tendrils, it ascended either straight up the stick or in an irregular spire. The tendrils are in some respects highly remarkable. In a young plant they were about 2½ inches in length and much branched, the five chief branches apparently representing two pairs of leaflets and a terminal one … with the points blunt yet distinctly hooked. …
Whilst the tendrils are revolving more or less regularly, another remarkable movement takes place, namely, a slow inclination from the light towards the darkest side of the house. I repeatedly changed the position of my plants, and some little time after the revolving movement had ceased, the successively formed tendrils always ended by pointing to the darkest side. When I placed a thick post near a tendril, between it and the light, the tendril pointed in that direction. In two instances a pair of leaves stood so that one of the two tendrils was directed towards the light and the other to the darkest side of the house; the latter did not move, but the opposite one bent itself first upwards and then right over its fellow, so that the two became parallel, one above the other, both pointing to the dark: I then turned the plant half round; and the tendril which had turned over recovered its original position, and the opposite one which had not before moved, now turned over to the dark side. Lastly, on another plant, three pairs of tendrils were produced at the same time by three shoots, and all happened to be differently directed: I placed the pot in a box open only on one side, and obliquely facing the light; in two days all six tendrils pointed with unerring truth to the darkest corner of the box, though to do this each had to bend in a different manner. Six wind-vanes could not have more truly shown the direction of the wind, than did these branched tendrils the course of the stream of light which entered the box. …
When a tendril has not succeeded in clasping a support, either through its own revolving movement or that of the shoot, or by turning towards any object which intercepts the light, it bends vertically downwards and then towards its own stem, which it seizes together with the supporting stick, if there be one. A little aid is thus given in keeping the stem secure. If the tendril seizes nothing, it does not contract spirally, but soon withers away and drops off. If it seizes an object, all the branches contract spirally.
I have stated that after a tendril has come into contact with a stick, it bends round it in about half an hour; but I repeatedly observed, as in the case of B. speciosa and its allies, that it often again loosed the stick; sometimes seizing and loosing the same stick three or four times. Knowing that the tendrils avoided the light, I gave them a glass tube blackened within, and a well-blackened zinc plate: the branches curled round the tube and abruptly bent themselves round the edges of the zinc plate; but they soon recoiled from these objects with what I can only call disgust and straightened themselves. I then placed a post with extremely rugged bark close to a pair of tendrils; twice they touched it for an hour or two, and twice they withdrew; at last one of the hooked extremities curled round and firmly seized an excessively minute projecting point of bark, and then the other branches spread themselves out, following with accuracy every inequality of the surface. I afterwards placed near the plant a post without bark but much fissured, and the points of the tendrils crawled into all the crevices in a beautiful manner. To my surprise, I observed that the tips of the immature tendrils, with the branches not yet fully separated, likewise crawled just like roots into the minutest crevices. In two or three days after the tips had thus crawled into the crevices, or after their hooked ends had seized minute points, the final process, now to be described, commenced.
This process I discovered by having accidentally left a piece of wool near a tendril; and this led me to bind a quantity of flax, moss, and wool loosely round sticks, and to place them near tendrils. The wool must not be dyed, for these tendrils are excessively sensitive to some poisons. The hooked points soon caught hold of the fibres, even loosely floating fibres, and now there was no recoiling; on the contrary, the excitement caused the hooks to penetrate the fibrous mass and to curl inwards, so that each hook caught firmly one or two fibres, or a small bundle of them. The tips and the inner surfaces of the hooks now began to swell, and in two or three days were visibly enlarged. After a few more days the hooks were converted into whitish, irregular balls, rather above the 1\20th of an inch (1.27 mm.) in diameter, formed of coarse cellular tissue, which sometimes wholly enveloped and concealed the hooks themselves. The surfaces of these balls secrete some viscid resinous matter, to which the fibres of the flax, &c., adhere. When a fibre has become fastened to the surface, the cellular tissue does not grow directly beneath it, but continues to grow closely on each side; so that when several adjoining fibres, though excessively thin, were caught, so many crests of cellular matter, each not as thick as a human hair, grew up between them, and these, arching over on both sides, adhered firmly together. As the whole surface of the ball continues to grow, fresh fibres adhere and are afterwards enveloped; so that I have seen a little ball with between fifty and sixty fibres of flax crossing it at various angles and all embedded more or less deeply. Every gradation in the process could be followed—some fibres merely sticking to the surface, others lying in more or less deep furrows, or deeply embedded, or passing through the very centre of the cellular ball. …
From the facts now given, we may infer that though the tendrils of this Bignonia can occasionally adhere to smooth cylindrical sticks and often to rugged bark, yet that they are specially adapted to climb trees clothed with lichens, mosses, or other such productions; and I hear from Professor Asa Gray that the Polypodium incanum abounds on the forest-trees in the districts of North America where this species of Bignonia grows. Finally, I may remark how singular a fact it is that a leaf should be metamorphosed into a branched organ which turns from the light, and which can by its extremities either crawl like roots into crevices, or seize hold of minute projecting points, these extremities afterwards forming cellular outgrowths which secrete an adhesive cement, and then envelop by their continued growth the finest fibres.
Cardiospermum halicacabum. Hand-colored engraving, drawn by Sydenham Edwards, from The Botanical Magazine 26: 1049.