CLIMBING PLANTS
Echinocystis lobata is an annual vine with delightfully fragrant flowers found throughout most of North America. It bears several common names, including wild cucumber and balsam apple, while its Latin name is derived from the Greek words echinos (“hedgehog”) and cystis (“bladder”), describing the small prickle-covered fruits. This sprawling plant drapes its broad, lobed leaves and spiked clusters of star-like flowers while relying on its tendrils to climb over shrubs.
Bur cucumber led to Darwin’s research on climbing plants. He had become curious about climbers after reading an article by Harvard botanist Asa Gray, reporting experiments on the touch-sensitivity of the tendrils of the related vine Sicyos angulatus.162 Encouraging his friend to study the climbers, Gray sent Darwin Echinocystis seeds while joking that he doubted there was “warmth and sunshine enough in England to a get a sensible movement.”63 But Darwin had some germination success, and it wasn’t long before he was able to observe the remarkable tendril movements, inspiring him to study well over 100 species of climbing plants in the next several years, more than half of them tendril-bearers.
It was by carefully watching Echinocystis that Darwin first noticed the gyrating motions he was to later dub circumnutation. “This may be common phenomenon for what I know,” he wrote to his friend Joseph Hooker at Kew, “but it confounded me quite when I began to observe the irritability [sensitivity] of the tendrils. … The result is pretty, for the plant every 1½ or 2 hours sweeps a circle (according to length of bending shoot and length of tendril) of from 1 foot to 20 inches in diameter, and immediately that the tendril touches any object its sensitiveness causes it immediately to seize it.”64
This incessant movement of the tendrils is behind the plant’s uncanny ability to “seek” and find a support to climb, giving the impression of an almost animal awareness and intentionality. A local gardener who helped Darwin with experiments certainly thought so—Darwin related to Hooker how “a clever gardener, my neighbour, who saw the plant on my table last night, said ‘I believe, Sir, the tendrils can see, for wherever I put the plant, it finds out any stick near enough.’ I believe the above is the explanation, viz that it sweeps slowly round and round.”65
When Echinocystis tendrils coil and catch a support, they slowly become twisted in opposite directions, resulting in two oppositely spiraling sections approximately equal in length, separated by a short straight segment. Bur cucumber is not the only vine to do this; Darwin’s son Francis illustrated the phenomenon in Climbing Plants with a Bryony tendril.
A caught tendril of Bryonia dioica, spirally contracted in reverse directions.
One effect of this reverse twisting is to pull the plant closer to the support, and another is to create a spring of the tendril, firmly anchored to the support yet allowing some give—presumably useful in windy conditions.
In keeping with Darwin’s emphasis on animal-like qualities of plants, he used a human analogy in his discussion of how Echinocystis tendrils slowly but surely grasp a support: “It drags itself onwards by an insensibly slow, alternate movement, which may be compared to that of a strong man suspended by the ends of his fingers to a horizontal pole, who works his fingers onwards until he can grasp the pole with the palm of his hand.”66
Echinocystis lobata.—Numerous observations were made on this plant (raised from seed sent me by Prof. Asa Gray), for the spontaneous revolving movements of the internodes and tendrils were first observed by me in this case, and greatly perplexed me. My observations may now be much condensed. I observed thirty-five revolutions of the internodes and tendrils; the slowest rate was 2 hrs., and the average rate, with no great fluctuations, 1 hr. 40 m. Sometimes I tied the internodes, so that the tendrils alone moved; at other times I cut off the tendrils whilst very young, so that the internodes revolved by themselves; but the rate was not thus affected. The course generally pursued was with the sun, but often in an opposite direction. Sometimes the movement during a short time would either stop or be reversed; and this apparently was due to interference from the light, as, for instance, when I placed a plant close to a window. In one instance, an old tendril, which had nearly ceased revolving, moved in one direction, whilst a young tendril above moved in an opposite course. The two uppermost internodes alone revolve; and as soon as the lower one grows old, only its upper part continues to move. The ellipses or circles swept by the summits of the internodes are about three inches in diameter; whilst those swept by the tips of the tendrils, are from 15 to 16 inches in diameter. During the revolving movement, the internodes become successively curved to all points of the compass; in one part of their course they are often inclined, together with the tendrils, at about 45° to the horizon, and in another part stand vertically up. There was something in the appearance of the revolving internodes which continually gave the false impression that their movement was due to the weight of the long and spontaneously revolving tendril; but, on cutting off the latter with sharp scissors, the top of the shoot rose only a little, and went on revolving. This false appearance is apparently due to the internodes and tendrils all curving and moving harmoniously together.
A revolving tendril, though inclined during the greater part of its course at an angle of about 45° (in one case of only 37°) above the horizon, stiffened and straightened itself from tip to base in a certain part of its course, thus becoming nearly or quite vertical. I witnessed this repeatedly; and it occurred both when the supporting internodes were free and when they were tied up; but was perhaps most conspicuous in the latter case, or when the whole shoot happened to be much inclined. The tendril forms a very acute angle with the projecting extremity of the stem or shoot; and the stiffening always occurred as the tendril approached and had to pass over the shoot in its circular course. If it had not possessed and exercised this curious power, it would infallibly have struck against the extremity of the shoot and been arrested. As soon as the tendril with its three branches begins to stiffen itself in this manner and to rise from an inclined into a vertical position, the revolving motion becomes more rapid; and as soon as the tendril has succeeded in passing over the extremity of the shoot or point of difficulty, its motion, coinciding with that from its weight, often causes it to fall into its previously inclined position so quickly that the apex could be seen travelling like the minute hand of a gigantic clock.
The tendrils are thin, from 7 to 9 inches in length, with a pair of short lateral branches rising not far from the base. The tip is slightly and permanently curved, so as to act to a limited extent as a hook. The concave side of the tip is highly sensitive to a touch; but not so the convex side. I repeatedly proved this difference by lightly rubbing four or five times the convex side of one tendril, and only once or twice the concave side of another tendril, and the latter alone curled inwards. In a few hours afterwards, when the tendrils which had been rubbed on the concave side had straightened themselves, I reversed the process of rubbing, and always with the same result. After touching the concave side, the tip becomes sensibly curved in one or two minutes; and subsequently, if the touch has been at all rough, it coils itself into a helix. But the helix will, after a time, straighten itself, and be again ready to act. …
The revolving movement of a tendril is not stopped by the curving of its extremity after it has been touched. When one of the lateral branches has firmly clasped an object, the middle branch continues to revolve. When a stem is bent down and secured, so that the tendril depends but is left free to move, its previous revolving movement is nearly or quite stopped; but it soon begins to bend upwards, and as soon as it has become horizontal, the revolving movement recommences. I tried this four times; the tendril generally rose to a horizontal position in an hour or an hour and a half; but in one case, in which a tendril depended at an angle of 45° beneath the horizon, the uprising took two hours; in half an hour afterwards it rose to 23° above the horizon and then recommenced revolving. This upward movement is independent of the action of light, for it occurred twice in the dark, and on another occasion the light came in on one side alone. The movement no doubt is guided by opposition to the force of gravity, as in the case of the ascent of the plumules of germinating seeds.
A tendril does not long retain its revolving power; and as soon as this is lost, it bends downwards and contracts spirally. After the revolving movement has ceased, the tip still retains for a short time its sensitiveness to contact, but this can be of little or no use to the plant.
Epipactis latifolia. Watercolor by Elizabeth Wharton, British Flowers.