CLIMBING PLANTS
Passion flowers, introduced to Europe from the New World in the sixteenth century, were hugely popular in Darwin’s era, cultivated in botanical gardens and private greenhouses where their showy, intricate, and colorful flowers captivated plant lovers and led to cultivation of countless varieties. There are some 550 species in the genus Passiflora worldwide, mainly from Central and South America, and most of them herbaceous climbers. Darwin’s interest in this group began with questions about hybridization and their inability to self-pollinate—a mystery at the time—and soon expanded to include pollinators and seed dispersal. Later, in the 1860s, he turned his attention to the climbing abilities of Passiflora, and there his interest turned into a passion.
The tendrils intrigued him, as they appeared to be unusual in their development. It might be natural to assume that the tendrils of different climbing plant groups are all derived from basically the same ancestral structure—leaves, for example—but Darwin became convinced that different parts became modified in the same way in different groups, converging on the tendril morphology. His inference grew out of comparative studies facilitated by Joseph Hooker at the Royal Botanic Gardens, Kew: “I am getting very much amused by my tendrils,” he wrote to Hooker. “Will you just think whether you know any plant, which you could give or lend me or I could buy with tendrils remarkable in any way, for development, for odd or peculiar structure or even for odd place in natural arrangement … It is really curious the diversification of irritability.”113 That “diversification” is key—cases of diversified structures evolving into the same strikingly convergent structure and function spoke to him of the power of selection.
Darwin worked on four species of passion flowers. He asked his son William to make detailed microscopic observations of their tendril development and concluded from his sketches that, in this group, tendrils are derived from peduncles, or flower stems.114 He was also struck by the rapidity of the circular searching movement (circumnutation) of some Passiflora tendrils and their exquisite sensitivity to touch. Passiflora gracilis, the champion on both counts, was further intriguing because the internodes of young shoots rotated to boot. Darwin thought that might be a sign of a shoot-twining past. Do any living passion flowers or their relatives have twining shoots, he wondered? He could find none, so he asked Daniel Oliver, keeper of the Herbarium at Kew, to put the question to his fellow botanists: “At any time when botanists congregate thickly, or you come across any one who has studied the order of Passiflorae, will you ask the assembly whether any member of the order climbs without the aid of tendrils i.e. spirally twines; I am really anxious to know.”115 This was very much in keeping with Darwin’s crowd-sourcing, and his enthusiasm and curiosity were infectious. “I have at last got a flower of Passiflora Princeps [now P. racemosa],” wrote another correspondent, Thomas Farrer, after being exhorted to make observations. “You were quite right, as you seem always to be. … We have been deeply interested in watching the wonderful motions of Passifloras in climbing. They seek and find and hold on and pull up like an animal.”116 Yes, Darwin surely thought, that was just it!
Passiflora gracilis.—This well-named, elegant, annual species differs from the other members of the group observed by me, in the young internodes having the power of revolving. It exceeds all the other climbing plants which I have examined, in the rapidity of its movements, and all tendril-bearers in the sensitiveness of the tendrils. The internode which carries the upper active tendril and which likewise carries one or two younger immature internodes, made three revolutions, following the sun, at an average rate of 1 hr. 4 m.; it then made, the day becoming very hot, three other revolutions at an average rate of between 57 and 58 m.; so that the average of all six revolutions was 1 hr. 1 m. The apex of the tendril describes elongated ellipses, sometimes narrow and sometimes broad, with their longer axes inclined in slightly different directions. The plant can ascend a thin upright stick by the aid of its tendrils; but the stem is too stiff for it to twine spirally round it, even when not interfered with by the tendrils, these having been successively pinched off at an early age.
When the stem is secured, the tendrils are seen to revolve in nearly the same manner and at the same rate as the internodes. The tendrils are very thin, delicate, and straight, with the exception of the tips, which are a little curved; they are from 7 to 9 inches in length. A half-grown tendril is not sensitive; but when nearly full-grown they are extremely sensitive. A single delicate touch on the concave surface of the tip soon caused one to curve; and in 2 minutes it formed an open helix. A loop of soft thread weighing 1/32nd of a grain (2.02 mg.) placed most gently on the tip thrice caused distinct curvature. A bent bit of thin platina wire weighing only 1/50th of a grain (1.23 mg.) twice produced the same effect; but this latter weight, when left suspended, did not suffice to cause a permanent curvature. These trials were made under a bell-glass, so that the loops of thread and wire were not agitated by the wind. The movement after a touch is very rapid: I took hold of the lower part of several tendrils, and then touched their concave tips with a thin twig and watched them carefully through a lens; the tips evidently began to bend after the following intervals—31, 25, 32, 31, 28, 39, 31, and 30 seconds; so that the movement was generally perceptible in half a minute after a touch; but on one occasion it was distinctly visible in 25 seconds. One of the tendrils which thus became bent in 31 seconds, had been touched two hours previously and had coiled into a helix; so that in this interval it had straightened itself and had perfectly recovered its irritability. …
Passiflora quadrangularis.—This is a very distinct species. The tendrils are thick, long, and stiff; they are sensitive to a touch only on the concave surface towards the extremity. When a stick was placed so that the middle of the tendril came into contact with it, no curvature ensued. In the hothouse, a tendril made two revolutions, each in 2 hrs. 22 m.; in a cool room, one was completed in 3 hrs., and a second in 4 hrs. The internodes do not revolve; nor do those of the hybrid P. floribunda. …
The tendrils of many kinds of plants, if they catch nothing, contract after an interval of several days or weeks into a spire; but in these cases, the movement takes place after the tendril has lost its revolving power and hangs down; it has also then partly or wholly lost its sensibility; so that this movement can be of no use. The spiral contraction of unattached tendrils is a much slower process than that of attached ones. Young tendrils which have caught a support and are spirally contracted, may constantly be seen on the same stem with the much older unattached and uncontracted tendrils. … A full-grown tendril of Passiflora quadrangularis which had caught a stick began in 8 hrs. to contract, and in 24 hrs. formed several spires; a younger tendril, only two-thirds grown, showed the first trace of contraction in two days after clasping a stick, and in two more days formed several spires. It appears, therefore, that the contraction does not begin until the tendril is grown to nearly its full length. Another young tendril of about the same age and length as the last did not catch any object; it acquired its full length in four days; in six additional days it first became flexuous, and in two more days formed one complete spire. This first spire was formed towards the basal end, and the contraction steadily but slowly progressed towards the apex; but the whole was not closely wound up into a spire until 21 days had elapsed from the first observation, that is, until 17 days after the tendril had grown to its full length. …
The spiral contraction which ensues after a tendril has caught a support is of high service to the plant; hence its almost universal occurrence with species belonging to widely different orders. When a shoot is inclined and its tendril has caught an object above, the spiral contraction drags up the shoot. When the shoot is upright, the growth of the stem, after the tendrils have seized some object above, would leave it slack, were it not for the spiral contraction which draws up the stem as it increases in length. Thus there is no waste of growth, and the stretched stem ascends by the shortest course.
Phaseolus coccineus. Water and bodycolor on vellum by Dame Ann Hamilton, Drawings of Plants.