CLIMBING PLANTS
A genus of about fifteen species, Cardiospermum is native to the American tropics, with a few species with pantropical distribution and several prized world-wide as ornamentals. All of the species are herbaceous tendrilled vines with flowers produced in compact axillary panicles, each bearing a pair of tendrils at the base. The distinctive feature of the genus is the fruit, an inflated three-lobed pod that inspired the common name “balloon vine,” but equally appealing are the black seeds hidden within, each bearing a distinctive white heart-shaped mark—hence “Love-in-a-puff” (and the genus name, Cardiospermum, derived from Latin for “heart-seed”).
Cardiospermum is in the soapberry family, and the seeds are certainly “loved” by beautiful soapberry bugs, a group of some sixty-five species making up a subfamily of the uninspiringly named scentless plant bug family (Rhopalidae). Darwin would have been intrigued to learn that certain soapberry bugs have become models for the study of rapid evolutionary adaptation, thanks to humans planting Cardiospermum, soapberry (Sapindus saponaria), and their relatives far and wide—within mere decades of introducing new hosts, native soapberry bugs evolve different beak lengths to feed on the new and different-sized fruits, growth rates, and even hostplant preferences.127
Darwin’s interest, however, was in the curious tendrils of these plants. At the height of his fascination with tendrils, Darwin wrote Joseph Hooker at the Royal Botanic Gardens, Kew asking if his friend could “by an extraordinary good chance give me now a plant of Cardiospermum halicacabum (or any other species if such has tendrils).”28 Hooker was always a great help, and Darwin got his plant. Over time he worked out that, like in Vitis (see p. 333), Cardiospermum tendrils are modified flower peduncles—a diagnosis clinched by finding rare cases where the tendrils themselves actually yielded flowers, a curious sport. He also speculated that, besides climbing, the tendrils have a secondary role in helping to secure the large pods so that they don’t get blown about and damaged by the wind.
Cardiospermum halicacabum.—In this family, as in Vitaceae, the tendrils are modified flower-peduncles. In the present plant the two lateral branches of the main flower-peduncle have been converted into a pair of tendrils, corresponding with the single “flower-tendril” of the common vine. The main peduncle is thin, stiff, and from 3 to 4½ inches in length. Near the summit, above two little bracts, it divides into three branches. The middle one divides and re-divides and bears the flowers; ultimately it grows half as long again as the two other modified branches. These latter are the tendrils; they are at first thicker and longer than the middle branch, but never become more than an inch in length. They taper to a point and are flattened, with the lower clasping surface destitute of hairs. At first they project straight up; but soon diverging, spontaneously curl downwards so as to become symmetrically and elegantly hooked, as represented in the diagram. They are now, whilst the flower-buds are still small, ready for action.
Cardiospermum halicacabum. Upper part of the flower-peduncle with its two tendrils.
The two or three upper internodes, whilst young, steadily revolve; those on one plant made two circles against the course of the sun in 3 hrs. 12 m.; in a second plant the same course was followed, and the two circles were completed in 3 hrs. 41 m.; in a third plant, the internodes followed the sun and made two circles in 3 hrs. 47 m. The average rate of these six revolutions was 1 hr. 46 m. The stem shows no tendency to twine spirally round a support; but the allied tendril-bearing genus Paullinia is said to be a twiner. The flower-peduncles, which stand up above the end of the shoot, are carried round and round by the revolving movement of the internodes; and when the stem is securely tied, the long and thin flower-peduncles themselves are seen to be in continued and sometimes rapid movement from side to side. They sweep a wide space, but only occasionally revolve in a regular elliptical course. By the combined movements of the internodes and peduncles, one of the two short, hooked tendrils, sooner or later catches hold of some twig or branch, and then it curls round and securely grasps it. These tendrils are, however, but slightly sensitive; for by rubbing their under surface only a slight movement is slowly produced. I hooked a tendril on to a twig; and in 1 hr. 45 m. it was curved considerably inwards; in 2 hrs. 30 m. it formed a ring; and in from 5 to 6 hours from being first hooked, it closely grasped the stick. A second tendril acted at nearly the same rate; but I observed one that took 24 hours before it curled twice round a thin twig. Tendrils which have caught nothing, spontaneously curl up to a close helix after the interval of several days. Those which have curled round some object, soon become a little thicker and tougher. The long and thin main peduncle, though spontaneously moving, is not sensitive and never clasps a support. Nor does it ever contract spirally, although a contraction of this kind apparently would have been of service to the plant in climbing. Nevertheless it climbs pretty well without this aid. The seed-capsules, though light, are of enormous size (hence its English name of balloon-vine), and as two or three are carried on the same peduncle, the tendrils rising close to them may be of service in preventing their being dashed to pieces by the wind. In the hothouse, the tendrils served simply for climbing.
The position of the tendrils alone suffices to show their homological nature. In two instances, one of two tendrils produced a flower at its tip; this, however, did not prevent its acting properly and curling round a twig. In a third case, both lateral branches, which ought to have been modified into tendrils, produced flowers like the central branch and had quite lost their tendril-structure.
Catasetum saccatum. Chromolithograph drawn by A. Goessens, from Jean Jules Linden, Lindenia, Iconography of Orchids.