CLIMBING PLANTS
Gloriosa, a genus of climbing plants with eleven species from tropical regions of Africa and Asia, is prized for its dramatically upswept scarlet to orange petals—glorious indeed. Its luscious color and six-stamen one-pistil arrangement of reproductive structures made Gloriosa a titillating example for Erasmus Darwin in his poetic (and racy, for the time) showcase of Linnaeus’s sexual system of botany, The Loves of the Plants, where the flower’s stamens and pistil were anthropomorphized into six swains as the “blushing captives” of a wily lady.174
Erasmus’s grandson Charles would comment on Gloriosa pollination too, but in decidedly less exciting terms: “In the Gloriosa lily, the stigma of the grotesque and rectangularly bent pistil is brought, not into any pathway from the outside towards the nectar secreting recesses of the flower, but into the circular route which insects follow in proceeding from one nectary to the other.”75 But the main reason Darwin grew the plant wasn’t so much pollination as its unusual means of climbing, with tendrils reaching out from the tips of the leaves. “I am getting very much amused by my tendrils,” he opened a letter to Joseph Hooker in the summer of 1863. He was hoping Hooker could recommend plants with unusual tendrils, “remarkable in any way, for development, for odd or peculiar structure or even for odd place in natural arrangement,”76 as part of his evolutionary project, and he soon gratefully received a specimen of fabulous Gloriosa plantii (now synonymized with G. simplex), which grows widely in sub-Saharan Africa. He was “amused” and more, excited that this species provided him with a clue to tendril evolution. Early in his studies of climbing plants, he struggled with the question of whether tendrils were derived from stems or leaves and initially leaned toward the latter as reflected in this note dated 31 January 1864:
[Bignonia unguis], with cases of Gloriosa, [Tropaeolum tricolorum] and Clematis makes me strongly suspect that all tendrils are first leaf climbers. Good because genesis of tendrils otherwise inexplicable.77
He eventually came to see that in some cases (see Passiflora, p. 245), flower-peduncles gave rise to tendrils, but in most cases, tendrils were modified leaves. Gloriosa and other leaf-tip tendril climbers (e.g., Flagellaria indica, Uvularia, and Nepenthes) thus represented an intermediate form in a graduated series, from leaf-climbers with sensitive petioles to true tendril-bearing plants. “Leaves,” he concluded in Climbing Plants, “may acquire all the leading and characteristic qualities of tendrils, namely, sensitiveness, spontaneous movement and subsequently increased strength.”78
Gloriosa Plantii.—The stem of a half-grown plant continually moved, generally describing an irregular spire, but sometimes oval figures with the longer axes directed in different lines. It either followed the sun, or moved in an opposite course, and sometimes stood still before reversing its direction. One oval was completed in 3 hrs. 40m.; of two horseshoe-shaped figures, one was completed in 4 hrs. 35m. and the other in 3 hrs. The shoots, in their movements, reached points between four and five inches asunder. The young leaves, when first developed, stand up nearly vertically; but by the growth of the axis, and by the spontaneous bending down of the terminal half of the leaf, they soon become much inclined, and ultimately horizontal. The end of the leaf forms a narrow, ribbon-like, thickened projection, which at first is nearly straight, but by the time the leaf gets into an inclined position, the end bends downwards into a well-formed hook. This hook is now strong and rigid enough to catch any object, and, when caught, to anchor the plant and stop the revolving movement. Its inner surface is sensitive, but not in nearly so high a degree as that of the many before-described petioles; for a loop of string, weighing 1.64 grain, produced no effect. When the hook has caught a thin twig or even a rigid fibre, the point may be perceived in from 1 hr. to 3hrs. to have curled a little inwards; and, under favourable circumstances, it curls round and permanently seizes an object in from 8 hrs. to 10 hrs.
The hook when first formed, before the leaf has bent downwards, is but little sensitive. If it catches hold of nothing, it remains open and sensitive for a long time; ultimately the extremity spontaneously and slowly curls inwards, and makes a button-like, flat, spiral coil at the end of the leaf. One leaf was watched, and the hook remained open for thirty-three days; but during the last week the tip had curled so much inwards that only a very thin twig could have been inserted within it. As soon as the tip has curled so much inwards that the hook is converted into a ring, its sensibility is lost; but as long as it remains open some sensibility is retained.
Whilst the plant was only about six inches in height, the leaves, four or five in number, were broader than those subsequently produced; their soft and but little-attenuated tips were not sensitive and did not form hooks; nor did the stem then revolve. At this early period of growth, the plant can support itself; its climbing powers are not required, and consequently are not developed. So again, the leaves on the summit of a full-grown flowering plant, which would not require to climb any higher, were not sensitive and could not clasp a stick. We thus see how perfect is the economy of nature.
Humulus lupulus. Watercolor by Elizabeth Wharton, British Flowers.