In the case of the sundew we are obviously face to face with an exceedingly complex adaptation, for not only is there a secretion of the peculiar digestive juices, which occur only in carnivorous plants, but the secreting tentacles are actively motile. That the tentacles more remote from the captive may be excited to curve towards it, it is necessary that the stimulus exerted by it on the heads of the tentacles connected with it be conveyed to the base, and thence to the tips of the other tentacles, for they curve throughout their whole length. The utility of the contrivance is obvious, but that an arrangement so divergent from the ordinary dispositions of plants could be brought about points to the length of time that the processes of natural selection must have gone on, preserving every new little variation, and adding it to the rest.
Fig. 28. Leaf of Venus Fly-trap
(Dionæa muscipula), after Kerner. A,
leaf-blade (Spr) open. St, leaf-stalk.
Stch, sensitive hairs. B, vertical section
through the closed leaf-blade.
Fig. 29. Aldrovandia vesiculosa, a branch with the
traps FA.
Two plants remain to be noticed in conclusion, both possessing movable, closing traps for catching animals. The so-called Venus fly-trap (Dionæa muscipula) is a marsh plant of North America, the leaves of which, like those of Pinguicula and Drosera, are arranged in a rosette on the ground. The individual leaf has a spatula-like stalk and a blade in two halves (Fig. 28, A), each edged with long and strong spinous processes, directed obliquely inwards. The halves of the blade, when the necessary stimulus is applied to the surface, can close together in a very short time, from 10 to 30 seconds. The two rows of marginal spines then cross, as the interlocking fingers of the hands do, and thus form a cage out of which the imprisoned insect cannot escape. The appropriate stimulus to set the mechanism in motion is a light touch, while a more violent shock, or strong pressure, or a current of air, does not cause the trap to close. But if a fly comes to creep about on the leaf, and in doing so touches one of six short jointed hairs rising erect from a minute cushion of cells, then the leaf closes, quickly indeed, but at the same time so gently and imperceptibly that the fly is unaware of danger and does not try to escape. Then numerous purple mucous glands begin to surround the victim with pepsin-containing, acid, digestive juice which gradually dissolves it.
One of the water-plants of Southern Europe, Aldrovandia vesiculosa, which is also to be found in swamps on the northern ridge of the Alps, possesses, in addition to the capturing and digesting apparatus proper, an active motile apparatus, which is set in motion through sensitive hairs. When I found the plant for the first time in a swamp at Lindau, on the Lake of Constance, I took it at first sight for an Utricularia, for the two plants resemble each other in external appearance (cf. Figs. [22] and 29), but the modification of the leaves into traps is quite different. On both halves of the leaf-blade there are numerous bristles (Fig. 30, A), and the lightest touch on these by a little water animal acts as a releasing stimulus to the motile elements of the leaf (Stch). As in the Venus fly-trap, the two halves of the leaf close together somewhat quickly, but quite quietly, and the animal is caught. Fig. 30 shows a section of one of these traps in its closed state. The captive animals cannot escape, because the margins of the leaf shut quite tightly on one another, and are beset with little teeth. Numerous little glands (Dr) secrete a digestive juice, and after some days, or even weeks, the insoluble remains of the minute animals may be found inside the trap.
Fig. 30. Aldrovandia: its trap apparatus. A, open. St, stalk of the leaf. Spr, blade of the leaf. Stch, sensitive bristles. Dr, glands. B, closed, a cross-section.