VENUS'S FLY-TRAP (Dionaea muscipula), a remarkable insectivorous plant, a native of North and South Carolina, first described in 1768 by the American botanist Ellis, in a letter to Linnaeus, in which he gave a substantially correct account of the structure and functions of its leaves, and even suggested the probability of their carnivorism. Linnaeus declared it the most wonderful of plants (miraculum naturae), yet only admitted that it showed an extreme case of sensitiveness, supposing that the insects were only accidentally captured and subsequently allowed to escape. The insectivorous habit of the plant was subsequently fully investigated and described by Charles Darwin in his book on insectivorous plants.
The plant is a small herb with a rosette of radical leaves with broad leaf-like footstalks. Each leaf has two lobes, standing at rather less than a right angle to each other, their edges being produced into spike-like processes (fig. 1). The upper surface of each lobe is covered with minute circular sessile glands, each consisting of from 20 to 30 cells filled with purplish fluid; it bears also three line pointed sensitive bristles arranged in triangle (fig. 3). These contain no fibro-vascular bundles, but present an articulation near their bases, which enables them to bend parallel to the surface of the leaf when the lobes close. When the bristles are touched by an insect the lobes close very sharply upon the hinge-like midrib, the spikes interlock, and the insect is imprisoned (fig. 2). If very minute, and so not worth digesting, it is able to escape between the interlocked spines;
Fig. 1.—Leaf of Venus's Fly-Trap (Dionaea muscipula), viewed laterally in its expanded state, slightly enlarged. (After Darwin.)
more usually, however, it is retained between the lobes, which gradually but firmly compress its until its form is distinguishable from without. The leaf thus forms itself into a temporary stomach, and the glands, hitherto dry, commence, as soon as excited by the absorption of a trace of nitrogenous matter, to pour out an acid secretion containing a ferment or enzyme, similar to that excreted by the leaves of the sundew, which rapidly dissolves the soft parts of the insect. This when Darwin made a small openin at the base of one lobe of a leaf which had closed over a large crushed fly, the secretion continued to run down the foot stalk during the whole time—nine days—during which the plant was kept under observation. The closing of the leaf is due to a redistribution of water in the cells brought about by a change in the protoplasm which follows the stimulation of the sensitive bristles. Though the bristles are exquisitely sensitive is produced in such abundance that,
Fig. 2.—Leaf of D. muscipula closed over Insect. A, viewed from the side; B, from above;
to the slightest contact with solid bodies, yet they are far less sensitive than those of the sundew (Drosera) to prolonged pressure,
Fig. 3—A, sensitive bristle and glands of D. muscipula; B, glands.
a singular difference in evident relation to the habits of the two plants. Like the leaves of Drosera, however, those of Dionaea are completely indifferent to wind and rain. The surface of the blade is very slightly sensitive; it may be roughly handled or scratched without causing movement, but closes when its surface or midrib is deeply pricked or cut. Irritation of the triangular area on each lobe enclosed by the sensitive bristles causes closure. The foot stalk is quite insensitive. Inorganic or non-nitrogenous bodies, placed on the leaves without touching the sensitive bristles, do not excite movement, but nitrogenous bodies, if in the least degree damp, cause after several hours the lobes to close slowly. So too the leaf which has closed over a digestible body applies a gradual pressure, which serves to bring the glands on both sides into contact with the body. Thus we see that there are two kinds of movement, adapted for different purposes, one rapid, excited mechanically, the other slow, excited chemically. Leaves made to close over insoluble bodies reopen in less than twenty-four hours, and are reads, even before being fully expanded, to shut again. But if they have closed over nitrogen-yielding bodies, they remain closely shut for many days, and after re-expanding are torpid, and never act again, or only after a considerable time. Even in a state of nature, the most vigorous leaves are very rarely able to digest more than twice, or at most thrice, during their life.