VENUS'S FLY-TRAP, a remarkable insectivorous plant (Dionaea niuscipula) of the family Droseraceae, 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 sub stantially correct account of the structure and functions of its leaves, and even suggested the probability of their insectivorous habit. Linnaeus declared it the most wonderful of plants (miracu ium 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 de scribed 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. The upper surface of each lobe is covered with minute circular sessile glands. It bears also three fine-pointed sensitive bristles. These contain no fibro-vascu lar bundles, but show a constriction 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—after a latent period of less than a second under suitable temperature conditions—close upon the hinge-like midrib, the spikes interlock, and the insect is imprisoned.
The leaf then forms itself into what may be called a temporary stomach, and the glands, hitherto dry, are stimulated by the presence of chemical substances passing out of the insect to pour out an acid secretion containing an enzyme (q.v.), similar to that
excreted by the leaves of the sundew, which rapidly dissolves the soft parts of the insect. This is produced in such abundance that, when Darwin made a small opening at the base of one lobe of a leaf which had closed over a large crushed fly, the secretion con tinued to run down the footstalk during the whole time—nine days—during which the plant was kept under observation. The closing of the leaf is due to alterations in the cell-structure of the leaf and is later fixed by growth. The closing is accompanied by electrical changes which have been compared with those occurring in stimulated muscle.
Though the bristles are exquisitely sensitive to the slightest contact with solid bodies, yet they are far less sensitive than those of the sundew (Drosera) to prolonged stimulation, a singular relation of 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. After the absorption of the products of digestion of the insect the leaf opens again by a process of growth and is ready for another meal. Dionaea and Mimosa show the two most striking cases of move ment in the plant kingdom.