Carnivorous Plants

Carnivorous Plants, a variety of plants belonging to widely-different groups and occurring in all parts of the world, though all established instances are dicotyledonous, and either aquatic or marsh-haunting forms, in which a considerable proportion of nitrogenous matter is obtained from animals captured by the leaves. These plants may be rootless, as are Aldrovanda and bladderworts (q.v.), or have a slightly-developed root system serving mainly for the absorption of pure water from the barren wet sand or sphagnum bog on which others, such as the sundews (q.v.), flourish. The leaves in the butterworts (q.v.) are not modified in form, but have glands secreting a viscid liquid, and margins that slowly roll inward. Those of Sarracenia (q.v.), Nepenthes, and others, are variously modified into pitchers, sometimes baited with honey-glands externally, and having generally a slippery neck, downward-pointing hairs, and glands secreting a liquid within. The bladderworts have numerous minute bladders with trap-doors, but no liquid secretion. The sundews, and some allied forms (Droseraceae) of exceptionally wide geographical distribution, have lobes or "tentacles" to their leaves, containing spiral vessels, and terminating in a gland secreting a viscid fluid; whilst Dionaea muscipula (q.v.), the Venus's Flytrap of Wilmington, Carolina, has dry eglandular tentacles, with hairs on the blade of the leaf electrically sensitive to the merest trace of nitrogen, the two halves of the leaf-blade closing on a fly like a rat-trap. In this last case rapid motion is substituted for viscosity. In the butterworts, sundews, and Nepenthes, the liquid secreted becomes acid on nitrogenous stimulation: in Dionaea on stimulation a liquid already acid is poured out; and in all these cases a process of true digestion occurs. Zymases or peptogenic ferments are present, and the soft digestible part of the fly or other nitrogenous food is converted into peptones and absorbed. The experiments on the sundew of Dr. Francis Darwin, whose father, Charles Darwin, first directed general investigation to these plants, proved that the plant gains in size, weight, number of shoots, flowers and seeds, and in weight of seed from nitrogenous food taken in this way. In the bladderworts and Sarracenia, on the other hand, there seems to be no digestion, the plant merely absorbing the liquid product of the decay of the captured organisms. As these, in the former, are largely minute crustaceans (water fleas, etc.), the term "insectivorous" is hardly so generally applicable as is "carnivorous." Any nitrogenous food can be taken, such as milk, beef, bacon, milk-biscuit, or even seeds. The delicacy of the test for nitrogen which they afford is one of the most marked peculiarities of the group. "One twenty-millionth of a grain of the phosphate of ammonia (including less than the one thirty-millionth of efficient matter) when absorbed by a gland" of the sundew "leads to a motor impulse being transmitted down the whole length of the tentacle, causing the basal part to bend, often through an angle of above 180 degrees" (Darwin). The captured fly is thus carried to the centre of the leaf: the protoplasm in the cells of the tentacle becomes contracted; and the secretion of all the tentacles becomes almost instantaneously acid. Many of these interesting plants are commonly and easily cultivated, and instructive experiments can be readily performed upon them.