Insectivorous Plants

Abstract

IN the Venus's Fly-trap, Dionœa muscipula (Fig. 5), we have a further differentiation of the organs of assimilation. The sensibility or irritability resides in three hairs—termed by Mr. Darwin “filaments”—on each half of the upper surface of the bilateral leaf; while the function of absorption appears to belong only to a number of small purplish almost sessile glands which thickly cover the whole of the upper face. These glands have also the power of secretion; but only—and here we have another variation from Drosera—when excited by the absorption of nitrogenous matter. The filaments are sensitive both to sudden impact and to contact with other substances, except water; the lobes of the leaf closing together, in the former case very suddenly, in the latter more slowly. If the leaf has closed in consequence of sudden impact or of the contact of non-nitrogenous matter, the two lobes remain concave, enclosing a considerable cavity; shortly re-open in perhaps twenty-four hours; and are at once again irritable. When, however, the irritating foreign substance contains soluble nitrogenous matter, the lobes of the leaf become gradually pressed closely together, and remain closed for a period of many (from nine to twenty-four) days; and when they again open, if they ever do so, are at first scarcely sensitive to renewed irritation. The mode in which (as Mr. Darwin shows) this arrangement is serviceable to the plant by securing the capture of large and permitting the escape of small insects, is highly curious, but too long to quote. The absorption of nitrogenous matter by the glands is accompanied by an aggregation of the protoplasm in the cells of the filaments, similar to that observed in Drosera, but this result does not follow the simple irritation of the filaments. The series of experiments described appears to prove the existence of an actual process of digestion in Dionœa, the closed leaf forming a temporary stomach, within which the acid secretion is poured out. The plant seems to be subject to dyspepsia, which is even fatal when it has indulged too freely in the pleasures of the table, or rather of the leaf. These observations, however, come from America, where, in its native land, its habits may possibly be more intemperate than in this country. Mr. Darwin believes the motor impulse to be transmitted in Dionœa as in Drosera, through the parenchymatous tissue of the leaf.

Insectivorous Plants.

By Charles Darwin, &c. With Illustrations. (London: J. Murray, 1875.)

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