[page] 760

In Botany, we have to record the appearance of the long-expected book by Darwin on insectivorous plants. Although much has been written about such plants both in Europe and this country, the present work far surpasses any previous publication in the number and variety of the experiments and the accuracy of the results recorded. The observations are confined principally to members of the Droseraceæ and Lentibulariaceæ, the greater part of the book being devoted to an account of experiments on Drosera rotundifolia (common sun-dew), Dionæa muscipula (Venus's fly-trap), and Utricularia neglecta. In the first-named plant the upper surface of the somewhat concave leaf blades is covered with glandular hairs, which secrete a sticky substance at their tips, by means of which insects are caught. If an object is placed on the hairs in the centre of the leaf, an impulse is communicated to the radial hairs which causes them to bend over until their tips touch the object. If an object is placed on a hair remote from the centre, the other hairs bend over toward it. At the same time that the hairs bend, the secretion from their tips increases in quantity and becomes acid. The rapidity with which they converge over an object is found to depend on the chemical character of the object itself, nitrogenous bodies acting more powerfully than non-nitrogenous bodies. No substances affect the hairs so strongly as salts of ammonia, and the amount of phosphate of ammonia required to cause the hairs to bend is so incredibly small that, were it not for the accuracy of Darwin's record, one would be inclined to doubt the fact. By means of the secretion nitrogenous substances, as insects and pieces of meat, are softened and dissolved, whereas little or no effect is produced on non-nitrogenous substances. Although the chemical analysis of the secretion is difficult, owing to the small amount produced by any plant, judging by its power of dissolving different substances Darwin concludes that it is very closely allied to, if not identical with, the gastric juice. After dissolving digestible matter which has been caught, the hairs straighten themselves into their original position; when an indigestible body is caught, the hairs recover their position much more quickly. Whereas the hairs of Drosera are adapted for catching small insects by means of a sticky substance, the two lobes of the leaves of Dionæa are furnished with three highly sensitive hairs, which when touched cause the lobes to shut up quickly. The margins of the lobes are furnished with teeth, which interlock as the lobes come together, and imprison any insect on the leaf, unless it be very small. An acid secretion is then poured out by glands on the upper surface of the leaf, and digestion takes place as in Drosera, but the process is more difficult to observe than in the last mentioned plant, since the leaf is folded together. In both Drosera and Dionæa, although the hairs are sensitive when touched, they do not seem to be affected by falling drops of rain or by strong currents of air. The species of Utricularia catch their food by means of little traps on the leaves, and the insects caught slowly putrefy. Frequent reference is made throughout the book to experiments by Cauley, Mellichanho, and Mrs. Treat on American species of insectivorous plants.