It is rather curious that so few cases of adaptive coloration have been described for plants. No one supposes that the slate color of the lichen is connected with the color of the rocks on which it grows, in the sense that the resemblance is of any use to the lichen. Nor does the color of the marine red algæ serve in any way to protect the plants so far as is known. The green color of nearly all the higher plants is obviously connected with the substance, chlorophyl, that is essential for the processes of assimilation, and has no relation to external objects. But when we come to the colors of flowers we meet with curious cases of adaptation, at least according to the generally accepted point of view. For it is believed by many naturalists that the color of the corolla of flowering plants is connected with the visits of insects to the flowers, and these visits are in many cases essential for the cross-fertilization of the flowers. This adaptation is one useful to the species, rather than the individual, and belongs to another category. The leaf of the Venus’s fly-trap, which suddenly closes together from the sides when a fly or other light body comes to rest on it, is certainly a remarkable adaptation. A copious secretion of a digestive fluid is poured out on the surface of the leaf, and the products of digestion are absorbed. There can be no question that this contrivance is of some use to the plant. In other insectivorous plants, the pitcher plants, the leaves are transformed into pitchers. In Nepenthes a digestive fluid is secreted from the walls. A line of glands secreting a sweet fluid serves to attract insects to the top of the pitcher, whence they may wander or fall into the fluid inside, and there being drowned, they are digested. A lidlike cover projecting over the opening of the pitcher is supposed to be of use to keep out the rain. In _Utricularia_, a submerged water-plant, the tips of the leaves are changed into small bladders, each having a small entrance closed by an elastic valve opening inwards. Small snails and crustaceans can pass into this opening, to which they are guided by small outgrowths; but once in the cup they cannot get out again, and, in fact, small animals are generally found in the bladders where they die and their substance is absorbed by forked hairs projecting into the interior of the bladder. The cactus is a plant that is well suited to a dry climate. Its leaves have completely disappeared, and the stem has become swollen into a water-reservoir. “It has been estimated that the amount of water evaporated by a melon cactus is reduced to one six-hundredth of that given off by any equally heavy climbing-plant.” [Illustration: Fig. 1.—The fertilization of _Aristolochia Clematitis_. A, portion of stem with flowers in axil of leaf in different stages. B and C, longitudinal sections of two flowers, before and after fertilization. (After Sachs.)]