If any mineral or other non-nutritious substance be placed on a leaf, the contact stimulates the little glands, causing them to discharge a larger quantity of fluid, but no change seems to take place in the character or composition of the secretion. But if any nitrogenous organic substance, such as an insect or a small piece of meat, be brought in contact with the glands, not only will the secretion increase in quantity, but it will also assume an acid character, and contain a ferment which is capable of digesting the nitrogenous material. In fact, the secretion produced under these circumstances possesses the same properties as the gastric fluid of the stomachs of animals. The animal food of the Butterworts consists of small insects and other little creatures. If an insect alights on the leaf, it is caught by the sticky secretion of the glands, and every effort to escape causes it to become more and more besmeared with the mucilage, till, at last, it is no longer able to move; and its death is probably hastened by the stoppage of its spiracles or breathing-holes. If the insect is a small one, and it settles near the edge of the leaf, the curved margin slowly bends over it until it is more or less enclosed, and the larger number of glands thus brought in contact with its body pour out their digestive secretion, which slowly dissolves the nourishing portions, leaving nothing but the legs, wings, and other indigestible parts. A larger insect, alighting similarly near the edge of the leaf, could hardly be enclosed by the bending of the margin near it; but it is pushed towards the middle as the edge curls over, and then the opposite side also bends over it, till the insect is more or less enclosed, when it is digested as mentioned above. The digestion of an insect and the absorption of nutrient matter by the cells of the leaf occupy from twenty to thirty hours, and when the whole is accomplished the leaf slowly expands, assuming its normal position, and exposing the indigestible residue of its prey to be blown away or washed off by the rain. It has been observed that the Butterworts are not exclusively animal feeders, for their leaves readily digest any pollen cells or the spores of the lower plants that are carried to them by the wind. [Illustration: THE ROUND-LEAVED SUNDEW.] Equally interesting are the habits of the Sundew (_Drosera_), of which there are three species, all readily distinguished from every other British plant by the glandular hairs that cover the long-stalked, radical leaves. They have leafless flower-stalks, each bearing a one-sided spike or raceme of white flowers. The sepals, petals, and stamens each number five; and the ovary, which ripens into a one-celled capsule of three or four valves, has three or four forked styles.