Among certain lower animals and plants symbiotic connection is often most intimate. For instance, in the body-wall of certain Sea Anemones and Holothurians there are small green cells which were long believed to be part of the animal, and which puzzled naturalists because they contained chlorophyll, that remarkable green substance characteristic of plants, which gives to them the power of forming food out of its raw inorganic materials. These cells are now known to be minute seaweeds (Algæ), which spend their lives in the animal tissues to the benefit of both organisms. The plant, by virtue of its chlorophyll, absorbs carbon dioxide, decomposes it, and gives out oxygen, which is eagerly seized on by the animal. The animal in its turn liberates carbon dioxide, which is required by the plant. Similar relations exist between Algæ and some of the lowly Radiolarians and Foraminifera; in these cases, the animals being very minute, the plant partner plays a more conspicuous rôle. It is noteworthy that these Algæ are quite capable of living and multiplying separately, free from the body of the animals, and the animals also are capable of pursuing an independent existence. Let us turn now to the relations existing between flowers and insects, which form one of the most picturesque and romantic features of field life, and of which the materials for study and observation are ever at our own doors. What is a flower? A flower is a group of modified leaves set apart for the business of sexual reproduction. The essential parts or _sporophylls_ are of two kinds, which may be borne on the same flower or on separate flowers on one plant, or on separate plants. These are the _stamens_, bearing _pollen grains_ (or _microspores_), from which _male cells_ arise; and _carpels_, which contain _ovules_, each enclosing an _embryo sac_ or _megaspore_, in which is an _ovum_ or _female cell_. Each stamen consists usually of a slender stalk, the _filament_, bearing an oblong head, the _anther_, which contains four chambers, or pollen sacs, filled with pollen grains; these, when mature, escape into the air by the rupturing of the walls of the chambers. Each carpel contains in its lower part an ovary, while its upper part presents to the air a surface charged with nutrient substance, the _stigma_, which is often raised on a slender stalk, the _style_.