Although the almost innumerable diversities in the form of leaves must produce corresponding diversities in the inflorescence, yet the general characteristics are the same or similar in both of the great botanical classes. The structure of the calyx and corolla which form the floral envelope of the fructification, is similar to that of leaves. The calyx consists, in its early stage, of several parts called sepals, which have all the characters of leaves; subsequently they are sometimes united by their edges, so as to form a tube, or are otherwise modified. The corolla only differs from a leaf by greater delicacy of organization; it has fewer stomates or breathing pores, and the veins have less woody fibre and sometimes many spiral vessels; the veins in the petals of chickweed and some other plants are entirely composed of spiral vessels. The calyx and corolla protect the fructifying organs consisting of stamens with their anthers, and the pistil or pistils. The stamens are formed of very fine filaments, and the anthers, when young and still enclosed in the unexpanded flower, are full of a liquid which is afterwards changed into a delicate homogeneous, cellular tissue. Then the internal part of that tissue becomes divided into two kinds of cells, one of which goes to form the walls of two lateral and parallel chambers or loculi, separated by a part more or less continuous with the filament. The other kind of cells are developed into pollen grains within the chambers. They gradually form a cylindrical assemblage of mother cells. Within each of these, four cells are ultimately formed, each containing a single pollen cell. In all plants, except the aquatic, the pollen cells are soon clothed by the deposition of one or more layers of cells, which form the outer membrane, on which are impressed figures or markings peculiar to each species of plant, such as slits, points, papillæ, sharply defined circles, pores, &c. The colour of the external layer is generally yellow, rarely green, blue, or red; the thread is usually white, except in the fuchsia and some others. When the pollen is ripe the chambers dehisce to emit it. The opening is usually a slit between the valves which close the chambers. The length of the slit is variable, and the form of the pollen grains is equally so; they are commonly ellipsoidal, and attenuated at the extremities; but in all cases they are beautiful microscopic objects.