These are situated just inside the corolla, and may vary from one to a hundred or more in one flower. They consist of a filament or stalk, comparable to a leaf stalk, and an anther on the top, corresponding to the blade of the leaf. The filament may be absent but the anther is essential, as it contains the powdery-looking pollen grains (fig. 50) that fertilize the embryo cell. The filaments may be free from one anotner, or united in one, two, or more bundles, these various conditions being indicated by special names in the textbooks, which may be consulted where necessary. They may also be seated on various parts of the flower, and the distinctions are very valuable in systematic botany.