Page:Popular Science Monthly Volume 25.djvu/179

Rh termed, becomes inclosed with one or more coats of varying thickness, and, when the whole structure has reached maturity and is ready to separate from the parent-plant, we have the familiar body known as a seed. The seed is an independent plant-structure designed to develop into a mature herb, shrub, or tree, when the conditions are favorable for its germination and growth.

The lowest class of flowering plants is the Gymnosperms—which includes the cycads and cone-bearing plants. The ovules are naked, and the embryo develops considerable endosperm. This corresponds to the prothallus of higher cryptogams, as in it the corpuscula, which correspond with archegonia, are formed. The pollen-grains are several-celled, thus suggesting a prothallus, especially as only a part of the grain takes part in the formation of the tube and the male fertilizing fluid. The gymnosperms evidently occupy an intermediate place between the higher cryptogams and the angiosperms or flowering plants with their ovules inclosed in an ovary. This last class contains the great mass of plants with evident floral organs, and is divided into the exogens, like the oak, apple, and rose, and the endogens, illustrated by the grasses and cereal grains.

The points that interest us most in the present consideration are, that, unlike the gymnosperms, the ovules are inclosed in an ovary; the endosperm forms in the embryo sac after fertilization, and the pollen-grain sends out its tube without previous cell-division. The pollen-grains often have no rudimentary prothallus. The first result of fertilization is the formation of a cellulose wall around the germ-cell. This cell soon divides, forming the suspensor at the lower end, from which the embryo plantlet is developed. The endosperm-cells form at the same time at the opposite end of the embryo sac. Fig. 17, A, shows a longitudinal section of a young ovule shortly after fertilization. The embryo sac is at e, with a small embryo at the left end, and free endosperm-cells formed at the other. The embryo is shown more magnified at B, and at C is seen the same more advanced. The endosperm is rich in food-materials for the growing embryo, and may be entirely absorbed and the space occupied by the latter.

It now remains for us to determine the extent of the sexual generation in the flowering plants. Among gymnosperms it is not difficult to see that it consists of the pollen-grain and the embryo sac with its