=657. Fertilization in plants is fundamentally the same as in animals.=—In all the great groups of plants as represented by spirogyra, œdogonium, vaucheria, peronospora, ferns, gymnosperms, and in the angiosperms, fertilization, as we have seen, consists in the fusion of a male nucleus with a female nucleus. Fertilization, then, in plants is identical with that which takes place in animals. =658. Embryo.=—After fertilization the egg develops into a short row of cells, the _suspensor_ of the embryo. At the free end the embryo develops. In figs. 397 and 398 is a young embryo of trillium. =659. Endosperm, the mature female prothallium.=—During the development of the embryo the endosperm nucleus divides into a great many nuclei in a mass of protoplasm, and cell walls are formed separating them into cells. This mass of cells is the _endosperm_, and it surrounds the embryo. It is the _mature female prothallium_, belated in its growth in the angiosperms, usually developing only when fertilization takes place, and its use has been assured. [Illustration: Fig. 393. Mature embryo sac (young prothallium) of lilium. _m_, micropylar end; _S_, synergids; _E_, egg; _Pn_, polar nuclei; _Ant_, antipodals. (Easter lily.)] [Illustration: Fig. 394. Section through nucellus and upper part of embryo sac of cotton at time of entrance of pollen tube. _E_, egg; _S_, synergids; _P_, pollen tube with sperm cell in the end. (Duggar.)] =660. Seed.=—As the embryo is developing it derives its nourishment from the endosperm (or in some cases perhaps from the nucellus). At the same time the integuments increase in extent and harden as the seed is formed. [Illustration: Fig. 395. Fertilization of cotton. _pt_, pollen tube; _Sn_, synergids; _E_, egg, with male and female nucleus fusing. (Duggar.)] [Illustration: Fig. 396. Diagrammatic section of ovary and ovule at time of fertilization in angiosperm. _f_, funicle of ovule; _n_, nucellus; _m_, micropyle; _b_, antipodal cells of embryo sac; _e_, endosperm nucleus; _k_, egg-cell and synergids; _ai_, outer integument of ovule; _ii_, inner integument. The track of the pollen tube is shown down through the style, walls of the ovary to the micropylar end of the embryo sac.] =661. Perisperm.=—In most plants the nucellus is all consumed in the development of the endosperm, so that only minute fragments of disorganized cell walls remain next the inner integument. In some plants, however, (the water-lily family, the pepper family, etc.,) a portion of the nucellus remains intact in the mature seed. In such seeds the remaining portion of the nucellus is the _perisperm_.