655. Embryo sac is the young female prothallium.—In figs. [391]-[393] are shown the different stages in the development of the embryo sac in lilium. The embryo sac at this stage is the young female prothallium, and the egg is the only remnant of the female sexual organ, the archegonium, in this reduced gametophyte.
656. Fertilization.—When the pollen tube has reached the embryo sac (paragraph 652) it opens and the two sperm cells are emptied near the egg. The first sperm nucleus enters the protoplasm surrounding the egg nucleus and uniting with the latter brings about fertilization. The second sperm nucleus often unites with the endosperm nucleus (or with one or both of the “polar nuclei”), bringing about what some call a second fertilization. Where this takes place in addition to the union of the first sperm nucleus with the egg nucleus it is called double fertilization. The sperm nucleus is usually smaller than the egg nucleus, but often grows to near or quite the size of the egg nucleus before union. See figs. [394] and [395].
Fig. 392.
Two-and four-celled stage of embryo sac of lilium. The middle one shows division of nuclei to form the four-celled stage. (Easter lily.)
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.
Fig. 393.
Mature embryo sac
(young prothallium) of
lilium. m, micropylar end;
S, synergids; E, egg;
Pn, polar nuclei; Ant,
antipodals. (Easter lily.)