ANGIOSPERMS. 



585 



Results of Fertilisation in the Embryo-sac ; Formation of the Endosperm ^ and 

 Embryo. [The first result of fertilisation seen in the embryo-sac is the division of its 

 nucleus. The action of the pollen-tube on the oosphere is apparent about the 

 same time : the oospore increases in size, elongating in the direction of the long axis 

 of the nucellus. The formation of the endosperm very commonly begins before the 

 division of the oospore, at the latest during the formation of the suspensor. The two 

 nuclei which are formed by the division of the nucleus of the embryo-sac divide again, 

 and this process is repeated until a number of nuclei are formed, lying in the proto- 

 plasm which lines the wall of the embryo-sac, the number of nuclei becoming greater 

 as the embryo-sac increases in size. When the embryo-sac has ceased to grow the 

 protoplasm becomes aggregated around the nuclei, and the area of protoplasm sur- 

 rounding each nucleus is marked out by an ectoplasmic layer, and in this layer a 

 cellulose membrane is formed which is attached externally to the wall of the embryo- 

 sac, and is free at its inner margin ; thus the cells are separated from each other. 

 The cells now increase in size, become vacuolated, and a cellulose wall is formed 

 on their internal surfaces. These processes of cell-formation begin usually at the 

 chalazal end of the embryo-sac and gradually extend to the micropylar. 



Fig. 400. — Viola tricolor; A longitudinal section through the anatropous ovule after fertilisation, // the placenta, w cushion 

 on the raphe, a outer, i inner integument, / the pollen-tube which has entered the micropyle, e the embryo-sac containing the 

 embryo (to the left) and a number of young endosperm cells ; B and C the apices of two embryo-sacs e with the embryo eb 

 attached to it ; the suspensor in B is two-celled. 



In many cases the development of endosperm does not proceed beyond this 

 point, for the growing oospore comes into contact with the parietal layer of cells and 

 prevents any further cell-multiplication : in other cases the cells of the parietal layer 

 multiply by division, and thus fill up the embryo-sac] If the sac increases greatly in 

 size, as, for instance, in Ricinus and in the large-seeded Papilionaceae, the filling up 

 with endosperm does not take place till later, and the centre of the sac is filled in the 

 unripe seed with a clear vacuole-fluid. In the embryo-sac of the Cocoa-nut, which 

 grows to an enormous size, this fluid— the cocoa-nut-milk— remains until the seed is 

 fully ripe, the tissue of the endosperm forming a layer only some millimetres in 

 thickness, which lines the inside of the testa. The very narrow elongated embryo- 

 sacs of plants with small seeds, as Pistia and Arum, are filled up by a single 

 longitudinal row of endosperm cells. In a large number of dicotyledonous plants (as 

 Loranthacege, Orobancheae, Labiatae, Campanulaceae, «&c.), with long narrow tubular 



^ [Hofmeister, Neue Beitrage, Abhand. d. k. sachs. Ges. d. Wiss. VI. — Strasburger, Befruchtung 

 und Zelltheilung ; id. Zellbildung und Zelllheilung, 3rd ed. ; id. Angiospermen und Gymnospermen. 

 — Hegelmaier, Vergl. Unters. ueb. Entwick, dikotyledoner Keime, 1878; id. Zur Embryogenie und 

 Endospermentwickelung von Lupinus, Bot. Zeitg. 1880. — Darapsky, Der Embryosackkern und das 

 Endosperm, Bot. Zeitg. 187^.] 



