204 



MORPHOLOGY OF GYMNOSPERMS 



and occasionally a multinucleate cell is observed; but in mature 

 endosperm tissue the cells are uninucleate. Whether this is due to 

 disorganization or fusion of nuclei, or to additional wall-formation, 

 was not determined. Centripetal growth of the endosperm is rapid 

 through July and August, the sac being filled with tissue by the last 

 week of August. In the closure of the tissue at the center of the sac, 

 there is no formation of a common wall, but two distinct abutting 

 walls are formed, so that even in the ripe seed the tissue of the endo- 

 sperm separates easily at the center by a cleavage plane. 



, As the centripetal development pro- 

 ceeds, the whole gametophyte increases 

 in bulk, the inner cells enlarging 

 greatly and occasionally dividing, and 

 those of the peripheral region multi- 

 plying. Soon after the appearance of 

 walls and long before the sac is filled 

 with tissue, the endosperm becomes 

 green; and in a few weeks it is the 

 greenest region of the ovule, the 

 spectroscope showing the presence of 

 chlorophyll. The cells are early filled 

 with starch, some of it doubtless 

 formed by the green gametophyte itself. 

 Outside of the gametophyte starch is 

 abundant only in the outer region of 

 the integument. The endosperm in 

 its growth encroaches upon the nucellar 

 tissues until it destroys not only the previously active tapetal zone, 

 but also most of the undifferentiated nucellar tissue, whose living 

 cells are separated from it by a mass of the dead cells of the tapetal 

 zone. It is probably this zone of crushed cells that has been 

 interpreted as a "tapetum" in certain fossil seeds (p. 37), but 

 whether it was previously active as a tapetum in such cases cannot 

 be determined. 



When the endosperm has reached its full development, the endo- 

 sperm membrane has become much thickened, and the megaspore 

 membrane has become extremely thick and dense. As the endosperm 



Fig. 234. — Ginkgo biloba: walls 

 of endosperm cells in contact with 

 the endosperm membrane, and not 

 extending to the megaspore mem- 

 brane; June 5; X650. — After 

 Miss Carothers (46). 



