. Morphology of gymnosperms. Gymnosperms; Plant morphology. 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 th
. Morphology of gymnosperms. Gymnosperms; Plant morphology. 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 bi
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