COTTON EMBRYOGENESIS: THE EARLY DEVELOPMENT OF THE FREE NUCLEAR ENDOSPERM

An electron microscope study was made of the central cell and the development of the free nuclear endosperm surrounding the zygote and synergids during the first three days after pollination. The cytoplasm of the central cell, concentrated around the partially-fused polar nuclei, contains many ribosomes, mitochondria and large, dense, starch-containing plastids, some dictyosomes and lipid bodies, and long, single cisternae of rough endoplasmic reticulum (RER) that frequently terminate in whorls. Dense, core-containing microbodies are closely associated with the RER. After fertilization the cytoplasm of the 2-and 4-nucleate endosperm shows an increase in number of dictyosomes, and in amount of RER which becomes stacked in arrays of parallel cisternae. Cup-shaped plastids are associated with many long, helical polysomes. Perinuclear aggregates of dense, granular material also appear after fertilization. Granular aggregates and helical polysomes disappear after the first few divisions of the primary endosperm nucleus. During the second and third days of development there is an increase in dictyosome number and RER proliferation, and endosperm nuclei become deeply lobed. Concurrently, there is a sharp decline in the starch and lipid reserves of the central cell and elaborate transfer walls are formed at the micropylar end of the embryo sac and on the outer surface of the degenerating synergid. The transfer walls contain groups of small, membrane-bound vesicles, and are associated with large numbers of mitochondria and with the smooth endoplasmic reticulum.