| Resumo: |
The ovule of C. arabica L. consists õf a single integument and a small nucellús which disappears as the ovule matures. Three of the four macrospores resulting from the'division of the macrosporocyte, degenerate. The remaining chalazal cell gives rise to a "normal'' embryo sac, which is ready for fertilization at the time of the flower opening. Double fertilization occurs, as a rule, the day the flower opens. The embryo sac then increases in volume and compresses the inner integument cells. The outer cells of the integument, however, multiply actively, giving rise to the "perisperm". After degeneration of the synergids and antipodals, the zygote stays near the micro-pyle in a resting stage, while the primary endosperm nucleus divides. This first division of the endosperm occurs from 21 to 27 days after flower opening. The cytoplasm condenses around the newly formed nuclei, permitting the adjacent tissues to sink into the embryo sac. Since the separating walls were not seen at the binueleate stage and were present at the four-nucleate stage, it seems that the endosperm belongs to the' "nuclear type". As the number of endosperm cells increases, the "perisperm" cells are again compressed and give more and more room to the new tissue. The first division in the zygote occurs from sixty to seventy days after flower opening, when the endosperm is already multinucleate. A differentiated embryo develops, with a hypocotyl and two small cotyledons in the ripe seed. In the ripe seed the "perisperm" disappears almost completely: its remains form the thin "silver skin" which envelops the endosperm. The parchment layer which envelops the seed is the endocarp.
|
