Morphological and Cytological Studies on Carica papaya

Abstract

1. In Carica papaya the primary sporogenous cell appears within the young ovule as the integuments are beginning to differentiate. One or two layers of parietal tissue are present. 2. The megaspore mother cell is formed simultaneous with the development of the two integuments. 3. Through two meiotic divisions the megaspore mother cell produces an axial row of four megaspores, the chalazal one of which develops into a normal seven-celled, eight-nucleate megagametophyte; the other three megaspores disintegrate. The haploid number of chromosomes is nine. 4. The mature megagametophyte is minute compared with the massive integuments and nucellus. The egg cell enlarges as the megagametophyte grows, and a filiform apparatus develops late in the differentiation of the synergids. The antipodal cells disintegrate almost immediately after their formation. 5. Under greenhouse conditions pollen tubes enter the ovules about 10 days after pollination. Fertilization occurs approximately 13-15 days after pollination. Zygotes persist for about 5-8 days before they divide. Divisions occur irregularly in the young embryos. There is a definite lag in the embryo development of this species, while endosperm development is very rapid, as is differentiation of the seed coat. 6. The large massive pollen tube persists within the embryo sac up to at least 64 days after pollination. The fact that it is closely pressed against the many-celled embryo may suggest a nutritive relationship between it and the young developing embryo. 7. At approximately 79 days after pollination the endosperm shows a transition from the free-nuclear state to a cellular condition, beginning at the micropylar end of the sac. Mature endosperm contains an abundance of oil and aleurone grains, but starch is absent. 8. The seed coat is derived chiefly from the outer integument of the ovule, which grows considerably to form the firm compact endotesta and a transparent gelatinous sarcotesta. 9. The vascular bundle in a more mature ovule extends from the funiculus along the outer integument into the inner integument at the chalazal end of the ovule, where it branches widely to form an umbrella-like structure. 10. Modified elongated nucellar cells at the chalazal end of the ovule, located relatively close to the endosperm and also to the vascular tissue, may play an essential role in some nutritive relationship existing between the entering food supply and the endosperm. 11. Only a very small percentage of the ovules in a given pistil produce embryos, even though endosperm and pollen tubes are present. However, an ovule may grow and develop to a certain extent after pollination, even though fertilization does not occur.