Cotton embryogenesis: The pollen cytoplasm

The ultrastructure and composition of cotton (Gossypium hirsutum) pollen, exclusive of the wall, was examined immediately before and after germination. The pollen grain before germination consists of two parts: the outer layer and a central core. The outer layer contains large numbers of mitochondria and dictyosomes as well as endoplasmic reticulum (ER). The core contains units made of spherical pockets of ER which are lined with lipid droplets and filled with small vesicles; the ER is rich in protein and may contain carbohydrate while the vesicles are filled with carbohydrate. Starch-containing plastids are also present in the core as are small vacuoles. The cytoplasm of the pore regions contains many 0.5 μ spherical bodies containing carbohydrate. After germination the ER pockets open and the lipid droplets and small vesicles mix with the other portions of the cytoplasm. With germination the pore region becomes filled with mitochondria and small vesicles. The vegetative nucleus is large, extremely dense and contains invaginations filled with coils of ER. A greatly reduced nucleolus is present in the generative cell which is surrounded by a carbohydrate wall. The cytoplasm of the generative cell is dense and contains many ribosomes, a few dictyosomes and mitochondria, many vesicles of several sizes, and some ER. No plastids were identified. The generative nucleus is also dense with masses of DNA clumped near the nuclear membrane. An unusual tubular structure of unknown origin or function was observed in the generative cell.