MICROGAMETOGENESIS IN PLUMBAGO-ZEYLANICA (PLUMBAGINACEAE) .1. DESCRIPTIVE CYTOLOGY AND 3-DIMENSIONAL ORGANIZATION

The generative cell is initiated as a small, lenticular, unpolarized c ell with a cell wall traceable to two origins: the external segment or iginates as intine, while an inner callose positive cell wall forms de novo. As the lenticular generative cell begins its migration into the pollen cytoplasm, the generative cell becomes polarized both external ly and internally, displaying a characteristic shape and patterns of o rganelle distribution oriented with respect to the Vegetative nucleus and independent of pollen aperture location. Separation of the generat ive cell from the pollen wall begins at the end opposite the vegetativ e nucleus and results in an elongating protuberance at the opposite en d of the generative cell; this becomes associated with a preformed gro ove located on the surface of the vegetative nucleus. The generative c ell subsequently separates from the intine near the vegetative nucleus and moves progressively toward the opposite end of the cell; during t his separation, the edge of the wall facing the intine becomes callose -positive and remains so until separating from the intine. The generat ive cell becomes a free cell within the pollen, which is in physical a ssociation with the vegetative nucleus. Generative cell organization a nd organelle content become increasingly polarized during maturation, with microtubules evident both in the elongating protuberance of the g enerative cell and in association with organelles. The generative nucl eus migrates away from the vegetative nucleus and toward the plastid-r ich end of the generative cell, whereas mitochondria are more generall y distributed within the cell. Generative cell polarization is made pe rmanent during mitotic division and cytokinesis, i.e., two sperm cells differing in morphology are formed: the larger cell associated with t he vegetative nucleus (S-ua) contains a majority of the mitochondria, and the smaller, unassociated sperm cell (S-ua) receives the plastids.