Male Gametophyte (i) Pre-pollination Development (a) The development of male gametophyte is more or less uniform in angiosperms. (b) The development initiates in pollen grains while it is still within the microsporangium or pollen sac. (c) The nucleus of microspore migrates from the centre to periphery and many vacuoles appear in between the nucleus and the wall of the microspore before cell division. (d) The nucleus of the microspore undergoes mitosis to form a larger vegetative cell (or tube cell) and a smaller generative cell. There is no distinct cell wall between these two cells. Both the cells are bound by the cell membrane only. A temporary callose wall is laid down between vegetative and generative cells. The callose wall (plug) spreads between generative celJ and intine to finally pinch the generative cell off.
(e) The callose wall then dissolves and the generative cells become free in the cytoplasm. (f) The larger vegetative cells contain stored food granules like fat, starch, etc.
(ii) Post-pollination Development (a) These changes occur when pollen grains fall on.the stigma of the pistil. (b) Pollen grains absorb the water and available nutrients on the surface of stigma. (c) Intine then protrudes through germ pores and forms a pollen tube. It pierces the stigmatic surface and moves down to ovary. (d) In generative cell, nucleus divides mitotically forming two male nuclei which develop into non-motile male gametes. (e) The nucleus of generative cell migrates to pollen tube. The formation of male gametes can also occur prior to formation of pollen tube. (f) The male gametophyte in flowering plants is thus a highly reduced structure.
