Generally all the angiospermic plants reproduce by sexual reproduction. It occurs as a result of fusion of male and female reproductive units. These are called as gametes and are haploid (n) in nature. Male gametes or microspores are also known as pollens or pollen grains. Normally, these are formed in large numbers in the anthers of the stamens.
The female gametes or megaspores in the angiosperms are borne in the structures called megasporangium or ovule. The ovules are attached to the ovary which is a part of carpel. The process of development of microspores or megaspores is called sporogenesis. If a male gamete or microspore develops in the anther, it is called microsporogenesis. Similarly, the process of development of magaspore or female gamete in the ovary is called megasporogenesis.
Mierosporogcnesis:
The male reproductive organ together in angiosperms is called androecium. Individual members of the androecium are the stamens. The stamens comprise of filaments and anthers. The anther lobes are connected by a connective.
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The microspores or the male gametes develop in the anthers. Generally, the anthers are four lobed and constituted of homogenous mass of parenchymatous cells. At each lobe, below the epidermis a row of cells become differentiated from the rest.
These are known as the archesporial cells or archesporium. Archesporium is perceptible from the rest of cells by its larger size, radial elongation and conspicuous nucleus. Archesporium divides forming outer primary parietal cells and inner primary sporogenous cells.
The parietal cell by further divisions forms the wall layers. Normally, there is formation a five layered wall of which the outermost forms the epidermis. The layer following this is called endothecium.
This is single layered and helps in dehiscence of the matured spores. The next two layers are generally called the middle layers, which get crushed up at the time of maturity of spores. The innermost of the wall layers is called tapetum. This layer provides nourishment to the developing spores.
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The primary sporogenous cells divide repeatedly to form ultimately the spore mother cells. In certain cases, the primaiy sporogenous cell directly functions as the spore mother cells. These cells are the last cells of the saprophytic generation.
They undergo reduction division to form microspore tetrads. Individual cells of the tetrads are called male gametes or pollens. When pollens mature, they get free from the tetrad wall, secrete a thick outer wall called exile and a thin inner wall, and intone. Intone is continuous and extensible whereas exile is discontinuous and inextensible. The discontinuous portions form germ pores. These haploid pollens after pollination develop male