can define the process of microgametogenesis ........????
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Answer:
Microsporogenesis in Angiosperms
Microsporangium
The terminal part of the stamen bears the anther. It consists of two lobes. Each lobe has two chambers or loculi, called the pollen sacs or microsporangia. Thus a typical anther is tetrasporangiate. The four pollen sacs appear to lie in the four corners of an anther. The pollen sacs or microsporangia develop hypodermally in anther from strips of archesporial cells.
The transverse section of a mature anther shows the following:
Wall of the anther: The anther wall consists of four layers. These are epidermis, endothecium, middle layers and tapetum.
Epidermis: This is the outermost layer. It is only one cell in thickness.
Endothecium: This layer is situated just below the epidermis. It is a single layer of radially elongated cell with fibrous thickening. Because of the presence of fibrous thickenings, the endothecium is also called fibrous layer. In between these cells, a few cells without thickening are also present. These thinned wall cells collectively form the stomium or line of dehiscence.
Middle layers: The middle layers consist of three to four layers of thinned wall cells. These layers are situated below the endothecium. These layers generally degenerate to provide nourishment to the growing microspore mother cells.
Tapetum: Tapetum is the innermost layer of the wall and is multinucleate. The tapetal cells provide nutrition to the developing microspores. Tapetum secretes both enzymes and hormones. Tapetum is of two types-Secretory or glandular tapetum.and amoeboid or plasmodial tapetum. Pollen chamber: The pollen chamber forms the central cavity of anther lobe. The process of microsporogenesis takes place in the pollen chamber.
Microsporogenesis
Microsporogenesis is the process in which each micropore mother cell divides meiotically to form four haploid microspores or pollen grains. The arrangement of pollen grain in a tetrad is affected by cytokinesis during meiosis. It is of following types:
Simultaneous type: In simultaneous type, the cytokinesis occurs only at the end of meiosis II. This results in the production of four cells by cell wall formation between the four nuclei. The resultant tetrad shows tetrahedral arrangement and is common in dicotyledons.
Successive type: In this type, cytokinesis occurs twice – once at the end of meiosis I, forming two cells and then again at the end of meiosis II to form four cells. The resultant tetrad shows isobilateral arrangement and is found in monocotyledons.
There are five types of tetrads – tetrahedral, isobilateral, decussate, T-shaped and linear. Among these, tetrahedral tetrad is the most common. In the members of family Cyperaceae, pollen mother cell divides meiotically to form four nuclei. Of these, three degenerate, the remaining nucleus form the only microspore. Thus, in the members of Cyperaceae, only one microspore is formed by one pollen mother cell instead of usual four.
In Drosera, Typha etc., all the four pollen grains do not separate but together form compound pollen grains. In Mimosaceae, 8 to 64 pollen grains remain united together. In Calotropis and some orchids, all the pollen grains of an anther lobe form a typical structure called pollinium.
Explanation: