give solution of 28 and 31
Dear Student,
Ans 31. Microsporogenesis
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It is the process of formation of microspore from PMC (Pollen Mother Cells).
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As development occurs in the anther, the sporogenous tissue undergoes meiosis to form microspore tetrad.
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Each cell of sporogenous tissue has capacity to give rise to a tetrad. Hence, each cell is a potential pollen or PMC.
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As the anther matures, the microspores get detached from each other and develop into pollen grains.
Pollen grains
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Represent the male gamete and are spherical, having a two-layered wall:
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Exine (outer) − Hard layer made of sporopollenin, which is extremely resistant and can withstand high temperatures, acidic and alkaline conditions, and enzymes
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Intine (inner) − Thin and continuous layer made up of cellulose and pectin
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Mature pollen grain contains two cells:
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Vegetative cell − Large with irregular nucleus, contains food reserves
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Generative cell − Small and floats in the cytoplasm of the vegetative cell
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In 60% of the angiosperms, pollen grains are shed at 2-celled stage while in others generative cell undergoes mitosis to form two male gametes (3-celled stage).
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The viability of pollen grains after they are shed depends upon temperature and humidity. It ranges from 30 minutes to few months.
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