give solution of 28 and 31

Dear Student,

Ans 31. Microsporogenesis

  • It is the process of formation of microspore from PMC (Pollen Mother Cells).

  • As development occurs in the anther, the sporogenous tissue undergoes meiosis to form microspore tetrad.

  • Each cell of sporogenous tissue has capacity to give rise to a tetrad. Hence, each cell is a potential pollen or PMC.

  • As the anther matures, the microspores get detached from each other and develop into pollen grains.
     

    Pollen grains

  • Represent the male gamete and are spherical, having a two-layered wall:

    • Exine (outer) − Hard layer made of sporopollenin, which is extremely resistant and can withstand high temperatures, acidic and alkaline conditions, and enzymes

    • Intine (inner) − Thin and continuous layer made up of cellulose and pectin

  • Mature pollen grain contains two cells:

    • Vegetative cell − Large with irregular nucleus, contains food reserves

    • Generative cell − Small and floats in the cytoplasm of the vegetative cell

  • 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).

  • 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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