note on formation of vegetative and generative cells with appropriate diagram in male gametophyte
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Answer:
Transformation of microspore mother cell (2n) into pollen grain (n) is the first gametophytic structure. On germination pollen grain develops into male gametophyte which starts even before pollination.
Development of Male Gametophyte
The entire process of development of male gametophyte can be divided into two phases:
a. Pre-pollination development and
b. Post-pollination development.
a. Pre-Pollination Development:
Pollen grains show in-situ germination, i.e. germination starts in pollen sac. This is known as precocious germination. In this process a large central vacuole is formed which pushes the nucleus to one side. Now the nucleus undergoes mitosis giving rise to two daughter nuclei. Out of the two cells so formed, the smaller one is called generative cell and the larger one is called vegetative cell.
Vegetative cell has plenty of cytoplasm which has reserve food for the development of male gametophyte. Generative cell moves to the middle of pollen grain. At this stage pollen grain falls on the stigma and the rest of the development takes place on the stigma.
b. Post-Pollination Development:
As the pollen grain falls on the stigma it absorbs nutrients from stigma through the germ pore. Due to this absorption vegetative cell enlarges and intine moves out through the germ pore to form pollen tube. Nucleus of generative cell and vegetative cell migrates to the pollen tube.
Now is the time for the generative cell to divide and give rise to two haploid, unicellular and non-motile male gametes. Hydrolytic enzymes are secreted by the pollen tube which dissolves the tissues of style for deeper penetration of tube. Size of mature male gametophyte is reduced and it gets nutrition from the tissues of style.
