Question

Describe in sequence the events that lead to the development of three celled pollen grain from microspore mother cell in angiosperms.

Answer 1

Hint: The pollen grains or the microspores tell about the first cell generation of the male gametophyte. After the further development of the male gametophyte or microsporogenesis gives rise to a large vegetative cell and a small generative cell.

Complete step by step answer:
Structure and development of male gametophyte:
Male gametophyte (MG) is the haploid structure formed by pollen grains or microspore and it produces the male gametes or antherozoids. The latter undergoes another mitotic division to form two gametes (sperms) either in the pollen grain or pollen tube.
Formation Of MG from pollen grains occurs as follows:
1) Preparation:
The nucleus of the pollen grain migrates from center to periphery and its cytoplasm becomes highly polarized.
2) Formation of vegetative and generative cells:
The migratetation of pollen nucleus divides mitotically to form two unequal cells. The larger cell is called vegetative cell and the smaller, spindle shaped cell is the generative cell. Later, the generative cell floats within the cytoplasm . This is a 2-celled Male gametophyte .
3) Formation of Male gametes:
The generative cell divides by mitosis forming two male cells which then give rise to two non-motile male gametes or antherozoids. This is a 3-celled Male gametophyte.
4) Pollen tube formation: Pollens are shed from the anther and deposited on the stigma(pollination), at the 3-celled stage, as in Beta and Hordeum.On the stigma, pollen grains germinate forming a narrow tube called pollen tube. The pollen tube grows down through the style. The male gametes and the vegetative nucleus along with the cytoplasm is transferred to the pollen tube and is carried at its growing tip.

Note:
Microspore:
- The newly formed pollen grains have very dense cytoplasm with distinct nuclei.
- Soon after its release from tetrad, it undergoes rapid increase in volume.
- The nucleus of the pollen may remain in rest from a couple of days to several weeks depending upon the species.
- The pollen grains contain reserve food materials in the form of carbohydrates, proteins or lipids.