Answer within 200 words : Give a structure of a typical pollen grain and its pre- and post - pollination changes .

Explanation

The Microspore or Pollen Grain It is a haploid, uninucleate and minute spore produced in large numbers by melosis in the microspore mother cell. They vary in their size, shape, colour, design, etc., from species to species.

Pollen grains are generally spherical measuring about 25-50 micrometer in diameter. It has a two layered wall also called sporoderm), outer hard layer exine is made up of sporopollenir. It is the most resistant organic material as it can withstand high temperature and strong acids and alkali.
No known enzyme can degrade it. Thus, pollens are well preserved as fossils. Pollen grains have prominent distal aperture for germination called germ pore, where sporopollenin is absent. The inner intine layer is thin and chiefly composed of cellulose and pectin.
In insect pollinated pollen grains, the exine is covered by a yellowish viscous and sticky substance called pollenkitt which emits smell.
Formation of Male Gametophyte
(i) Pre-pollination development Development of male gametophyte starts in pollen grains while still present in the microsporangium or pollen sac. The nucleus of pollen grain grows in size, moves to one side near the wall and divides mitotically to form a vegetative cell and a generative cell. The shape of generative cell is elongated which helps in its passage through pollen tube. The vegetative cell contains stored food and also contains protein granules. The microspore containing the two cells, i.e. vegetative and generative cell, is shed at this stage from microsporangia.
(ii) Post-pollination development On reaching the stigma, pollen grain absorbs water and nutrients from the stigmatic secretion through its germ pores and the tube (vegetative) cell enlarges. Now in the generative cell, the nucleus divides mitotically to form two male nuclei which become surrounded by a thin cytoplasmic sheath and appear as distinct non-motile male gametes. The nucleus of the generative cell, migrates to pollen tube. As the pollen tube elongates, the distal part becomes highly vacuolated and becomes separated from the anterior part containing the three nuclei, by formation of callose plug.