Class 12 Biology Chapter 2 Microsporogenesis & Pollen Grain Important Notes 2023

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CHAPTER 2

SEXUAL REPRODUCTION IN FLOWERING PLANTS 

Microsporogenesis : As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to  form microspore tetrads. As each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad. Each one is a potential pollen or microspore mother cell. 

• The process of formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis. The microspores, as they are formed, are arranged in a cluster of four cells - the microspore tetrad. 

• As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains.

• Inside each microsporangium several thousands of microspores or pollen grains are formed that are released with the dehiscence of anther.

Pollen grain : The pollen grains represent the male gametophytes. Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. It has a prominent two-layered wall. The hard outer layer called the exine is made up of sporopollein which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known. Pollen grain exine has prominent apertures called germpores where sporopollenin is absent. Pollen grains are well preserved as fossils because of the presence of sporopollenin. 
The inner wall of the pollen grains is called the intine. It is a thin and continuous layer made up of cellulose and pectin. The cytoplasm of pollen grains is surrounded by a plasma membrane. When the pollen grains is mature it contains two cells, the vegetative cell and generative cell.

• The vegetative cell is bigger has abundant food reserve and a large irregularly shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle shaped with dense cytoplasm and a nucleus. In over 60 per cent of angiosperms, pollen grains are shed at thi 2-celled stage. In the remaining species, the generative cell divide mitotically to give rise to the two male gametes  before pollen grains are shed (3-celled stage).

• Pollen grains of many species cause severe allergies and bronchial affections  in some people often leading to chronic respiratory disorders - asthma, bronchitis,  etc.

• The period for which pollen grains remain viable is highly variable and to some extent depends on the prevailing temperature and humidity. In some cereals such as rice and wheat, pollen grains lose viability within 30 minutes of their release, and in some members of Rosaceae, Leguminoseae and solanaceace, they maintain viability for months. 

• It is possible to store pollen  grains of a large number of species for years in liquid nitrogen (-196°C). Such stored pollen can be used as pollen banks, similar to seed banks, in crop breeding programmes.

CHAPTER 2 PREVIOUS NOTES LINK HERE👇

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