01} Reproduction in lower & higher plants part 3 ~ Microsporogenesis.
01 Reproduction in Lower and Higher Plants - part 03 - Microsporogenesis
Microsporogenesis :
- Each microspore mother cell divides meiotically to form tetrad of haploid microspores (pollen grains).
- Typical pollen grain is a non-motile, haploid, unicellular body with single nucleus.
- It is surrounded by a two layered wall called sporoderm.
- The outer layer exine is thick and made up of complex, non-biodegradable, substance called sporopollenin.
- It may be smooth or with a sculptured pattern (characteristic of the species).
- It is resistant to chemicals. At some places exine is very thin showing thin areas known as germ-pores.
- These are meant for the growth of emerging pollen tube during germination of pollen grain.
- The inner wall layer, intine consists of cellulose and pectin.
- Pollen viability (viability is the functional ablity of pollen grain to germinate to develop male gametophyte) depends upon environmental conditions of temperature and humidity.
- It is 30 minutes in rice and wheat.
- But in some members of family Solanaceae, Rosaceae, Leguminosae, it lasts even for months.
- Pollen grain marks the begining of male gametophyte.
- It undergoes first mitotic division to produce bigger, naked vegetative cell and small, thin walled generative cell.
- The generative cell floats in the cytoplasm of vegetative cell.
- The second mitotic divisionis concerned with generative cell only and gives rise to two non-motile male gametes.
- The mitotic division of generative cell takes place either in pollen grain or in the pollen tube.
- The pollen grains are shed from the anther, at this two- celled stagein most of the angiosperms
- Female reproductive whorl of flower is gynoecium (Pistil).
- Individual member of gynoecium is called carpel (megasporophyll).
- A flower with many, free carpels is called apocarpous (e.g. Michelia).
- A syncarpous flower is one that has many carpels fused together (e.g. Brinjal).
- Typical carpel has three parts viz, -
- Ovary
- Style and
- Stigma.
- The number of ovules in the ovary varies e.g. paddy, wheat and mango are uniovulate whereas tomato and lady's finger are multiovulate.
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