Represent the male gamete and are spherical, having a two-layered wall as;
Exine (outer) − Hard layer made of sporopollenin, which is extremely resistant and can withstand high temperatures, acidic and alkaline conditions, and enzymes
Intine (inner) − Thin and continuous layer made up of cellulose and pectin
Mature pollen grain contains two cells:
Vegetative cell − Large with irregular nucleus, contains food reserves.
Generative cell − Small and floats in the cytoplasm of the vegetative cell.
In 60% of the angiosperms, pollen grains are shed at 2-celled stage while in others generative cell undergoes mitosis to form two male gametes (3-celled stage).
The viability of pollen grains after they are shed depends upon temperature and humidity.
It ranges from 30 minutes to few months.
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