The pollen grain

A pollen grain is the male gametophyte. Grains are usually spherical and about 25 to 50 micrometres across.

The wall has two layers. The hard outer exine is made of sporopollenin, among the most resistant organic materials known: it survives high temperature and strong acids and alkalis, and no enzyme is known to break it down.

The exine has germ pores, gaps where sporopollenin is absent; the pollen tube later grows out through one of them. Sporopollenin is why pollen is so well preserved as fossils, and the exine carries species-typical patterns.

The inner wall, the intine, is thin and continuous and is made of cellulose and pectin. Inside it the cytoplasm is bounded by the plasma membrane.

A mature grain has two cells. The vegetative cell is larger, holds plenty of food reserve and a large, irregular nucleus. The generative cell is small, spindle-shaped with dense cytoplasm and a nucleus, and floats inside the vegetative cell.

In over 60 per cent of angiosperms pollen is shed at this 2-celled stage. In the rest the generative cell divides by mitosis before shedding, so pollen is shed 3-celled: one vegetative cell and two male gametes.

Pollen of many species triggers allergies and bronchial problems such as asthma and bronchitis. Parthenium (carrot grass), which reached India as a contaminant of imported wheat, has spread everywhere and causes pollen allergy.

Pollen is rich in nutrients and is sold as tablets and syrups as a food supplement; claims have been made that eating it improves the performance of athletes and race horses.

Viability varies with species, temperature and humidity: rice and wheat pollen die within about 30 minutes of release, while pollen of some members of Rosaceae, Leguminosae and Solanaceae stays viable for months. Pollen can be kept for years in liquid nitrogen at −196 °C, in pollen banks used by crop breeders.