Q.Pollen grains are able to withstand extremes of temperature and desiccation, because their exine is composed of

A mature pollen grain is generally spherical, about 25-50 micrometres in diameter, and represents the immature male gametophyte. It has a two-layered wall: a hard outer exine made of sporopollenin (one of the most resistant organic substances known, unaffected by strong acids, alkalis, enzymes or high temperature, which is why pollen exine fossilises well and is used in palynology), interrupted at soft, thin regions called germ pores where sporopollenin is absent; and a thin inner intine made of cellulose and pectin, continuous over the germ pores. On maturity the pollen grain's generative cell divides mitotically to give two non-motile male gametes, while the vegetative cell remains large, food-filled, and irregular. Because it lacks sporopollenin, some pollen (from grasses, ragweed, etc.) can provoke pollen allergy, a hypersensitivity reaction in the respiratory …