Flowering is a major phase in the life cycle of a plant and takes place when the climate is congenial for pollination and the formation of seeds. This phase is, however, preceded by several physiological and morphological changes induced by hormones such as florigen present in the leaves. These changes lead to the differentiation of vegetative buds into floral buds and the development of the floral primordium – the rudimentary or the preliminary stage of the flower or the flowering shoot. The process of formation of microspores meiotically from a microspore mother cell is called microsporogenesis. The vegetative cell and generative cell represent the two-celled stage of the pollen grain. Later, the generative cell undergoes mitotic division to produce two male gametes. The pollen grain is now said to be at the 3-celled stage.
A typical pollen grain, however, is spherical in shape and has a diameter measuring about 25-50 micrometers. It is surrounded by two layers of wall – exine and intine. Intine, on the other hand, is a thin and continuous inner layer of wall composed of cellulose and pectin. On the inner side of the intine is the plasma membrane that surrounds the pollen grain’s cytoplasm. When pollen grains land on the stigma, they germinate into a pollen tube that carries the male gametes towards the embryo sac. This ability of the pollen grain to deliver the male gametes to the embryo sac is called pollen viability. However, factors such as temperature and humidity affect pollen viability. Moreover, pollen grains of different plants have different periods of viability.
Pollen grains of cereals such as wheat and rice lose their viability within 30 minutes of their release whereas pollen grains of some members of Rosaceae, Leguminoseae and Solanaceae remain viable for months. Pollen grains, which play a vital role in plant reproduction, are also often consumed by human beings as they are rich in nutrients. Pollen products, in the form of tablets and syrups, are commonly spotted on shelves of supermarkets Moreover, pollen is also stored in pollen banks in liquid nitrogen at 196 degrees centigrade. The pollen is later used in plant breeding programmes.
Pollen grains, however, can trigger allergies in some people and can cause asthma and bronchitis. In fact, the weed carrot grass or Parthenium hysterophorus and its pollen found in non-cultivated lands in Punjab causes allergies such as eczema, dermatitis and other skin diseases. Pollen, formed inside the stamen, plays an important role in plant propagation and also serves as a source of nutrition although in some cases, it may trigger allergies.