Describe the structure of pollen grain and explain the development of male gametophyte in it

Explanation

The pollen grains represent the male gametophytes that is located in microspores. By touching the opened anthers of Hibiscus or any other flower, deposition of yellowish powdery pollen grains are found on fingers. Sprinkling these grains on a drop of water taken on a glass slide and observing under a microscopes, variety of architecture sizes, shapes, colours, designs are seen on the pollen grains from different species.
Structure of Pollen grains: Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. It has a promient two layered wall. The hard outer layer called the exine is made up of sporopollenim which is one of the most resistant organic material

It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known.

Pollen grain exine has prominent apertures called germ proes. Where sporopollenn is absent. Pollen grains are well preserved as fossils because of the presence of sporopollenin. The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin.
The development of male gametophyte in pollen grain. The cytoplasm of pollen grain is surrounded by a plasma membrane. When the pollen grain is mature it contains two cells, the vegetative cell and generative cell. The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle shaped with dense cytoplasm and a nucleus. In over 60 percent of angiosperms, pollen grains are shed at this 2-celled stage. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled). Pollen grains of many species cause severe allergies and bronchial affictions in some people often leading to chronic respiratory disorders-asthma bronchitis etc.
Note: It may be mentioned that Parthenium of carrot grass that came into India as contaminant with imported wheat, has become ubiquitous in occurrence and causes pollen allergy.