Essential Parts of Flower: Androecium - Microspores and Pollen Grains

Microspores and Pollen Grains:

• Palynology is the study of pollen grains. It helps to identify the distribution of coal and to locate oil fields. Pollen grains reflect the vegetation of an area.
• Microspores are the immediate product of meiosis of the microspore mother cell whereas the pollen grain is derived from the microspore.
• Pollen grains develop from the diploid microspore mother cells in pollen sacs of the anther.
• The pollen grain is haploid, unicellular, uninucleate, and a spherical structure measuring about 25-50 µm in diameter.
• The microspores have protoplast surrounded by a wall that is yet to be fully developed. The pollen protoplast consists of dense cytoplasm with a centrally located nucleus. The wall is differentiated into two layers, namely, the inner layer called intine and the outer layer called exine.

1. Intine: Intine is thin, uniform, and made up of pectin, hemicellulose, cellulose, and callose together with proteins. 
2. Exine: Exine is the outer, thick, and resistant layer. It is composed of sporopollenin which provides resistance to a pollen grain from high temperatures, strong acids, and alkalis. 
   

   Structure of Pollen grain

• The exine is not uniform and is thin at certain areas. When these thin areas are small and round it is called germ pores or when elongated it is called furrows.
• It is associated with the germination of pollen grains. Pollen grain or microspore is the first cell of male gametophyte or immature male gametophyte.
• The sporopollenin is generally absent in germ pores. The surface of the exine is either smooth or sculptured in various patterns (rod-like, grooved, warty, punctuate, etc.) The sculpturing pattern is used in plant identification and classification.
• The shape of a pollen grain varies from species to species. It may be globose, ellipsoid, fusiform, lobed, angular, or crescent-shaped. The size of the pollen varies from 10 micrometers in Myosotis to 200 micrometers in members of the family Cucurbitaceae and Nyctaginaceae.
  

  Electron micrographs of a few pollen grains