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Parts of a Pollen Grain

Pollen grains cause allergies and provide nutrition, but their original purpose is reproduction. Flowering plants disperse pollen grains to disseminate their sperm cells to other members of the species. Each pollen grain consists of two to three cells surrounded by a protective wall. The grains come from the anthers of flowers, which have loculi that serve as the centers of pollen generation.

Germ Cell
- This cell forms the sperm cells that fertilize the egg in the flower. In the loculi of the anthers, cells produce pollen mother cells. They undergo the process of mitosis to form sperm cells: each mother cell creates four sperm cells. The original germ cell divides into two sperm cells. This split can occur before or after the grain reaches the stigma, depending on the species.
Tube Cell
- The tube cell is named for its function. Upon pollination, the grain absorbs water, which triggers new processes in its cells. The tube cell develops into a tube. The sperm cells move through the tube to the egg. While the parent flower is forming pollen grains in its anthers, the tube cell is added to the grain by a layer of cells called the tapetum, which also encase the cells in a wall called the sporoderm.
Sporoderm
- The sporoderm is the wall surrounding the pollen grain. It protects the cells as the grain travels from the anther to a stigma. Once the grain pollinates a flower, apertures in the sporoderm allow the cells to exit the covering. The outer structures of pollen grains persist in the fossil record.
  
  This wall is made up of various layers. There are two main layers in the sporoderm: the exine and the intine. The intine is made of cellulose; the exine is mostly made of sporopollenin, a tough polymer. The exine itself is made up of five layers. The outer three form the sexine, while the inner two are called the nexine.
  
  The outside layer is the supra tactal layer, which has sculpture elements on the surface. This distinctive sculpting allows scientists to identify the species from which a pollen grain came. Sometimes, the surface of the grain has substances that allow it to adhere to objects to facilitate transportation of the pollen to a flower. The next layer is the tectum, a thick protective layer. Underneath the tectum is the columella layer, which is composed of rods that lend strength to the pollen structure. Beneath the collumellae are the foot layer and the endexine layer, which comprise the nexine.

Nature