Pollen

Plants characteristically undergo a cyclic alternation of generations in which a spore producing generation, called the sporophyte (spore plant), is followed by a gamete (sex cell) producing generation, called the gametophyte (gamete plant), and so on. In evolutionarily higher plants the gamete producing generation has been progressively reduced in size and in the duration of its existence, in seed plants the male gametophyte has been diminished to the tiny pollen grain.

Pollen grains (male gametophytes) are produced on highly modified leaves called microsporophylls (megasporophylls produce female gametophytes). In cone-bearing plants, such as pine trees, the microsporophyll is a scale of a male cone. In flowering plants the microsporophyll is the stamen of the flower. A stamen consists of a pollen producing section, called the anther, and a stalk. As anther matures, four groups of specialized cells develop within it; these specialized cells are called microspore mother cells (or pollen mother cells, or microsporocytes). Each of the four groups of microspore mother cells is surrounded by nutritive tissue and supporting cells and is collectively referred to as an anther sac, a pollen sac, or a microsporangium. Each microspore mother cell divides to form four microspore, which is why the flower bearing plant is called the sporophyte. While still in the anther sac and shorty before being released, each microspore begins to germinate; that is, its nucleus divides into two nuclei, the generative nucleus divides into two nuclei, the generative nucleus and the tube nucleus.

The two nuclei structure is the pollen grain, or male gametophyte. The pollen grain's generative nucleus will later divide into male sex cells, or gametes, which is why the pollen grain is called the gametophyte. The two male sex cells are known as sperm nuclei and correspond to the sperm of animals.

Pollen may be produced in relatively small amounts, as few as several dozen grains per another sac, or in tremendous numbers, as in the wind pollinated coniferous trees, where a single cone may produce millions of grains. Pollen grains are commonly yellow in color but may also be orange, green, or other colors, they range from about 2 microns (8/100,000 in) to several hundred micron or more. Pollen grain contain protein and sugars and serve as an attractant to insects and other animals, which aid in pollination. Because pollen grains are distinctive in shape and structure, their plants of origin can be identified from them; a branch of botany, called palynology, specialized in this study.