Life of a Pollen Grain

Pollen is the male gametophyte component used in the sexual reproduction of seed plants. It is produced within the male organ of plants and then is transferred to the female organs of other plants. Before pollen grains, plants depended on spores for reproduction. The first appearance of the pollen grain, more than 300 million years ago, freed plants from their reliance on standing water for fertilization.

Formation

Pollen grains are formed through a process called meiosis by the mother cells in the middle of the pollen sacs in plants. These pollen sacs are located within the anther of angiosperms (flowering plants) and within the microsporangiate cones in gymnosperms (cone-bearing plants). Meiosis is the process of cell division where the nucleus divides into four nuclei. Each of those nuclei contains half the usual number of chromosomes. After meiosis takes place, the resulting cells are called microspores but later become pollen grains. These one-celled microspores then divide by mitosis, forming pollen grains containing two nuclei. Mitosis is when a cell divides to produce two daughter cells, each one having the same number of chromosomes as the original cell. These mature pollen grains then begin to form elaborate walls.

Pollen Wall Characteristics

The pollen wall functions to protect the sperm nucleus from damage during transfer from the anther of one plant to the stigma of another. The pollen grain is coated with waxes and proteins within a two-layer structure. The outer wall is called the exine, and the inner wall is called the intine. The two layers allow for further protection from crushing or collapsing. The exine makes each species of pollen grains characteristic because it can be smooth or covered with spines, warts, particles, pores or grooves.

Transfer

Pollen grains can be transferred by wind (anemophily) or by animals (zoophyly). Once the pollen grain reaches the stigma of a receptive flower, one that has not already been fertilized, the pollen germinates and begins the formation of a pollen tube.

Germination

When germination occurs, one of the pollen grain's two original nuclei divides by mitosis to produce two sperm nuclei. At this point, there are now two sperm nuclei as well as one of the original nuclei. This germinated pollen grain with the pollen tube and three nuclei represents the mature male gametophyte.

Fertilization

This pollen tube grows through hollow center of the style toward the ovary, where the egg is housed. The three nuclei travel down the pollen tube as it grows. The ovary is where fertilization occurs, marking the end of the independent pollen grain.

References

• University of Tulsa: What Is Pollen?
• University of Arizona: Palynology
• Ohio State University: Flowering Plant Reproduction