If you suffer from seasonal allergies, spring pollen means misery. For car drivers, it means washing a yellow-green sludge from the windshield each morning. However, pollen serves a vital function for plants. Its tiny size lets it carry easily on the wind or even be transported by animals such as bees, helping plants to reproduce and increase their diversity in a variety of environments.
Pollen cells enclose the sperm cells produce by a plant. Pollen cells are easily transported and when blown on the wind or carried by a pollinator, they stick to the female structure of the flower, called the pistil. The pistil leads to the ovary, where the sperm inside of the pollen grain fertilizes the egg to produce a seed.
Pollen develops on male structures called anthers, the club shapes atop filaments at the center of the flower. Retired biology professor John W. Kimball explains on his website how special cells, called "mother cells," divide to produce pollen. Each pollen grain contains three cells, one of which is the tube cell that delivers the sperm to the egg. The other two cells are sperm cells.
When pollen lands on the pistil of a flower, the cells in the pollen grain get right to work. The pistil is usually quite distant from the ovary, which sits at the bottom of the flower, so the tube cell first forms a pollen tube. As Dr. Kimball explains, this extends into an ovule until it reaches the egg. The sperm cells migrate with the pollen tube, and when the end of the tube breaks open, are released into the ovule. One fertilizes the egg, producing a zygote that will germinate from the seed as a new plant. The second sperm cell fuses with other female cells to produce a substance called endosperm, which nurtures the seedling as it grows.
Early in their evolutionary history, plants relied on water to carry sperm cells to egg cells. When fertilization occurred, it produced spores, which developed into adult plants. One of the first evolutionary steps toward modern flowering seed plants occurred when sex cells began to develop as male microspores and female megaspores, according to the Seed Biology Place. The microspores would continue evolving to become pollen, effectively liberating plants from reliance on water for fertilization to occur. Microspores, like pollen, could be carried by the wind.
According to botanists Pam Soltis, Doug Soltis and Christine Edwards writing on the Tree of Life website, flowering plants are the most numerous and diverse plant species on Earth, owing to their advanced evolutionary development, part of which is the system of pollination they use. Before pollen, plants needed to grow where a source of water was regularly available for reproductive purposes. Pollen freed plants from those constraints, allowing them to survive a greater variety of climates. Because the wind or animals carry pollen, it allowed plants to reproduce with individuals located at great distances from each other, resulting in greater genetic diversity.