For some allergy sufferers the word pollen brings to mind something designed to cause pain and suffering; small particles having a dust-like consistency that are produced by plants, and which must be the result of a curse placed on us for eating these plants. For many people the word pollen conjures up an image of that yellow to yellow-green stuff that covers our cars and porches in the spring for days and weeks on end. But what is pollen really? Why is it produced? Why is it so plentiful in the spring? What function does it have? Today’s blog will hopefully answer some of the questions you might have about this substance called pollen, and give you some understanding of why pollen, botanically speaking, is a really remarkable substance and why it is essential for the survival of most plants.
Pollen is produced in the anthers of angiosperms (flowering plants) and in the male cones of gymnosperms (cone-bearing plants). Pollen contains nuclei (the organelles in cells that contain the DNA) that are produced by the process of meiosis. Meiosis is a special type of cell division which serves to produce the gametes or sex cells in plants and animals. In humans, the egg cells are produced in the ovaries, while sperm cells are produced in the testes. In flowers, the egg cells are present in structures called ovules, and the sperm cells are present in pollen grains. The fusion of a sperm nucleus from a pollen grain with the nucleus inside of an ovule forms the zygote (fertilized egg). The zygote then develops into the plant embryo, the part of the seed sometimes called the germ. (You are familiar with this terminology since it is used in discussions of whole grain products; think about whole grain flour which contains the wheat germ versus processed white flour which doesn’t.) So what is pollen? It is the male component of sexual reproduction in flowering and cone-producing plants. And why is pollen produced? It serves to transfer the sperm nuclei from a flower or cone of one plant to a flower or cone of another plant of the same species in order to effect formation of the zygote.
After pollen is produced in the anthers of flowers and male cones of gymnosperms, it is released from these structures in a process called dehiscence. Once pollen is released from the anthers or cones, it is transported in some way to another plant, the process of pollination. If a plant has flowers that are pollinated by insects, or hummingbirds, or bats, then the pollen released from the anthers has to adhere to the bodies of these animals so that it can be transferred from the anthers of one flower to the pistil of another flower. Plants that have their pollination carried out by wind on the other hand, release their pollen from the anthers such that it readily gets into the airstream and is transported to another plant of the same species via wind. It should be obvious that wind-pollination is a much more random, hit-or-miss process than is pollination carried out by an animal pollinator. Hence, wind pollinated plants produce extremely large amounts of pollen to ensure that some of this pollen will reach the stigma of an appropriate flower, and effect fertilization of egg cells in the ovules of that flower. Most tree species reproduce in the spring and many trees use wind as the transporter of their pollen. Trees such as oaks, elms, and pines produce tremendous numbers of pollen grains that fill the air on dry days in the spring and create the yellow to yellow-green layer that covers our cars and patios.
Just think; the reason for all of that pollen in the air during the spring is to ensure successful reproduction for the plants producing it.