From Seed to Seed:
During the following discussion, keep in mind that we are talking specifically about flowering plants-rather than the more primitive mosses, for example. Although some of the processes are the same for both groups, it will save us from having to add qualifiers such as "except for this or that group of plants."
In plants, there are some intermediate steps in the process. Rather than producing egg and sperm, plants use meiosis to produce spores. (Mitosis and meiosis are covered in detail in Section C, Plant Growth.) Let's take a closer look.
In the anthers of a flower, certain diploid cells undergo meiosis, forming haploid spores (called microspores-"little spores"). These microspores divide by mitosis to form two-celled pollen, consisting of a tube cell and a generative cell.
If the pollen reaches a receptive stigma, the pollen germinates. The stigma has a sticky surface so that pollen can easily adhere to it. This transfer of pollen from anther to stigma is known as pollination.The tube cell germinates and produces a pollen tube which grows down the style. The long, hollow pollen tube will eventually penetrate the pistil's tissues in search of the egg. At the same time, the generative cell divides by mitosis to produce two sperm. The two sperm will travel down the pollen tube and enter the ovule, which contains the egg.
What egg? The female gametes, or eggs, are formed in
the ovules, within the ovary in the flower. Here,
as above, a certain diploid cell undergoes meiosis. However, in this case,
the result is four haploid megaspores
("large spores"). Three of the megaspores die and disintegrate. The
remaining cell undergoes mitosis several times. One of these daughter
cells then functions as an egg, ready to be united with a sperm.
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Copyright 2001, National Gardening
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