While the flower is developing in the bud, a diploid megasporocyte cell differentiates from all the other cells in the ovule (Fig. 23.3). This megasporocyte undergoes meiosis, producing four haploid megaspores. Soon after they are produced, three of these megaspores degenerate and disappear, but the nucleus of the fourth undergoes mitosis, and the cell enlarges. While the cell is growing larger, its two haploid nuclei divide once more. The resulting four nuclei then divide yet another time. Consequently, eight haploid nuclei in all are produced (without walls being formed between them). By the time these three successive mitotic divisions have been completed, the cell has grown to many times its original volume. At the same time, two outer layers of cells of the ovule differentiate. These layers, called integuments, later become the seed coat of the seed. As they develop, they leave a pore, or gap, called the micropyle, at one end.
At this stage, there are eight haploid nuclei in two groups, four nuclei toward each end of the large cell. One nucleus from each group then migrates toward the middle of the cell. These two central cell nuclei may become a binu-cleate cell, or they may fuse together, forming a single diploid nucleus. Cell walls also form around the remaining nuclei. In the group closest to the micropyle, one of the cells functions as the female gamete, or egg. The other two cells, called synergids, either are destroyed or degenerate during or after events that occur later. At the other end, the remaining three cells, called antipodals, have no apparent function, and later they also degenerate. The large sac, usually containing eight nuclei in seven cells, constitutes the female gametophyte (megagametophyte), formerly known as the embryo sac (Fig. 23.4).
Usually while the megagametophyte is developing, a parallel process that leads to the formation of male gametophytes takes place in the anthers. As an anther develops, four patches of tissue differentiate from the main mass of cells. These patches of tissue contain many diploid microsporocyte cells, each of which undergoes meiosis, producing a quartet (also referred to as a tetrad) of microspores. Four chambers or cavities (pollen sacs) lined with nutritive tapetal cells are visible in an anther cross section by the time the microspores have been produced. As the anther matures, the walls between adjacent pairs of chambers break down so that only two larger sacs remain.
Figure 23.3 Life cycle of a typical floweringplant.
After meiosis, the haploid microspores in the pollen sacs undergo several changes more or less simultaneously, the processes usually taking from a day or two to a couple of weeks. The following three changes are the most important:
1. the nucleus in each microspore divides once by mitosis;
2. the members of each quartet of microspores separate from one another (in some species, the separation does not occur, but this is unusual);
3. a two-layered wall, whose outer layer is often finely sculptured, develops around each microspore.
When these events are complete, the microspores have become pollen grains (Fig. 23.5). The outer layer of the pollen grain wall, called the exine, which is often sculptured, contains chemicals that may later react with other chemicals in the stigma of a flower. As a result of these reactions, the pollen grain may germinate or its further develop three megaspores degenerate three successive divisions of the remaining megaspore nucleus take place three megaspores degenerate
three successive divisions of the remaining megaspore nucleus take place the eight nuclei produced by the three successive divisions of the megaspore nucleus become rearranged in what is now called the megagametophyte. It constitutes the female gametophyte.
ment may be blocked, depending on whether or not it originated from the same plant, another plant of the same species, or a plant of a different species.
The cytoplasm of pollen grains is often rich in vitamins, and pollen is often collected and sold in health-food stores as a food supplement. The exine of many pollen grains is virtually indestructible, and pollen has been found preserved in deposits thousands of years old. This has proved invaluable in archaeological investigations. The sculpturing of pollen grain exine of various species is also distinctive enough to allow identification to family, genus, and even species in some instances. This feature has resulted in its use in forensic investigations of criminal cases.
Many pollen grains have three boat-shaped or porelike thin areas (apertures) in the wall, but the number of apertures may range from one to many. The development of the male gametophyte, discussed in the next section,
446 Chapter 23
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