Before reading this section, you may wish to review the section in Chapter 29 entitled "Life cycles of plants feature alternation of generations" (pages 571-572). Central to understanding plant reproduction is the concept of alternation of generations, in which a multicellular diploid generation alternates with a multicellular haploid generation.
In angiosperms, the diploid sporophyte generation is the larger and more conspicuous one. The sporophyte generation produces flowers. The flowers produce spores, which develop into tiny gametophytes that begin and, in the case of the megagametophyte, end their development enclosed by sporo-phyte tisue.
The haploid gametophytes—the gamete-producing gen-eration—of flowering plants develop from haploid spores in sporangia within the flower (Figure 39.1):
► Female gametophytes (megagametophytes), which are called embryo sacs, develop in megasporangia.
► Male gametophytes (microgametophytes), which are called pollen grains, develop in microsporangia.
Within the ovule, a megasporocyte—a cell within the megas-porangium—divides meiotically to produce four haploid megaspores. In most plants, all but one of these megaspores then degenerate. The surviving megaspore usually undergoes three mitotic divisions, producing eight haploid nuclei, all initially contained within a single cell—three nuclei at one end, three at the other, and two in the middle. Subsequent cell wall formation leads to an elliptical, seven-celled megagameto-phyte with a total of eight nuclei (see Figure 39.1):
► At one end of the elliptical megagametophyte are three tiny cells: the egg and two cells called synergids. The egg is the female gamete, and the synergids participate indirectly in fertilization by attracting and accepting the pollen tube.
39.1 Development of Gametophytes and Nuclear Fusion The embryo sac is the female gametophyte; the pollen grain is the male gametophyte.The male and female nuclei meet and fuse within the embryo sac. Most angiosperms have double fertilization, in which a zygote and an endosperm nucleus form from separate fusion events—the zygote from one sperm and the egg and the endosperm from the other sperm and two polar nuclei.
► At the opposite end of the megagametophyte are three antipodal cells, which eventually degenerate.
► In the large central cell are two polar nuclei.
The embryo sac (megagametophyte) is the entire seven-cell, eight-nucleus structure. You can review the development of the embryo sac in Figure 39.1.
The pollen grain (microgametophyte) consists of fewer cells and nuclei than the embryo sac. The development of a pollen grain begins when a microsporocyte within the anther divides meiotically. Each resulting haploid microspore develops a spore wall, within which it normally undergoes one mitotic division before the anthers open and release these two-celled pollen grains. The two cells are the tube cell and the generative cell. Further development of the pollen grain, which we will describe shortly, is delayed until the pollen arrives at a stigma. In angiosperms, the transfer of pollen from the anther to the stigma is referred to as pollination.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.