Formation Of A Male Gametophyte

Microspores

v Filament

Pollen grain (male gametophyte)

figure 30-7

A cross section of an anther of a flower reveals four pollen sacs (microsporangia). Microspore mother cells within the microsporangia undergo meiosis and produce microspores. Each microspore then develops into a two-celled pollen grain (a male gametophyte).

v Filament

Pollen grain (male gametophyte)

One nucleus from each group migrates to the center of the cell. These two nuclei are called polar nuclei because they came from the ends, or poles, of the cell. Cell walls then form around each of the remaining six nuclei. One of the three cells that are nearest to the micropyle enlarges and becomes the egg. The two cells on either side of the egg will later help attract the pollen tube toward the egg. The function of the three cells at the other end of the cell is unclear. These five non-egg cells will eventually die after fertilization occurs.

The resulting structure, which usually contains eight nuclei and seven cells (the egg, the five non-egg cells, and the large central cell enclosing these cells), is the embryo sac. The embryo sac is the mature female gametophyte, or megagametophyte. The embryo sac is another feature that is seen in angiosperms but is not seen in gymnosperms. The surrounding integuments and the embryo sac now form a mature ovule, which may eventually develop into a seed.

Pollen Grain Formation

An anther contains four microsporangia, or pollen sacs, as shown in Figure 30-7. Initially, the pollen sacs contain many diploid cells. These diploid cells are called microspore mother cells. Each of these microspore mother cells undergoes meiosis and produces four haploid microspores. A microspore undergoes mitosis and produces two haploid cells, but these cells do not separate. A thick wall then develops around the microspore. The resulting two-celled structure is a pollen grain, which is the male gameto-phyte, or microgametophyte. The larger of the two cells is the tube cell, from which the pollen tube will form. The generative cell, which is enclosed in the tube cell, will divide by mitosis to form two sperm.

figure 30-7

A cross section of an anther of a flower reveals four pollen sacs (microsporangia). Microspore mother cells within the microsporangia undergo meiosis and produce microspores. Each microspore then develops into a two-celled pollen grain (a male gametophyte).

figure 30-8

These long, tubular flowers are adapted for pollination by hummingbirds, which have long beaks adapted for reaching the nectar located deep in the flowers.

figure 30-8

These long, tubular flowers are adapted for pollination by hummingbirds, which have long beaks adapted for reaching the nectar located deep in the flowers.

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