Figure 306

A cross section of an immature ovule from a flower reveals a single large cell, a megaspore mother cell. This cell undergoes meiosis and produces four megaspores. One of the megaspores undergoes a series of mitotic divisions that results in the formation of an embryo sac (a female gametophyte).

One or more carpels fused together make up the structure called a pistil (also shown in Figure 30-6). The enlarged base of a pistil is called the ovary. A style, which is usually stalklike, rises from the ovary. The tip of the style is called the stigma. Generally a stigma is sticky or has hairs, enabling it to trap pollen grains.

| GAMETOPHYTE FORMATION

In angiosperms, gametophytes develop within the reproductive structures of flowers. Embryo sacs, which are the female gametophytes in angiosperms, form within the ovary of the pistil. Pollen grains, the male gametophytes, form within the anthers of the stamens.

Embryo Sac Formation

In flowering plants, ovules form in the ovary of a pistil. As Figure 30-6 shows, an angiosperm ovule consists of a megasporangium surrounded by two integuments. These two integuments do not completely enclose the megasporangium. At one end of the ovule is the micropyle, through which a pollen tube can enter.

An ovule contains a large diploid cell called a megaspore mother cell. A megaspore mother cell undergoes meiosis and produces four haploid megaspores. In many species of flowering plants, only one megaspore enlarges. The other three degenerate. The remaining megaspore undergoes three mitotic divisions, which produce a cell that has eight haploid nuclei. These nuclei migrate to certain locations within the cell. The nuclei are initially arranged in two groups of four, with one group of nuclei at each end of the cell.

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