BIO 203


As a flowering plant A. nervosa's reproductive organs are located inside the flower of the plant. These flowers contain both male and female organs. The male organ the is called the stamen and is composed of the anther and the filament. The female part of the flower is called the carpel, or pistil, and is made up of three parts, the stigma, style, and ovary.

To begin reproduction gametophytes must be developed. The male gametophytes are developed in the anther and come from microsporocytes that undergo meiosis to form microspores. These microspores eventually develop into grains of pollen which are made up of two cells, the tube cell, and the generative cell. At the time of pollination the generative cell divides and produces two sperm cells.

The female gametophyte, or megagametophyte, begins developing at roughly the same time as the male gametophyte within the ovaries. Inside the ovaries there are ovules that contain one megasporocyte. These megasporocytes undergo meiosis and produce four haploid megaspores, three of which disintegrate soon after development. The remaining megaspore undergoes three mitotic divisions and produce eight nuclei in an embryo sac within the megaspore. These nuclei begin to cluster in groups of four on opposite sides of the sac, then one nuclei from each cluster moves into the center. These nuclei are called polar nuclei, and will form a polar cell after cells walls develope around them. Cell walls also form around one of the clusters of three nuclei at the end of the sac forming antipodals. The opposite cluster forms the egg apparatus, one becoming the egg cell the others become synergids, or helpers. These three cells look very similar, but only the egg will continue development. Then the antipodals and the synergids deteriorate and we are finally left with the female gametophyte containing one egg cell and the polar cell.

Now the pollen must reach the stigma of flower. This process, called pollination, is usually accomplished by an outside source such as wind or insects. After pollen grains have come into contact with the stigma fertilization can begin. Assuming the pollen is compatable, moisture is absorbed into the pollen and a pollen tube comes out through a pore in the wall. This pollen cell with its three nuclei and pollen tube extended is now considered mature. The tube grows toward the ovary of the flower through the style and eventually penetrates the embryo sac and discharges its contents. One of the sperm cells fuses with the egg forming a diploid zygote, the other sperm fuses with the polar cell which then will form the endosperm nucleus. Meanwhile the tube nucleus disintegrates. This entire process is called double fertilization, a process unique to flowering plants and results in polyploid endosperm tissue.



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