The Process of Pollination in Flowering Plants
Investigating Sexual Reproduction in Angiosperms
Aug 17, 2009
The first step in the production of seeds by a flowering plant is the formation of gametes (sex cells). Through pollination gametes are brought together so that fertilization may occur. From this fertilization a seed eventually develops.
The Formation of Gametes
Sexual reproduction, whether in plant or animal, is basically the fusion of male and female gametes – sperm or pollen and egg – to produce a zygote, which will develop into a new individual.
Gametes are unique cells because they are haploid. That is, they contain only half a set of genes. Other body cells are diploid. That is, they contain a full set of genes. In plants, when the haploid pollen (1/2 set) fertilizes the haploid egg (1/2 set), the zygote that results is diploid (full set).
In flowering plants, pollen grains form in the anthers of stamens. An anther contains four microsporangia, or pollen sacs. Initially, the pollen sacs contain many diploid cells known as microspore mother cells. These cells are diploid containing a full set of genes and designated 2n.
Pollen formation occurs in a series of steps: (1) Each of microspore mother cell undergoes meiosis (a type of cell division in which the chromosome number is reduced) and produces four microspores. These cells are haploid. They contain 1/2 set of chromosomes and are designated 1n. (2) A microspore undergoes mitosis (a type of cell division in which the chromosome number is maintained) and divides into two haploid cells. (3) A thick wall then develops around these two cells. The resulting structure is a pollen grain or the male gamete.
The larger of the two cells inside a pollen grain is the tube cell. If the pollen grain lands on a female stigma, this cell will begin to grow a tube down toward the ovary. The smaller of the two cells is the generative cell. This cell will eventually divide by mitosis again to form two sperm.
In flowering plants, ovules (structure that gives rise to and contains the female reproductive cells) form in the ovary of a pistil. An ovule contains a large diploid (2n) megaspore mother cell surrounded by two integuments (flaps). At one end of the ovule is the micropyle, a hole in the integuments through which a pollen tube can enter.
Egg formation occurs in a series of steps: (1) The megaspore mother cell undergoes meiosis and produces four haploid (1n) megaspores. One of these megaspores will enlarge while the other three die. (2) The remaining megaspore undergoes three consecutive mitotic divisions to produce eight separate haploid nuclei in a single cell (the ovule). (3) These nuclei then migrate to certain locations within the ovule:
- Two nuclei migrate to the center of the cell. These two are called polar nuclei because they came from the ends, or poles, of the cell.
- Three nuclei move to the top of the cell and are termed antipodals.
- The remaining three nuclei travel to the bottom of the cell. The outside two become the synergidswhile the middle one matures, enlarges and becomes the egg. The antipodals and synergids are essentially functionless and disappear shortly after fertilization.
- The resulting structure, which is microscopic and usually contains seven cells and eight nuclei is called an embryo sac.
Pollination Brings Gametes Together
Once pollen and egg have been formed, the trick is to get them together (a process called pollination) and affect fertilization (fusion of male and female gametes). Pollination between the same flower or flowers of the same plant is known as self-pollination. Transfer of pollen from the anther of one flower to the stigma of a flower of another plant of the same or related species is known as cross-pollination. Plants produced by cross-pollination are referred to as hybrids.
Self-pollination occurs in many plants. Among the economically important self-pollinating plants are oats, wheat, barley and rice, peas, and beans. Cross-pollination, however, is more common than self-pollination and probably occurs, at least occasionally, in self-pollinated species. Cross-pollination brings about a more diverse combination of the hereditary units of the two parents.
Without pollination fertilization cannot occur and without fertilization seeds are not formed. Thus, the process of pollination is critical to the survival of any sexually reproducing plant species.
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