Life Cycle of the Flowering Plant
|Plant Life Cycles
Unlike animals, plants have multicellular haploid and multicellular diploid stages in their life cycle. Gametes develop from the multicellular haploid gametophytes (Greek phyton, "plant"). Fertilization gives rise to a multicellular, diploid sporophyte that produces haploid spores via meiosis. This type of life cycle is called a haplodiplontic life cycle. It differs from our own diplontic life cycle, where only the gametes are in the haploid state. Haplodiplontic life cycles are best understood by focusing on where meiosis occurs. Gametes are not the direct result of a meiotic division. Diploid sporophyte cells undergo meiosis to produce haploid spores. Each spore goes through mitotic divisions to yield a multicellular, haploid gametophyte. Subsequent mitotic divisions within the gametophyte produce gametes. The diploid sporophyte results from the fusion of two gametes. Among the Plantae, the gametophytes and sporophytes of a species have distinct morphologies (in some algae they look alike). How a single genome can be used to create two unique morphologies is an intriguing puzzle. Aside from the difference in ploidy level between the gametophyte and sporophyte, there may be some interesting parallels to draw with metamorphosis in insects and amphibians.
The life cycle of an angiosperm is represented here by a pea plant (genus Pisum). The sporophyte is the dominant generation, but multicellular male and female gametophytes are produced within the flower of the sporophyte. Cells of the microsporangium within the anther undergo meiosis to produce microspores. Subsequent mitotic divsions are limited, but the end result is multicellular pollen. The megasporangium is protected by two layers of integuments and the ovary wall. Within the megasporangium, meiosis yields four megaspores - three small and one large. Only the large megaspore survives to produce the embryo sac. Fertilization occurs when the pollen germinates and the pollen tube grows toward the embryo sac. The sporophyte generation may be maintained in a dormant state, protected by the seed coat.
At first glance, angiosperms may appear to have a diplontic life cycle because the gametophyte generation of the angiosperm is reduced to just a few cells. However, mitotic division still follows meiosis, and a multicellular gametophyte produces egg or sperm. Male and female gametophytes have distinct morphologies (i.e., angiosperms are heterosporous), but the gametes they produce no longer rely on water for fertilization. Rather, wind or members of the animal kingdom deliver the male gametophyte - pollen - to the female parent. Pollen is not merely a cell, but a multicellular complex. Another innovation is the production of a seed coat, which adds an extra layer of protection around the embryo. The seed coat is also found in the gymnosperms. A further protective layer, the fruit, is unique to the angiosperms and aids in dispersal of the enclosed embryos by wind or animals.
|Revised from a page on Plant Life Cycles and Angiosperm Development.
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