The Life Cycle of an Angiosperm

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 The parts of the flower are shown. The base of the perianth, which includes petals and sepals, is called the flora axis. A narrowing called the articulation separates the floral axis from the lower pedicel, which attached the flower to a stem. Microsporangia are in the anthers. Microspores, or mother cells form inside the microsporangia. The microspore undergoes meiosis, producing four cells, each of which becomes a grain of pollen with a hard coating. The pollen grain undergoes mitosis, producing a generative cell and a tube cell. Macrospores form inside vase-like carpel, in the ovules, which are in the ovaries. The macrospores undergo meiosis, producing four cells. The cells then undergo mitosis, producing three antipodals, two polar nuclei, and egg and two synergids, each with a nucleus. Together, these cells are called the megagametophyte, or embryo sac. Pollination occurs when a pollen grain lands on the stigma, the flat structure at the top of the carpel. The tube nucleus grows into the long style, to the ovary. There, the generative cell of the sperm fertilizes the egg.
Angiosperm life cycle. The life cycle of an angiosperm is shown. Anthers and carpels are structures that shelter the actual gametophytes: the pollen grain and embryo sac. Double fertilization is a process unique to angiosperms. (credit: modification of work by Mariana Ruiz Villareal)

The Life Cycle of an Angiosperm (OpenStax Biology 2e)

The adult or sporophyte phase is the main phase of an angiosperm’s life cycle. Like gymnosperms, angiosperms are heterosporous. Therefore, they produce microspores, which will generate pollen grains as the male gametophytes, and megaspores, which will form an ovule that contains female gametophytes. Inside the anther’s microsporangia, male sporocytes divide by meiosis to generate haploid microspores, which, in turn, undergo mitosis and give rise to pollen grains. Each pollen grain contains two cells: one generative cell that will divide into two sperm and a second cell that will become the pollen tube cell.

The ovule, sheltered within the ovary of the carpel, contains the megasporangium protected by two layers of integuments and the ovary wall. Within each megasporangium, a diploid megasporocyte undergoes meiosis, generating four haploid megaspores—three small and one large. Only the large megaspore survives; it divides mitotically three times to produce eight nuclei distributed among the seven cells of the female gametophyte or embryo sac. Three of these cells are located at each pole of the embryo sac. The three cells at one pole become the egg and two synergids. The three cells at the opposite pole become antipodal cells. The center cell contains the remaining two nuclei (polar nuclei). This cell will eventually produce the endosperm of the seed. The mature embryo sac then contains one egg cell, two synergids or “helper” cells, three antipodal cells (which eventually degenerate), and a central cell with two polar nuclei. When a pollen grain reaches the stigma, a pollen tube extends from the grain, grows down the style, and enters through the micropyle: an opening in the integuments of the ovule. The two sperm are deposited in the embryo sac.

A double fertilization event then occurs. One sperm and the egg combine, forming a diploid zygote—the future embryo. The other sperm fuses with the polar nuclei, forming a triploid cell that will develop into the endosperm—the tissue that serves as a food reserve for the developing embryo. The zygote develops into an embryo with a radicle, or small root, and one (monocot) or two (dicot) leaf-like organs called cotyledons. This difference in the number of embryonic leaves is the basis for the two major groups of angiosperms: the monocots and the eudicots. Seed food reserves are stored outside the embryo, in the form of complex carbohydrates, lipids, or proteins. The cotyledons serve as conduits to transmit the broken-down food reserves from their storage site inside the seed to the developing embryo. The seed consists of a toughened layer of integuments forming the coat, the endosperm with food reserves, and at the center, the well-protected embryo.

Most angiosperms have perfect flowers, which means that each flower carries both stamens and carpels. In monoecious plants, male (staminate) and female (pistillate) flowers are separate, but carried on the same plant. Sweetgums (Liquidambar spp.) and beeches (Betula spp. are monoecious. In dioecious plants, male and female flowers are found on separate plants. Willows (Salix spp.) and poplars (Populus spp.) are dioecious. In spite of the predominance of perfect flowers, only a few species of angiosperms self-pollinate. Both anatomical and environmental barriers promote cross-pollination mediated by a physical agent (wind or water), or an animal, such as an insect or bird. Cross-pollination increases genetic diversity in a species.

Image is of a river birch, Baskauf Betula. Seed pods hang from a branch, and appear to have the same composition and appearance of a cluster of grapes.
Beech inflorescences. The female inflorescence is at the upper left. The male inflorescence is at the lower right. (credit: Stephen J. Baskauf, 2002. http://bioimages.vanderbilt.edu/baskauf/10593. Morphbank :: Biological Imaging (http://www.morphbank.net/, 29 June 2017). Florida State University, Department of Scientific Computing, Tallahassee, FL 32306-4026 USA)

Related Topic: The Evolution of Angiosperms

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e


Related External Links:

The origin and diversification of angiosperms

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