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Animation: Sexual Reproduction in Angiosperms

by Pearson
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The dominant plants on Earth are the angiosperms, the flowering plants. Flowers, the reproductive structures of angiosperms, produce fruits, which enclose the seeds. A flower is generally composed of four parts -- sepals, petals, stamens, and one or more carpels. Although the sepals and petals can play an indirect role in reproduction (for example, attracting pollinators), here we'll concentrate on the stamens and carpel, the parts directly involved in reproduction. The stamen, the pollen-containing structure, is composed of two parts: a stalk-like filament and an enlarged anther. Inside the anther are chambers containing diploid cells called microsporocytes. Each microsporocyte undergoes meiosis to produce four haploid microspores. These microspores divide by mitosis to form male gametophytes, or pollen grains. The immature pollen grain consists of a small generative cell enclosed within a large vegetative cell called the tube cell. The generative cell will later divide to form sperm. When conditions are right, the anther opens to release the pollen grains. Pollen is produced in large quantities because most of it will not reach a receptive flower. Some falls to the ground, some is eaten by insects, and so on. The carpel is made up of three parts: the stigma, the style, and the ovary. A carpel may contain more than one ovary. The stigma, at the top of the carpel, is specialized for receiving pollen. The stalk-like style supports the stigma. At the base of the carpel is the enlarged ovary. The ovary contains developing ovules. After fertilization, the ovary becomes the fruit. Each ovule contains a single diploid megasporocyte and is surrounded by a continuous covering except for a narrow canal at one end, called the micropyle. The megasporocyte undergoes meiosis to produce four haploid megaspores. Three of the megaspores degenerate, leaving a single functional megaspore in each ovule. The megaspore divides by mitosis three times, producing a total of eight nuclei, which segregate to form seven cells. The most important elements of the female gametophyte are the egg, located at the micropyle, and the two polar nuclei. The polar nuclei are found in the largest cell of this structure, at the center of the female gametophyte. Sexual reproduction in angiosperms involves the transfer of pollen from an anther to a stigma, a process called pollination. In self-compatible flowers, the stigma is receptive to pollen from the same flower. Self-incompatible flowers require pollen from a different plant. In both cases, pollen can be dispersed by living vectors (such as insects) or nonliving vectors (such as the wind). In this example, pollen from the anther of one flower is transferred to the stigma of another flower. After the pollen grain lands on a receptive stigma, it germinates, producing a pollen tube. The pollen tube grows down through the tissue of the style. At some point during its journey, the generative cell of the pollen grain divides by mitosis to form two sperm nuclei, or male gametes. The pollen tube continues to grow until it reaches the ovary. It then enters an ovule through the micropyle. It ruptures one of the cells next to the egg and discharges the two sperm nuclei. In most plant groups, fertilization is straightforward: One sperm nucleus fuses with the egg to form a diploid zygote. In angiosperms, however, an unusual event called double fertilization takes place. The other sperm nucleus fuses with the two polar nuclei to form a large triploid (3n) cell. The triploid cell resulting from this second fertilization begins a series of mitotic divisions that forms a tissue called the endosperm. The function of the endosperm is to store nutrients.
The dominant plants on Earth are the angiosperms, the flowering plants. Flowers, the reproductive structures of angiosperms, produce fruits, which enclose the seeds. A flower is generally composed of four parts -- sepals, petals, stamens, and one or more carpels. Although the sepals and petals can play an indirect role in reproduction (for example, attracting pollinators), here we'll concentrate on the stamens and carpel, the parts directly involved in reproduction. The stamen, the pollen-containing structure, is composed of two parts: a stalk-like filament and an enlarged anther. Inside the anther are chambers containing diploid cells called microsporocytes. Each microsporocyte undergoes meiosis to produce four haploid microspores. These microspores divide by mitosis to form male gametophytes, or pollen grains. The immature pollen grain consists of a small generative cell enclosed within a large vegetative cell called the tube cell. The generative cell will later divide to form sperm. When conditions are right, the anther opens to release the pollen grains. Pollen is produced in large quantities because most of it will not reach a receptive flower. Some falls to the ground, some is eaten by insects, and so on. The carpel is made up of three parts: the stigma, the style, and the ovary. A carpel may contain more than one ovary. The stigma, at the top of the carpel, is specialized for receiving pollen. The stalk-like style supports the stigma. At the base of the carpel is the enlarged ovary. The ovary contains developing ovules. After fertilization, the ovary becomes the fruit. Each ovule contains a single diploid megasporocyte and is surrounded by a continuous covering except for a narrow canal at one end, called the micropyle. The megasporocyte undergoes meiosis to produce four haploid megaspores. Three of the megaspores degenerate, leaving a single functional megaspore in each ovule. The megaspore divides by mitosis three times, producing a total of eight nuclei, which segregate to form seven cells. The most important elements of the female gametophyte are the egg, located at the micropyle, and the two polar nuclei. The polar nuclei are found in the largest cell of this structure, at the center of the female gametophyte. Sexual reproduction in angiosperms involves the transfer of pollen from an anther to a stigma, a process called pollination. In self-compatible flowers, the stigma is receptive to pollen from the same flower. Self-incompatible flowers require pollen from a different plant. In both cases, pollen can be dispersed by living vectors (such as insects) or nonliving vectors (such as the wind). In this example, pollen from the anther of one flower is transferred to the stigma of another flower. After the pollen grain lands on a receptive stigma, it germinates, producing a pollen tube. The pollen tube grows down through the tissue of the style. At some point during its journey, the generative cell of the pollen grain divides by mitosis to form two sperm nuclei, or male gametes. The pollen tube continues to grow until it reaches the ovary. It then enters an ovule through the micropyle. It ruptures one of the cells next to the egg and discharges the two sperm nuclei. In most plant groups, fertilization is straightforward: One sperm nucleus fuses with the egg to form a diploid zygote. In angiosperms, however, an unusual event called double fertilization takes place. The other sperm nucleus fuses with the two polar nuclei to form a large triploid (3n) cell. The triploid cell resulting from this second fertilization begins a series of mitotic divisions that forms a tissue called the endosperm. The function of the endosperm is to store nutrients.