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Flowering Plants: Fertilization
When and if a pollen grain reaches the pistil (pollination), it sticks to the surface of the stigma . There the tube nucleus causes a pollen tube to form and dig its way down through the style. It eventually reaches a small opening in the ovule called the micropyle. Meanwhile, the generative nucleus divides (by mitosis) to produce two haploid sperm nuclei that are carried down the pollen tube into the ovule. (The nucleus of the pollen tube and the two sperm nuclei make up the male gametophyte generation.)
Within the ovule, one of the sperm nuclei joins with the two polar nuclei to form a triploid, or 3n, structure called the endosperm nucleus. The other sperm nucleus fertilizes the egg to form the diploid zygote. These two fusions are known as double fertilization. Zygote formation marks the beginning of a new sporophyte generation.
After fertilization, the zygote develops into an embryo. The endosperm. Nucleusdivides many times to form a mass of tissue called endosperm. Endosperm is the food source for the embryo. The outer covering of the ovule hardens into a protective device, the seed coat. The ovule is now a seed. Within each seed is the plant embryo and a food source.
In summary, a flowering plant has a life cycle in which the sporophyte generation is dominant. Within a seed is a diploid embryo. The embryo grows and develops into a multicellular sporophyte and produces flowers. By meiosis, diploid cells within the ovules produce haploid megaspores, and cells within anthers produce haploid microspores. These structures are the beginning of a gametophyte generation. Cells derived from the megaspores and microspores undergomitosis to became eggs and sperm. Fertilization results in the formation of a diploid zygote, marking the beginning of a sporophyte generation and completing the lifecycle.
แหล่งข้อมูล: BIOLOGY living system. Page 421.
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