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Plants reproduce sexually, often relying on pollinators to bring sperm and egg together. The sporophyte (diploid) develops from the zygote by mitosis; it consists of roots, stems, leaves, and flowers Meiosis of cells within flowers produces the small haploid gametophytes, which in turn produce either sperm or eggs. The specialized parts of the flower grow from the modified end of the floral shoot – the receptacle. Sepals: (collectively called the calyx) are the outermost green, leaflike parts. Petals (collectively called the corolla) are the colored parts of located between the reproductive structures and the sepals. Flowers differ from the other tissues of the plant in their fragrance and colors (Carotenoids and anthocyanins), which are attractive to pollinators. Male parts – called stamens are located inside the corolla Stamen consist of a slender stalk (filament) capped with an anther Inside the anthers are pollen sacs in which pollen grains develop Look at the previous picture to see the male parts in a diagram Female parts are located in the central part of the flower Carpel is the vessel-shaped structure with an expanded lower chamber (ovary), slender column (style) and upper surface (stigma) for pollen landing. In ovary eggs develop, fertilization occurs and seeds mature. Refer to diagram Perfect flowers have both male and female parts (may or may not be on the same plant) Imperfect flowers is missing one of the sex In anthers, each diploid “mother” cell divides by meiosis to form four haploid microspores. Each microspores will divide to form pollen grains One cell in each pollen grain will produce the sperm; the other will form the pollen tube In carpel, a mass of tissue forms an ovule (potential seed) enclosed by integuments. A diploid “mother” cells divides by meiosis to produce haploid megaspores, one of which will undergo mitosis three times to produce a cell with eight nuclei The nuclei migrate resulting in an embryo sac (female gametophyte) with seven cells; one cell has two nuclei and will become the endoperm (nutrition for embryo); another cell will be the egg Pollination is the transfer of pollen to the surface of the stigma by the actions of insects, birds, or other agents. In germination, a pollen tube forms producing a path that the two sperm will follow to the ovule. When the pollen tube reaches an ovule, it penetrates the embryo sac, and the two sperm are released. One sperm fuses with the egg nucleus to form a diploid zygote The other sperm nucleus fuses with the two endoperm nuclei to yield a triploid “Primary endosperm cell” that will nourish the young sporophyte seedling. The zygote undergoes repeated divisions to form an embryo sporophyte as a part of an ovule and is accompanied by formation of fruit. Cotyledons (seed leaves) develop for the purpose of utilizing the endosperms during germination. From zygote to embyro, the plant supplies nutrition until the time when the connection between the ovule and ovary wall is broken The mature ovule integuments thicken inot seed coats around the seed (a mature ovule containing embryo and food reserve) Embryo is inactive Seed dehydrates (5 -15% water) Embryo surrounded by endosperm, enlarged cotyledons or both A fruit mature ovary with seeds (ovules) inside; they may be classified as simple aggregate, multiple or accessory. The pericarp of a fruit consists collectively of endocarp (around the seed), mesocarp (fleshy portion), and exocarp (the skin) Immature fruit protects the seed (green, bitter, sour, etc.) Mature fruit aids in dispersal (colorful and tasty) Forms from ovary Does not provide nutrient for seeds Seeds have coevolved with particular dispersing agents – currents of air or water, or animals passing by Example: Pericarp of maple seeds extends out like wings to catch the wing and be transported Some fruits are dispersed by sticking on animal bodies or by passing through the digestive tract to be deposited in the feces Humans are perhaps the grand dispersing agents by virtue of the long distances to which they carry seeds Seed frequently undergo a period of dormancy. WHY??? Requires water, oxygen, and warm temperatures Root emerges first. WHY AGAIN? Vegetative growth modes includes: runners, rhizomes, corms, tubers, and bulbs. Parthenogenesis, embryo development from an unfertilized egg, can produce new orange plants. Vegetative propagation (“Cuttings”) can result in new plants produced from leaves that form roots. Tissue Culture propagation can result in whole plants produced from a group of cells. THE END!!
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