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CHAPTER 38 PLANT REPRODUCTION AND BIOTECHNOLOGY Introduction It has been said that an oak is an acorn’s way of making more acorns. In a Darwinian view of life, the fitness of an organism is measured only by its ability to replace itself with healthy, fertile offspring. Sexual reproduction is not the sole means by which flowering plants reproduce. Many species can also reproduce asexually, creating offspring that are genetically identical to themselves. A. Sexual Reproduction 1. Sporophyte and gametophyte generations alternate in the life cycles of plants: a review The life cycles of angiosperms and other plants are characterized by an alternation of generations , in which haploid (n) and diploid (2n) generations take turns producing each other. The diploid plant, the sporophyte , produces haploid spores by meiosis. These spores divide by mitosis, giving rise to multicellular male and female haploid plants - the gametophyte . The gametophytes produce gametes - sperm and eggs. Fertilization results in diploid zygotes, which divide by mitosis and form new sporophytes. In angiosperms, the sporophyte is the dominant generation, the conspicuous plant we see. Over the course of seed plant evolution, gametophytes became reduced in size and dependent on their sporophyte parents.
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Angiosperm gametophytes consist of only a few cells. In angiosperms, the sporophyte produces a unique reproductive structure, the flower. Male and female gametophytes develop within the anthers and ovaries, respectively, of a sporophyte flower. Pollination by wind or animals brings a male gametophyte (pollen grain) to a female gametophyte. Union of gametes (fertilization) takes place within the ovary. Development of the seeds containing the sporophyte embryos also occurs in the ovary, which itself develops into the fruit around the seed. 2. Flowers are specialized shoots bearing the reproductive organs of the angiosperm sporophyte Flowers, the reproductive shoots of the angiosperm sporophyte, are typically composed of four whorls of highly modified leaves called floral organs, which are separated by very short internodes. Unlike the indeterminate growth of vegetative shoots, flowers are determinate shoots in that they cease growing once the flower and fruit are formed. The four kinds of floral organs are the sepals, petals, stamens, and carpals . Their site of attachment to the stem is the receptacle . Sepals and petals are nonreproductive organs. Sepals, which enclose and protect the floral bud before it opens, are usually green and more leaflike in appearance. In many angiosperms, the petals are brightly colored and advertise the flower to insects and other pollinators. Stamens and carpels are the male and female reproductive organs,
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