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Most angiosperms (flowering plants) have evolved to reproduce sexually because this strategy has the selective advantage of producing the genetic diversity that is the raw material for evolution. Sexual reproduction in angiosperms involves mitosis, meiosis, and the alternation of haploid and diploid generations (see Figure 11.14).
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The embryo sac is the female gametophyte and the pollen grain is the male gametophyte.
A pollen grain germinates to form a pollen tube, which grows through the style to the embryo sac.
Angiosperms prevent self-
The embryo and endosperm develop within the seed, enclosed within a fruit derived from the ovary wall.
There are several important differences between sexual reproduction in angiosperms and in vertebrate animals:
In most angiosperms, there are multicellular diploid (sporophyte) and haploid (gametophyte) life stages (alternation of generations); in animals, there is no multicellular haploid stage.
In angiosperms, the cells that will form gametes are determined in the adult organism, usually in response to environmental conditions; in animals, the germ line cells are determined before birth.
Meiosis in plants produces spores, after which mitosis produces gametes; in animals, meiosis usually produces gametes directly.