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spores through meiosis, and those spores develop into multicellular haploid gametophytes. In pteridophytes (ferns, club mosses, and horestails) the gametophyte is free-living. In seed plants (gymnosperms and angiosperms) the female gametophyte is borne within the ovule and only the male gametophyte (pollen) leaves the structure in which it was produced. Gametophytes produce haploid egg and sperm through mitosis, and these unite to form diploid zygotes from which new sporophytes develop. Asexual reproduction in plants, as in animals, occurs when offspring are produced through modifications of the sexual life cycle that do not include meiosis and syngamy (see Fig. 1).

When vascular plants reproduce asexually, they may do so either by budding, branching, or tillering (vegetative reproduction) or by producing spores or seed genetically identical to the sporophytes that produced them (agamospermy in seed plants, apogamy in pteridophytes). Vegetative reproduction is extremely common in perennial plants, especially in grasses and aquatic plants, and it can have dramatic consequences. The water-weed Elodea canadensis, for example, was introduced into Britain in about 1840 and spread throughout Europe by 1880 entirely by vegetative reproduction (Gustafsson, 1947). Exclusive reliance on vegetative reproduction is, however, the exception rather than the rule. More commonly,

FIGURE 1 Diagram of basic vascular plant life cycles. Asexual life cycles are indicated with dashed lines. The sexual life cycle is indicated with solid lines. See Mogie (1992) for a more complete description of asexual life cycles.

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