although monoecious plants can receive self-pollen from male flowers on the same plant. Many monoecious species produce male and female flowers at different times, and the probability of selfing is reduced. Similarly, in plants with hermaphrodite flowers, self-pollination within flowers is avoided when the male and female floral parts mature at different times. In some species, the chance of self-pollination is reduced because the male and female parts of the same flowers are separated. In a subset of those species, the male and female parts of the flower move closer together as the flower ages, allowing self-pollination as a “last resort” before the flower is too old to set fruit.

As a further deterrent to selfing, many flowering plant species are self-incompatible—that is, pollen that is deposited on a stigma within the same flower (or another flower on the same plant) is unable to achieve fertilization. Self-incompatibility is controlled in complex and variable ways, and it involves the interplay of incompatibility alleles (of which there may be many) and their effects in the two parent plants (Matton et al., 1994). The effectiveness of self-incompatibility mechanisms ranges from absolute to weak, and the mechanisms for blocking self-fertilization can break down as a result of aging or external factors, especially heat.

Breaking those barriers down ensures sexual reproduction (seed set and fruit set) even when cross-pollination is not possible. It is important to note, however, that despite the ubiquity of outbreeding, many species persist exclusively and successfully with self-pollinating and self-fertile flowers. Moreover, some self-fertile plants that can self-pollinate (including some legumes) are of agricultural importance. They can establish themselves in nonindigenous areas where their natural pollinators are absent. The nature and evolutionary biology of plant-breeding systems are presented in detail by Richards (1997).

to allowing organisms to adapt to spatially and temporally variable environments (Box 1-1). Genetic variability in plant populations could help to facilitate the evolution of resistance to pathogens and herbivores.

After fertilization is complete, the production of fruit ensues. A flower’s ovary may contain a single ovule and produce a fruit that bears only a single seed (as in the almond, avocado, coconut, plum, or cherry), or it may contain hundreds of ovules and produce a fruit bearing hundreds of seeds (as in the tomato, kiwi fruit, cucumber, watermelon, or squash). Because each seed results from the union of a sperm cell from a pollen grain and an egg cell, some plants require many hundreds of pollen grains to fertilize all of the available egg cells. If a flower receives an inadequate number of pollen grains, some of the egg cells will not be fertilized and accordingly seeds will not develop. Economic consequences of such incomplete fertilization include

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