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Plant sexuality deals with the wide variety of sexual reproduction systems found across the plant kingdom. This article describes morphological aspects of sexual reproduction of plants. That plants employ many different strategies to engage in sexual reproduction was used, from just a structural perspective, by Carolus Linnaeus (1735 and 1753) to propose a system of classification of flowering plants. Later this subject received attention from Christian Konrad Sprengel (1793) who described plant sexuality as the "revealed secret of nature" and, for the first time understood the biotic and abiotic interactions of the pollination process. Charles Darwin's theories of natural selection are based on his work. Flowers, the reproductive structures of angiosperms, are more varied than the equivalent structures of any other group of organisms, and flowering plants also have an unrivalled diversity of sexual systems (Barrett, 2002). But sexuality and the significance of sexual reproductive strategies is no less important in all of the other plant groups. The breeding system is the single most important determinant of the mating structure of nonclonal plant populations. The mating structure in turn controls the amount and distribution of genetic variation, a central element in the evolutionary process (Costich, 1995).
Terminology The complexity of the systems and devices used by plants to achieve sexual reproduction has resulted in botanists and evolutionary biologists proposing numerous terms to describe structures and strategies. Dellaporta and Calderon-Urrea (1993) list and define a variety of terms used to describe the modes of sexuality at different levels in flowering plants. This list is reproduced here (taken from Molner, 2004), generalized to fit more than just plants that have flowers, and expanded to include other terms and better definitions. Individual reproductive unit (a flower in angiosperms) Individual plant
Plant population Some plants use a method known as self-incompatibility to ensure genetic diversity within the species. In these plants, the male organs cannot fertilize the female parts of the same plant. Flower morphology A species, such as the ash tree (Fraxinus excelsior L.), demonstrates the possible range of variation in morphology and functionality exhibited by flowers with respect to gender. Flowers of the ash are wind-pollinated and lack petals and sepals. Structurally, the flowers may be either male, female, or hermaphrodite, the latter consisting of two anthers and an ovary ('c' below). A male flower can be morphologically male ('a' below) or a hermaphrodite flower with anthers and a rudimentary gynoecium ('b' below; functionally 'male'). Ash flowers can also be morphologically female ('e' below) or hermaphrodite and functionally female ('d' below; with vestigial anthers). (Illustration from Binggeli and Power, 1999) Angiosperms It is thought that flowering plants evolved from a common hermaphrodite ancestor, and that dioecy evolved from hermaphroditism. Hermaphroditism is very common in flowering plants; about 70% are hermaphroditic, while only about 5% are dioecious and 7% are monoecious. About 7% of species exhibit gynodioecy or androdioecy, while 10% contain both unisexual and bisexual flowers (Molner, 2004). A fair degree of correlation (though far from complete) exists between dioecy/sub-dioecy and plants that have seeds dispersed by birds (both nuts and berries). It is hypothesized that the concentration of fruit in half of the plants increases dispersal efficiency; female plants can produce a higher density of fruit as they do not expend resources on pollen production, and the dispersal agents (birds) need not waste time looking for fruit on male plants. | ||||||||||
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