- Also called:
- heterogamy
- Related Topics:
- heterostyly
- dichogamy
- chasmogamy
- protogyny
- protandry
cross-pollination
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What is cross-pollination in botany?
Cross-pollination in botany is the transfer of pollen from one plant's flowers or cones to another. This process, found in both angiosperms and gymnosperms, promotes cross-fertilization and outbreeding.
Compared to self-pollination, cross-pollination offers evolutionary advantages. The resulting seeds can combine traits from both parents, leading to more varied offspring. This genetic variability is crucial in changing environments, ensuring the survival of the species. Many plants have also developed structural adaptations to prevent self-pollination and facilitate cross-pollination. Chemical self-incompatibility is another method that prevents self-fertilization.
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How does cross-pollination differ from self-pollination?
Cross-pollination is the transfer of pollen from one plant's flowers to another, enabling cross-fertilization and outbreeding. Conversely, self-pollination occurs when a flower is fertilized by pollen from the same flower or another flower on the same plant.
Cross-pollination has evolutionary advantages because the resulting seeds combine traits from both parents, increasing genetic diversity, which can be crucial for survival in changing environments. In species where both methods are possible, cross-pollination often yields more and better-quality seeds. To facilitate cross-pollination and limit self-pollination, many flowering plants have evolved specific adaptations, such as chemical self-incompatibility, flower structure, and timing of stamen and pistil maturation.
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What are the main agents that help plants with cross-pollination?
Cross-pollination involves transferring pollen from one plant to another, mainly facilitated by wind and animals.
- Wind Conifers rely on wind for cross-pollination.
- Animals Insects, birds, bats, and other animals facilitate cross-pollination while feeding on nectar, carrying pollen from plant to plant. Bees are the most significant group of animal pollinators. Flowers have a wide range of adaptations to attract pollinators.
- Water Water is a less common agent and is only used by aquatic plants.
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Why is cross-pollination important for plants?
Cross-pollination is when pollen grains move from one plant to another, enabling cross-fertilization and outbreeding. Compared to self-pollination, cross-pollination creates more varied offspring by combining hereditary traits of both parents.
Genetic variability in a changing environment allows some individuals to adapt, ensuring the species' survival. Cross-pollination often yields more and better quality seeds. Many plants limit self-pollination using flower structure, self-incompatibility, or timing of stamen and pistil maturation. Animal-pollinated flowers attract insects with their structure, color, scent, or nectar.
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How do flowers adapt their structures to encourage cross-pollination?
Many flowering plants have evolved to limit self-pollination and facilitate cross-pollination. This is achieved through various structural adaptations. Some plants have separate "male" and "female" flowers, while others have hermaphroditic flowers where the pollen is shed either before or after the period during which the stigmas on the same plant are receptive.
Another structural adaptation is heterostyly, where there is variation in the length of the style (neck of the pistil), which discourages self-fertilization. Some plants also use mimicry to attract insects to aid in pollination.
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What are some examples of plants that depend mainly on cross-pollination, and how does this affect their genetic diversity?
Plants like date palms and willows rely on cross-pollination because they are dioecious, having separate male and female plants. Other plants, such as white clover and cabbage, use chemical self-incompatibility to prevent self-fertilization.
Compared to self-pollination, cross-pollination creates more genetic variability because the resulting seeds combine hereditary traits from two parents. This genetic diversity can help populations adapt to changing environments, increasing the likelihood of species survival.
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cross-pollination, type of pollination in which sperm-laden pollen grains are transferred from the cones or flowers of one plant to egg-bearing cones or flowers of another.Cross-pollination is found in both angiosperms (flowering plants) and gymnosperms (cone-bearing plants) and facilitates cross-fertilization and outbreeding. This movement of pollen may occur by wind, as in conifers, or via symbiotic relationships with various animals (e.g., bees and certain birds and bats) that carry pollen from plant to plant while feeding on nectar. See also major types of pollinators.
Advantages of cross-pollination
When compared with self-pollination (the transfer of pollen within a flower or between flowers on the same plant), cross-pollination clearly has certain evolutionary advantages. The seeds formed by outbreeding may combine the hereditary traits of both parents, and the resulting offspring generally are more varied than would be the case after self-pollination. In a changing environment, the genetic variability within a cross-pollinated population may enable some individuals to be adapted to their new situation, ensuring survival of the species, whereas the individuals resulting from self-pollination might all be unable to adjust. Self-pollination, or selfing, although foolproof in a stable environment, thus is an evolutionary cul-de-sac. There also is a more direct, visible difference between selfing and outbreeding: in those species where both methods work, cross-pollination usually produces more, and better quality, seeds. A dramatic demonstration of this effect is found with hybrid corn (maize), a superior product that results from cross-breeding of several especially bred lines. About half of the more important cultivated plants are naturally cross-pollinated.
Mechanisms that prevent self-pollination
Many flowering plants have evolved to limit self-pollination and facilitate cross-pollination. Self-pollination can be reduced or nearly eliminated by the structure of the flower, self-incompatibility, and the timing of the maturation of stamens and pistils of the same flower or plant. Cross-pollination may then be brought about by a number of agents, chiefly insects and wind. Wind-pollinated flowers generally can be recognized by their lack of color, odor, or nectar and their stigmas that are arranged to optimize the capture of airborne pollen. Animal-pollinated flowers are conspicuous by virtue of their structure, color, or the production of scent or nectar, all of which evolved in tandem with animal pollinators.
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Structural
Many species of plants have developed physical and structural mechanisms that prevent self-pollination. Some—e.g., date palms (Phoenix dactylifera) and willows (Salix species)—have become dioecious; that is, some plants produce only “male” (staminate) flowers, with the rest producing only “female” (pistillate or ovule-producing) ones. In species in which staminate and pistillate flowers are found on the same individual (monoecious plants) and in those with hermaphroditic flowers (flowers possessing both stamens and pistils), a common way of preventing self-fertilization is to have the pollen shed either before or after the period during which the stigmas on the same plant are receptive, a situation known as dichogamy. The more usual form of dichogamy, which is found especially in such insect-pollinated flowers as fireweed (Epilobium angustifolium) and salvias (Salvia species), is protandry, in which the stamens ripen before the pistils. Protogyny, the situation in which the pistils mature first, occurs in arum lilies and many wind-pollinated plants, such as grasses—although several grasses are self-pollinated, including common varieties of wheat, barley, and oats. Avocado has both protogynous and protandrous varieties, and these often are grown together to encourage cross-fertilization.
A structural feature of flowers that discourages selfing is heterostyly, or variation in the length of the style (neck of the pistil). This occurs in the common primrose (Primula vulgaris) and species of wood sorrel (Oxalis) and flax (Linum usitatissimum). In most British primrose populations, for example, approximately half the individuals have so-called “pin” flowers, which possess short stamens and a long style, giving the stigma a position at the flower’s mouth, whereas the other half have “thrum” flowers, in which the style is short and the stamens are long, forming a “thrumhead” at the opening of the flower. Bees can hardly fail to deposit the pollen they receive from one type of flower onto the stigmas of the other type. The genetic system that regulates flower structure in these primroses is so constituted that cross-pollination automatically maintains a 50:50 ratio between pins and thrums. In the flowers of purple loosestrife (Lythrum salicaria), the stamens and styles are of three different lengths to limit self-fertilization.
Chemical
Chemical self-incompatibility is another device for preventing self-fertilization. In this phenomenon, which depends on chemical substances within the plant, the pollen may fail to grow on a stigma of the same flower that produced it or, after germination, the pollen tube may not grow normally down the style to effect fertilization. This occurs in white clover (Trifolium repens), cabbage (Brassica oleracea), and many other species. The process is controlled genetically; it need not be absolute and can change in degree during the flowering season. Not surprisingly, chemical incompatibility usually is not found in those plants that have strong structural or temporal barriers against self-pollination. Formation of one such mechanism during evolution apparently was enough for most plant species.