Pollination
Pollination
Pollination is the process of moving pollen grains, which contain male sex cells, from the anthers (the pollen-containing part of floral stamens, the male reproductive structure) of flowers to the stigma (the glandular female receptive portion) in the pistil (female reproductive organ). When a pollen grain lands on the female part of the flower, the male sex cell joins with the female sex cells in the flower in a process called fertilization to form a seed from which a new plant can grow. The anthers and stigma can be on the same flower (self-pollination) or on different flowers (crosspollination), but must be of the same species . All higher plants, including flowers, herbs, bushes, grasses, conifers, and broad-leaved trees, use pollination for sexual reproduction.
Pollination can be accomplished by abiotic means such as wind and water. Many pollen grains are small (less than 0.05 mm, or 0.002 in). Thus wind can carry the pollen grains to other members of their species. Many plants, to ensure pollination, grow in dense stands and produce millions of pollen grains. Wind-pollinated plants generally have small, inconspicuous flowers that dangle in the wind (e.g., willow catkins). Grasses have wispy plume-like flowers that catch grains floating in the air. Some water plants such as the hornwort have their pollen transferred by water currents.
Many plants use animals such as insects, birds, and bats to transport pollen grains. This process, referred to as biotic pollination, requires a relationship between the pollinator and the flower to be pollinated. Such a relationship is usually established by some kind of direct attractant, such as nectar, sweet-tasting pollen, odor, or visual attraction (e.g., brightly-colored flowers). There may also be an indirect attraction, such as when insects of prey visit flowers to catch other visiting pollinators.
Insects, including bees, beetles, flies, wasps, ants, butterflies, and moths, are common biotic pollinators. As an insect crawls in and out of flowers in its search for nectar or other food source, it receives a dusting of pollen grains from the anther, the male part. When the insect visits another flower, the pollen rubs off on the stigma, the female part. If the pollen is left on the same species of flower, a long tube grows from each pollen grain down the stalk (style) of the stigma and into the ovule at the base, which contains the female egg cells. The male cells from the pollen grains pass along the tubes to the female cells and fertilize them. Plants with trumpet-shaped flowers, such as petunias, have nectar at the bottom, so only insects with long tube-like tongues can act as their pollinators.
The best-known and best-adapted biotic pollinator is the bee. The bee is a relatively large insect with a large demand for food both for itself and for its carefully lookedafter brood. It normally gets all of its food from flower blossoms. The bee has an ability to remember plant forms, which aids in its ability to find flowers. Social bees live together in a communal nest and often share foraging and nest activities. The amount of food carried into a hive by a honeybee has been estimated to be 100 times its own requirements. Social bees have developed communication systems that permit them to inform each other about the location and sources of food. These communication systems include odor paths, special buzz tones to alert other individuals, and dances that can indicate direction, distance to, and yield of a source of food. However, most of the world's 40,000 species of bees are solitary. The female bee mates and then constructs a nest underground or within woody stems containing many smooth-walled cells. The cells are filled with a mixture of nectar and pollen, which provides all the food required for larvae to complete their development into adult bees.
Biotic pollination may also be accomplished by such animals as birds and bats. For example, hummingbirds feeding from the hibiscus flower carry pollen on their beak and heads. Bats hover in front of flowers that open at night, licking nectar and covering their faces with pollen. Successful pollination, often mediated by animals but also accomplished abiotically, is extremely important for food production as well as maintenance of biological plant diversity. Pollination of plants is necessary for seed set, fruit yields, and reproduction of most food crops.
Many threats to animal pollinators and pollination processes have been identified. These include use of toxic chemicals , decline in pollinator populations, habitat loss, and migratory corridor fragmentation. Toxic chemicals can kill pollinators, and wild pollinators are often more vulnerable to insecticides and herbicides than domestic honeybees. Laws regulate and control the use of many pesticides during periods when pollinators are foraging, and use of toxic chemicals near pollinators' nesting sites should also be controlled. Pesticides that are known to be less toxic to pollinators can be used to reduce stress on pollinator populations. Fewer pollinators will result in fewer plants. When factors such as the use of pesticides and habitat fragmentation reduce populations of pollinators, plants will have low reproductive success. Some endangered plant species may even become extinct. Appropriate pesticide spraying set-back distances should be based on on-site determinations made by pollination ecologists familiar with the plant and pollinator species involved.
Another threat to pollination processes is the decline in honeybee populations. Since 1990, U.S. beekeepers have lost one-fifth of their domestic managed honeybee colonies for reasons that include two kinds of mite infestations, diseases, spraying of pesticides, and several other factors. The Varroa mite is an external parasite that was identified in the United States in 1987 and affects bee colonies in thirty states. This mite lives and feeds on developing bee larvae so that when the bees hatch, they are small and deformed. Varroa mites can be controlled by placing medicated plastic strips inside hives to kill the mites. The bees walk on the strips and then carry the medicine on their feet to the larvae growing in the honeycomb. The tracheal mite infects the respiratory system of adult honeybees. These mites were found in the United States in 1984 and are now present in most states. These tracheal mites make bees weak and can kill an entire colony. To control the mites, an antibiotic powder, such as terramycin, is mixed into sugar and oil and is placed inside the bee hive.
Diseases and use of pesticides also take their toll on bee populations. There are several diseases that can kill bees; these include American foul brood, chalk brood, European foul brood, paralysis virus , sacbrood disease and kashmir virus. Some bacterial diseases can be treated by stirring antibiotics into feed sugar. Often, if a hive is badly infected, the hive is burned to prevent the infection of other hives. Also, the spraying of pesticides (e.g., by farmers for crop protection or by towns and communities for control of mosquitoes) when bees are foraging can result in their death. Some pesticides kill bees directly while they are in the crops, while others are carried back to the hives with the pollen, where they are stored in the honeycombs. The bees and larvae die when they eat the pollen, which can be at any time of year. Often the pesticide does not kill the entire colony, but makes the colony susceptible to mite infections or freezing in cold weather.
Several other factors have contributed to declining bee populations. Africanized bees , a type of highly defensive bee that is also known as the killer bee, became established in the United States in 1990, after years of northward migration from South America where they were first released. Beekeepers often are forced to abandon their hives when Africanized bees move into an area. Also, bee populations that have been weakened by other factors are in danger of freezing in winter, due to an insufficient number of bees to provide necessary warmth; or an insufficient supply of food to convert to heat energy. Finally, loss of agricultural subsidies and price supports in the United States adversely affects the economics of managing bee colonies.
Habitat loss and the severing of migratory corridors also constitute threats to animal pollinators and pollination processes. Habitat fragmentation is the division of natural ecosystems into smaller areas due to land conversion for agriculture, forestry, and urbanization. As habitat areas become smaller and widely scattered, they may be insufficient to provide an adequate diversity of host plants and nectar sources that their pollinators require. Habitat fragmentation may also cause reduction in pollinator populations due to loss of nesting habitats. Ecologists need to monitor populations of pollinators and habitat fragmentation trends to determine possible causes of pollinator decline and to develop land use plans to protect pollinator populations such as maintenance of habitat set-asides or greenbelts near agricultural fields and timber areas.
The severing of migratory corridors can also disrupt pollination processes. Some pollinating animal species, such as nectarivorous bats, navigate through a variety of nectar-providing plants as they migrate from tropical to arid and temperate environments. A bat species may utilize a single type of flower in each local environment , but a series of plants are linked into a nectar corridor of successive flowering times along the bat's migration route. For example, a type of long-nosed bat flies a loop of 3,200 mi (5,152 km) to follow the sequential flowering of at least 16 flowering plants species, including tree morning glories, several century plants, and giant columnar cactus. Severing of migratory corridors by habitat and vegetation destruction or by spraying of toxic pesticides may adversely affect the success of the migration. For example, migratory monarch butterflies require critical habitats through their migratory cycle and can be affected if habitat is lost due to activities such as development. Monitoring of plant/pollinator changes in migratory corridors is required to develop appropriate protection strategies.
[Judith L. Sims ]
RESOURCES
BOOKS
Buchmann, S. L., and G. P. Nabhan. Forgotten Pollinators. Washington, DC: Island Press, 1996.
Faegri, K., and L. van der Pijl. The Principles of Pollination Ecology, 3rd ed (revised).
Meeuse, R. J. D. The Story of Pollination. New York: Ronald Press, 1961.
Pollination
Pollination
Pollination is the transfer of pollen from the male reproductive organs to the female reproductive organs of a plant , and it precedes fertilization, the fusion of the male and the female sex cells. Pollination occurs in seed-producing plants, but not in the more primitive spore-producing plants, such as ferns and mosses. In plants such as pines , firs , and spruces (the gymnosperms), pollen is transferred from the male cone to the female cone. In flowering plants (the angiosperms), pollen is transferred from the flower's stamen (male organ ) to the pistil (female organ). Many species of angiosperms have evolved elaborate structures or mechanisms to facilitate pollination of their flowers.
History of pollination studies
The German physician and botanist Rudolf Jakob Camerarius (1665-1721) is credited with the first empirical demonstration that plants reproduce sexually. Camerarius discovered the roles of the different parts of a flower in seed production. While studying certain bisexual (with both male and female reproductive organs) species of flowers, he noted that a stamen (male pollen-producing organ) and a pistil (female ovule-producing organ) were both needed for seed production. The details of fertilization were discovered by scientists several decades after Camerarius's death.
Among the many other scientists who followed Cam erarius's footsteps in the study of pollination, one of the most eminent was Charles Darwin. In 1862, Darwin published an important book on pollination: The Various Contrivancesby which Orchids Are Fertilized by Insects. In part, Darwin wrote this book on orchids in support of his theory of evolution proposed in The Origin of Species, published in 1859.
Darwin demonstrated that many orchid flowers had evolved elaborate structures by natural selection in order to facilitate cross-pollination. He suggested that orchids and their insect pollinators evolved by interacting with one another over many generations, a process referred to as coevolution.
One particular example illustrates Darwin's powerful insight. He studied dried specimens of Angraecum sesquipedale, an orchid native to Madagascar. The white flower of this orchid has a foot-long (30 cm) tubular spur with a small drop of nectar at its base. Darwin claimed that this orchid had been pollinated by a moth with a foot-long tongue. He noted, however, that his statement "has been ridiculed by some entomologists." And indeed, around the turn of the century, a Madagascan moth with a one-foot-long tongue was discovered. Apparently, the moth's tongue uncoils to sip the nectar of A. sesquipedale as it cross-pollinates the flowers.
Darwin continued his studies of pollination in subsequent years. In 1876, he wrote another important book on pollination biology , The Effects of Cross and Self Fertilization in the Vegetable Kingdom.
The Austrian monk and botanist Johann Gregor Mendel (1822-1884) also conducted important pollination studies in Brno (now in the Czech Republic) in the mid-1800s. He studied heredity by performing controlled cross-pollinations of pea plants thereby laying the foundation for the study of heredity and genetics .
Evolution of pollination
Botanists theorize that seed plants with morphologically distinct pollen (male) and ovules (female) evolved from ancestors with free-sporing heterospory, where the male and the female spores are also morphologically distinct.
The evolution of pollination coincided with the evolution of seed. Fossilized pollen grains of the seed ferns , an extinct group of seed-producing plants with fern-like leaves, have been dated to the late Carboniferous period (about 300 million years ago). These early seed plants relied upon wind to transport their pollen to the ovule. This was an advance over free-sporing plants, which were dependent upon water , as their sperm had to swim to reach the egg. The evolution of pollination therefore allowed seed plants to colonize terrestrial habitats.
It was once widely believed that insect pollination was the driving force in the evolutionary origin of angiosperms. However, paleobotanists have recently discovered pollen grains of early gymnosperms, which were too large to have been transported by wind. This and other evidence indicates that certain species of early gymnosperms were pollinated by insects millions of years before the angiosperms had originated.
Once the angiosperms had evolved, insect pollination became an important factor in their evolutionary diversification. By the late Cretaceous period (about 70 million years ago), the angiosperms had evolved flowers with complex and specific adaptations for pollination by insects and other animals. Furthermore, many flowers were clearly designed to ensure cross-pollination, exchange of pollen between different individuals. Cross-pollination is often beneficial because it produces offspring which have greater genetic heterogeneity, and are better able to endure environmental changes. This important point was also recognized by Darwin in his studies of pollination biology.
Wind pollination
Most modern gymnosperms and many angiosperms are pollinated by wind. Wind-pollinated flowers, such as those of the grasses , usually have exposed stamens, so that the light pollen grains can be carried by the wind.
Wind pollination is a primitive condition, and large amounts of pollen are usually wasted, because it does not reach female reproductive organs. For this reason, most wind-pollinated plants are found in temperate regions, where individuals of the same species often grow close together. Conversely, there are very few wind pollinated plants in the tropics, where plants of the same species tend to be farther apart.
Pollination by animals
In general, pollination by insects and other animals is more efficient than pollination by wind. Typically, pollination benefits the animal pollinator by providing it with nectar, and benefits the plant by providing a direct transfer of pollen from one plant to the pistil of another plant. Angiosperm flowers are often highly adapted for pollination by insect and other animals.
Each taxonomic group of pollinating animals is typically associated with flowers which have particular characteristics. Thus, one can often determine which animal pollinates a certain flower species by studying the morphology, color , and odor of the flower. For example, some flowers are pure red, or nearly pure red, and have very little odor. Birds , such as hummingbirds , serve as pollinators of most of these flowers, since birds have excellent vision in the red region of the spectrum, and a rather undeveloped sense of smell . Interestingly, Europe has no native pure red flowers and no bird pollinated flowers.
Some flowers have a very strong odor, but are very dark in color. These flowers are often pollinated by bats , which have very poor vision, are often active during the night, and have a very well developed sense of smell.
The flowers of many species of plants are marked with special ultraviolet absorbing pigments (flavonoids), which appear to direct the pollinator toward the pollen and nectar. These pigments are invisible to humans and most animals, but bees' eyes have special ultraviolet photoreceptors which enable the bees to detect patterns and so pollinate these flowers.
See also Gymnosperm; Sexual reproduction.
Resources
books
The American Horticultural Society. The American Horticultural Society Encyclopedia of Plants and Flowers. New York: DK Publishing, 2002.
Gould, S.J. The Panda's Thumb. New York: W. W. Norton, 1980.
Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.
Peter A. Ensminger
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Angiosperm
—A plant which produces seeds within the mature ovary of a flower, commonly referred as a flowering plant.
- Coevolution
—Evolution of two or more closely interacting species, such that the evolution of one species affects the evolution of the other(s).
- Gametophyte
—The haploid, gamete-producing generation in a plant's life cycle.
- Gymnosperm
—Plant which produces its seed naked, rather than within a mature ovary.
- Haploid
—Nucleus or cell containing one copy of each chromosome.
- Ovule
—Female haploid gametophyte of seed plants, which develops into a seed upon fertilization by a pollen grain.
- Pollen
—Male haploid gametophyte of seed plants, which unites with the ovule to form a seed.
Pollination
Pollination
History of pollination studies
Pollination is the transfer of pollen from the male reproductive organs to the female reproductive organs of a plant. Pollination comes before fertilization (the fusion of the male and the female sex cells). Pollination occurs in seed-producing plants, but not in the more primitive spore-producing plants, such as ferns and mosses. In plants such as pines, firs, and spruces (the gymnosperms), pollen is transferred from the male cone to the female cone. In flowering plants (the angiosperms), pollen is transferred from the flower’s stamen (male organ) to the pistil (female organ). Many species of angiosperms have evolved elaborate structures or mechanisms to facilitate pollination of their flowers.
History of pollination studies
The German physician and botanist Rudolf Jakob Camerarius (1665-1721) is credited with demonstrating that plants reproduce sexually. Camerarius discovered the roles of the different parts of a flower in seed production. While studying certain bisexual (with both male and female reproductive organs) species of flowers, he noted that a stamen (male pollen-producing organ) and a pistil (female ovule-producing organ) were both needed for seed production. The details of fertilization were discovered by scientists several decades after Camerarius’s death.
In 1862, Charles Darwin published an important book on pollination: The Various Contrivances by which Orchids Are Fertilized by Insects. In part, Darwin wrote this book on orchids in support of his theory of evolution proposed in The Origin of Species, published three years before.
Darwin demonstrated that many orchid flowers had evolved elaborate structures by natural selection in order to facilitate cross-pollination. He suggested that orchids and their insect pollinators evolved by interacting with one another over many generations, a process referred to as coevolution.
The Austrian monk and botanist Johann Gregor Mendel (1822-1884) also conducted important pollination studies in the mid-1800s. He studied heredity by performing controlled cross-pollinations of pea plants thereby laying the foundation for the study of heredity and genetics.
Evolution of pollination
The evolution of pollination coincided with the evolution of seed. Fossilized pollen grains of the seed ferns, an extinct group of seed-producing plants with fern-like leaves, have been dated to the late Carboniferous period (about 300 million years ago). These early seed plants relied upon wind to transport their pollen to the ovule. This was an advance over free-sporing plants, which were dependent upon water, as their sperm had to swim to reach the egg. The evolution of pollination therefore allowed seed plants to colonize terrestrial habitats.
It was once widely believed that insect pollination was the driving force in the evolutionary origin of angiosperms. However, paleobotanists have recently discovered pollen grains of early gymnosperms, which were too large to have been transported by wind. This and other evidence indicates that certain species of early gymnosperms were pollinated by insects millions of years before the angiosperms had originated.
Once the angiosperms had evolved, insect pollination became an important factor in their evolutionary diversification. By the late Cretaceous period (about 70 million years ago), the angiosperms had evolved flowers with complex and specific adaptations for pollination by insects and other animals. Furthermore, many flowers were clearly designed to ensure cross-pollination, exchange of pollen between different individuals. Cross-pollination is often beneficial because it produces offspring which have greater genetic heterogeneity, and are better able to endure environmental changes. This important point was also recognized by Darwin in his studies of pollination biology.
Wind pollination
Most modern gymnosperms and many angiosperms are pollinated by wind. Wind-pollinated flowers, such as those of the grasses, usually have exposed stamens, so that the light pollen grains can be carried by the wind.
Wind pollination is a crude mechanism; large amounts of pollen are usually wasted, because they do not reach female reproductive organs. For this reason, most wind-pollinated plants are found in temperate regions, where individuals of the same species often grow close together. Conversely, there are very few wind pollinated plants in the tropics, where plants of the same species tend to be farther apart.
KEY TERMS
Angiosperm— A plant which produces seeds within the mature ovary of a flower, commonly referred as a flowering plant.
Coevolution— Evolution of two or more closely interacting species, such that the evolution of one species affects the evolution of the other(s).
Gametophyte— The haploid, gamete-producing generation in a plant’s life cycle.
Gymnosperm— Plant which produces its seed naked, rather than within a mature ovary.
Haploid— Nucleus or cell containing one copy of each chromosome.
Ovule— Female haploid gametophyte of seed plants, which develops into a seed upon fertilization by a pollen grain.
Pollen— Male haploid gametophyte of seed plants, which unites with the ovule to form a seed.
Pollination by animals
In general, pollination by insects and other animals is more efficient than pollination by wind. Typically, pollination benefits the animal pollinator by providing it with nectar, and benefits the plant by providing a direct transfer of pollen from one plant to the pistil of another plant. Angiosperm flowers are often highly adapted for pollination by insect and other animals.
Each taxonomic group of pollinating animals is typically associated with flowers which have particular characteristics. Thus, one can often determine which animal pollinates a certain flower species by studying the morphology, color, and odor of the flower. For example, some flowers are pure red, or nearly pure red, and have very little odor. Birds, such as hummingbirds, serve as pollinators of most of these flowers, since birds have excellent vision in the red region of the spectrum, and a rather undeveloped sense of smell. Interestingly, Europe has no native pure red flowers and no bird pollinated flowers.
Some flowers have a very strong odor, but are very dark in color. These flowers are often pollinated by bats, which have very poor vision, are often active during the night, and have a very well developed sense of smell.
The flowers of many species of plants are marked with special ultraviolet absorbing pigments (flavonoids), which appear to direct the pollinator toward the pollen and nectar. These pigments are invisible to humans and most animals, but bees’ eyes have special ultraviolet photoreceptors which enable the bees to detect patterns and so pollinate these flowers.
Resources
BOOKS
Nabors, Murray. Introduction to Botany. New York: Benjamin Cummings, 2003.
Pollan, Michael. The Botany of Desire: A Plant’s-Eye View of the World. New York: Random House, 2002.
Proctor, Michael, Peter Yeo, and Andrew Lack. The Natural History of Pollination. Portland: Timber Press, 2003.
Peter A. Ensminger