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 Page 3467 | Top of Articleangiosperms), 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 crosspollination, 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.
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 Page 3468 | Top of Articledo 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.
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