In spite of a common perception that pollen grains are gametes, like the sperm cells of animals, this is incorrect; pollination is a phase in the alternation of generations. Each pollen grain is a male haploid plant, a gametophyte, adapted to being transported to the female gametophyte, where it can achieve fertilization by producing the male gamete (or gametes, in the process of double fertilization).
As such the Angiosperm successful pollen grain (gametophyte) containing the male gametes (sperm) gets transported to the stigma, where it germinates and its pollen tube grows down the style to the ovary. Its two gametes travel down the tube to where the gametophyte(s) containing the female gametes are held within the carpel. One nucleus fuses with the polar bodies to produce the endosperm tissues, and the other with the ovum to produce the embryo Hence the term: "double fertilization".
In gymnosperms the ovule is not contained in a carpel, but exposed on the surface of a dedicated support organ such as the scale of a cone, so that the penetration of carpel tissue is unnecessary. Details of the process vary according to the division of Gymnosperms in question.
The receptive part of the carpel is called a stigma in the flowers of angiosperms. The receptive part of the gymnosperm ovule is called the micropyle. Pollination is a necessary step in the reproduction of flowering plants, resulting in the production of offspring that are genetically diverse.
The study of pollination brings together many disciplines, such as botany, horticulture, entomology, and ecology. The pollination process as an interaction between flower and vector was first addressed in the 18th century by Christian Konrad Sprengel. It is important in horticulture and agriculture, because fruiting is dependent on fertilisation, which is the result of pollination.
Abiotic pollination refers to situations where pollination is mediated without the involvement of other organisms. Only 10% of flowering plants are pollinated without animal assistance. The most common form of abiotic pollination, anemophily, is pollination by wind. This form of pollination is predominant in grasses, most conifers, and many deciduous trees. Hydrophily is pollination by water, and occurs in aquatic plants which release their pollen directly into the surrounding water. About 80% of all plant pollination is biotic. In gymnosperms, biotic pollination is generally incidental when it occurs, though some gymnosperms and their pollinators are mutually adapted for pollination. The best-known examples probably are members of the order Cycadales and associated species of beetles. Most conifera are anemophilous; they depend on wind pollination. Of the abiotically pollinated species, 98% are anemophilous and 2% hydrophilous, being pollinated by water.
More commonly, the process of pollination requires pollinators: organisms that carry or move the pollen grains from the anther to the receptive part of the carpel or pistil. This is biotic pollination. The various flower traits (and combinations thereof) that differentially attract one type of pollinator or another are known as pollination syndromes. Roughly 200,000 varieties of animal pollinators are in the wild, most of which are insects.
Entomophily, pollination by insects, often occurs on plants that have developed colored petals and a strong scent to attract insects such as, bees, wasps and occasionally ants (Hymenoptera), beetles (Coleoptera), moths and butterflies (Lepidoptera), and flies (Diptera). The existence of insect pollination dates back to the dinosaur era.
In zoophily, pollination is performed by vertebrates such as birds and bats, particularly, hummingbirds, sunbirds, spiderhunters, honeyeaters, and fruit bats. Plants adapted to using bats or moths as pollinators typically have white petals and a strong scent, while plants that use birds as pollinators tend to develop red petals and rarely develop a scent (few birds rely on a sense of smell to find plant-based food).
Insect pollinators such as honeybees (Apis mellifera), bumblebees (Bombus terrestris), and butterflies (Thymelicus flavus) have been observed to engage in flower constancy, which means they are more likely to transfer pollen to other conspecific plants. This can be beneficial for the pollinators, as flower constancy prevents the loss of pollen during interspecific flights and pollinators from clogging stigmas with pollen of other flower species. It also improves the probability that the pollinator will find productive flowers easily accessible and recognisable by familiar clues.
Pollination can be accomplished by cross-pollination or by self-pollination :
- Cross-pollination, also called allogamy, occurs when pollen is delivered to a flower from a different plant. Plants adapted to outcross or cross-pollinate often have taller stamens than carpels or use other mechanisms to better ensure the spread of pollen to other plants' flowers.
- Self-pollination occurs when pollen from one flower pollinates the same flower or other flowers of the same individual. It is thought to have evolved under conditions when pollinators were not reliable vectors for pollen transport, and is most often seen in short-lived annual species and plants that colonize new locations. Self-pollination may include autogamy, where pollen moves to the female part of the same flower; or geitonogamy, when pollen is transferred to another flower on the same plant. Plants adapted to self-fertilize often have similar stamen and carpel lengths. Plants that can pollinate themselves and produce viable offspring are called self-fertile. Plants that cannot fertilize themselves are called self-sterile, a condition which mandates cross pollination for the production of offspring.
- Cleistogamy: is self-pollination that occurs before the flower opens. The pollen is released from the anther within the flower or the pollen on the anther grows a tube down the style to the ovules. It is a type of sexual breeding, in contrast to asexual systems such as apomixis. Some cleistogamous flowers never open, in contrast to chasmogamous flowers that open and are then pollinated. Cleistogamous flowers by necessity are self-compatible or self-fertile plants. Many plants are self-incompatible, and these two conditions are end points on a continuum.
Pollination also requires consideration of pollenizers. The terms "pollinator" and "pollenizer" are often confused: a pollinator is the agent that moves the pollen, whether it be bees, flies, bats, moths, or birds; a pollenizer is the plant that serves as the pollen source for other plants. Some plants are self-fertile or self-compatible and can pollinate themselves (e.g., they act as their own pollenizer). Other plants have chemical or physical barriers to self-pollination.
In agriculture and horticulture pollination management, a good pollenizer is a plant that provides compatible, viable and plentiful pollen and blooms at the same time as the plant that is to be pollinated or has pollen that can be stored and used when needed to pollinate the desired flowers. Hybridization is effective pollination between flowers of different species, or between different breeding lines or populations. see also Heterosis.
Peaches are considered self-fertile because a commercial crop can be produced without cross-pollination, though cross-pollination usually gives a better crop. Apples are considered self-incompatible, because a commercial crop must be cross-pollinated. Many commercial fruit tree varieties are grafted clones, genetically identical. An orchard block of apples of one variety is genetically a single plant. Many growers now consider this a mistake. One means of correcting this mistake is to graft a limb of an appropriate pollenizer (generally a variety of crabapple) every six trees or so.
Pollen vectors 
Biotic pollen vectors are animals, usually insects, but also reptiles, birds, mammals, and sundry others, that routinely transport pollen and play a role in pollination. This is usually as a result of their activities when visiting plants for feeding, breeding or shelter. The pollen adheres to the vector's body parts such as face, legs, mouthparts, hair, feathers, and moist spots; depending on the particular vector. Such transport is vital to the pollination of many plant species.
Any kind of animal that often visits or encounters flowers is likely to be a pollen vector to some extent. For example, a crab spider that stops at one flower for a time and then moves on, might carry pollen incidentally, but most pollen vectors of significant interest are those that routinely visit the flowers for some functional activity. They might feed on pollen, or plant organs, or on plant secretions such as nectar, and carry out acts of pollination on the way. Many plants bear flowers that favour certain types of pollinator over all others. This need not always be an effective strategy, because some flowers that are of such a shape that they favour pollinators that pass by their anthers and stigmata on the way to the nectar, may get robbed by ants that are small enough to bypass the normal channels, or by short-tongued bees that bite through the bases of deep corolla tubes to extract nectar at the end opposite to the anthers and stigma. However, in general, plants that rely on pollen vectors tend to be adapted to their particular type of vector, for example day-pollinated species tend to be brightly coloured, but if they are pollinated largely by birds or specialist mammals, they tend to be larger and have larger nectar rewards than species that are strictly insect-pollinated. They also tend to spread their rewards over longer periods, having long flowering seasons; their specialist pollinators would be likely to starve if the pollination season were too short.
As for the types of pollinators, reptile pollinators are known, but they form a minority in most ecological situations. They are most frequent and most ecologically significant in island systems, where insect and sometimes also bird populations may be unstable and less species-rich. Adaptation to a lack of animal food and of predation pressure, might therefore favour reptiles becoming more herbivorous and more inclined to feed on pollen and nectar. Most species of lizards in the families that seem to be significant in pollination seem to carry pollen only incidentally, especially the larger species such as Varanidae and Iguanidae, but especially several species of the Gekkonidae are active pollinators, and so is at least one species of the Lacertidae, Podarcis lilfordi, which pollinates various species, but in particular is the major pollinator of Euphorbia dendroides on various Mediterranean islands.
Mammals are not generally thought of as pollinators, but some rodents, bats and marsupials are significant pollinators and some even specialise in such activities. In South Africa certain species of Protea (in particular Protea humiflora, P. amplexicaulis, P. subulifolia, P. decurrens and P. cordata) are adapted to pollination by rodents (particularly Cape Spiny Mouse, Acomys subspinosus) and elephant shrews (Elephantulus species). The flowers are borne near the ground, are yeasty smelling, not colourful, and sunbirds reject the nectar with its high xylose content. The mice apparently can digest the xylose and they eat large quantities of the pollen. In Australia pollination by flying, gliding and earthbound mammals has been demonstrated.
Examples of pollen vectors include many species of wasps, that transport pollen of many plant species, being potential or even efficient pollinators.
Evolution of plant/pollinator interactions 
The first fossil record for abiotic pollination is from fern-like plants in the late Carboniferous period. Gymnosperms show evidence for biotic pollination as early as the Triassic period. Many fossilized pollen grains show characteristics similar to the biotically dispersed pollen today. Furthermore, the gut contents, wing structures, and mouthpart morphologies of fossilized beetles and flies suggest that they acted as early pollinators. The association between beetles and angiosperms during the early Cretaceous period led to parallel radiations of angiosperms and insects into the late Cretaceous. The evolution of nectaries in late Cretaceous flowers signals the beginning of the mutualism between hymenopterans and angiosperms.
In agriculture 
Pollination management is a branch of agriculture that seeks to protect and enhance present pollinators and often involves the culture and addition of pollinators in monoculture situations, such as commercial fruit orchards. The largest managed pollination event in the world is in Californian almond orchards, where nearly half (about one million hives) of the US honey bees are trucked to the almond orchards each spring. New York's apple crop requires about 30,000 hives; Maine's blueberry crop uses about 50,000 hives each year.
Bees are also brought to commercial plantings of cucumbers, squash, melons, strawberries, and many other crops. Honey bees are not the only managed pollinators: a few other species of bees are also raised as pollinators. The alfalfa leafcutter bee is an important pollinator for alfalfa seed in western United States and Canada. Bumblebees are increasingly raised and used extensively for greenhouse tomatoes and other crops.
The ecological and financial importance of natural pollination by insects to agricultural crops, improving their quality and quantity, becomes more and more appreciated and has given rise to new financial opportunities. The vicinity of a forest or wild grasslands with native pollinators near agricultural crops, such as apples, almonds or coffee can improve their yield by about 20%. The benefits of native pollinators may result in forest owners demanding payment for their contribution in the improved crop results - a simple example of the economic value of ecological services.
The American Institute of Biological Sciences reports that native insect pollination saves the United States agricultural economy nearly an estimated $3.1 billion annually through natural crop production; pollination produces some $40 billion worth of products annually in the United States alone.
Pollination of food crops has become an environmental issue, due to two trends. The trend to monoculture means that greater concentrations of pollinators are needed at bloom time than ever before, yet the area is forage poor or even deadly to bees for the rest of the season. The other trend is the decline of pollinator populations, due to pesticide misuse and overuse, new diseases and parasites of bees, clearcut logging, decline of beekeeping, suburban development, removal of hedges and other habitat from farms, and public concern about bees. Widespread aerial spraying for mosquitoes due to West Nile fears is causing an acceleration of the loss of pollinators.
The US solution to the pollinator shortage, so far, has been for commercial beekeepers to become pollination contractors and to migrate. Just as the combine harvesters follow the wheat harvest from Texas to Manitoba, beekeepers follow the bloom from south to north, to provide pollination for many different crops.
Environmental impacts 
Loss of pollinators, also known as Pollinator decline (of which colony collapse disorder is perhaps the most well known) has been noticed in recent years. Observed losses would have significant economic impacts. Possible explanations for pollinator decline include habitat destruction, pesticide, parasitism/diseases, and others.
See also 
|Wikimedia Commons has media related to: pollination|
- Canadian Pollination Initiative
- Cheating (biology)
- Fruit tree pollination
- Hand pollination
- Hermann Müller (botanist)
- Paul Knuth (botanist)
- Pollination syndrome
- Fritsch, Felix Eugene; Salisbury, Edward James; An introduction to the structure and reproduction of plants. Publisher: G. Bell, 1920. Downloadable from:http://www.archive.org/details/cu31924001698905
- Mauseth, James D. Botany: An Introduction to Plant Biology. Publisher: Jones & Bartlett, 2008 ISBN 978-0-7637-5345-0
- Hill, P.S.M., P.H. Wells, and H. Wells. 1997. Spontaneous flower constancy and learning in honey bees as a function of colour. Animal Behavior, 54: 615-627.
- Stout, J.C., J.A. Allen, and D. Goulson. 1998. The influence of relative plant density and floral morphological complexity on the behaviour of bumblebees. Oecologia, 117: 543-550.
- Chittka, L., A. Gumbert, and J. Kunze. 1997. Foraging dynamics of bumble bees: correlates of movement within and between plant species. Behavioral Ecology, 8(3): 239-249
- Goulson, D., J. Ollerton and C. Sluman. 1997. Foraging strategies in the small skipper butterfly, Thymelicus flavus: when to switch? Animal Behavior, 53: 1009-1016
- Harder, L. D., N.M. Williams, C.Y. Jordan, and W.A. Nelson. "The effects of Floral design and display on pollinator economics and pollen dispersal". 297-317.
- Editors, L. Chittka and J.D. Thomson. Cognitive Ecology of Pollination: Animal Behavior and Floral Evolution. 2001. Cambridge University Press.
- Chittka, L., J.D. Thomson, and N.M. Waser. 1999. Flower constancy, insect psychology, and plant evolution. Naturwissenschaften, 86: 361-177.
- Cronk, J. K.; Fennessy, M. Siobhan (2001). Wetland plants: biology and ecology. Boca Raton, Fla.: Lewis Publishers. p. 166. ISBN 1-56670-372-7.
- Glover, Beverly J. (2007). Understanding flowers and flowering: an integrated approach. Oxford University Press. p. 127. ISBN 0-19-856596-8
- Culley, Theresa M.; Klooster, Matthew R. (JAN-07). "The cleistogamous breeding system: a review of its frequency, evolution, and ecology in angiosperms". The Botanical Review
- Potts, Brad, Gore Peter. Reproductive Biology and Controlled Pollination of Eucalyptus. School of Plant Science, University of Tasmania 1995
- Olesen, Jens M. & Valido, Alfredo. Lizards as pollinators and seed dispersers: an island phenomenon. TRENDS in Ecology and Evolution Vol.18 No.4 April 2003
- GODÍNEZ-ÁLVAREZ HÉCTOR Pollination and seed dispersal by lizards. Revista Chilena de Historia Natural 77: 569-577, 2004
- Wiens, Delbert; Rourke, John P.; Casper, Brenda B.; Rickart, Eric A.; LaPine, T.R.; Peterson,J.; Channing, A: Nonflying Mammal Pollination of Southern African Proteas. Annals of the Missouri Botanical Garden Vol 70, number 1, 1983
- P.A. Fleming & S.W. Nicolson. Arthropod fauna of mammal-pollinated Protea humiflora: ants as an attractant for insectivore pollinators? African Entomology 11(1): 9–14 (2003)
- R. L. Goldingay, S. M. Carthew and R. J. Whelan. The Importance of Non-Flying Mammals in Pollination. Oikos Vol. 61, No. 1 (May, 1991), pp. 79-87. Published by: Wiley-Blackwell Article Stable URL: http://www.jstor.org/stable/3545409
- Sühs, R.B.; Somavilla, A.; Putzke, J.; Köhler, A. 2009. Pollen vector wasps (Hymenoptera, Vespidae) of Schinus terebinthifolius Raddi (Anacardiaceae), Santa Cruz do Sul, RS, Brazil. Brazilian Journal of Biosciences 7, n. 2, p. 138-143. Link: http://www.ufrgs.br/seerbio/ojs/index.php/rbb/article/view/1123
- BioScience, April 2006, Vol. 56 No. 4, pp. 315-317
- US Forest Department: Pollinator Factsheet
- "What is the male, pollen-producing part of a plant called?". CNN. 2000-05-05. Retrieved 2010-05-22.
- Crepet WL, Friis EM, Nixon KC. 1991. Fossil evidence for the evolution of biotic pollination [and discussion]. Philosophical Transactions: Biological Sciences 333(1267) 187-195.
- Dafni, Amots; Kevan, Peter G.; and Husband, Brian C. (2005). Practical Pollination Biology. Enviroquest, Ltd. ISBN 978-0-9680123-0-7.
- Labandeira CC, Kvacek J, & Mostovski MB. 2007. Pollination drops, pollen and insect pollination of Mesozoic gymnosperms. Taxon 56(3) 663-695.
- Sihag, R.C. 1997.Pollination Biology: Basic and Applied Principles. Rajendra Scientific Publishers,Hisar, 210p.
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- New "Pollinator Garden Wheel" from the National Academies provides information on pollination and tips on building a pollinator-friendly garden.
- Insect Pollination Of Cultivated Crop Plants by S. E. McGregor USDA 1972 (needs updating but still valuable)
- The Pollination Home page
- Pollination in Hydroponics
- Pollination syndromes images at bioimages.vanderbilt.edu