Tip of a tulip stamen with many grains of pollen

Closeup image of a cactus flower and its stamens

Scanning electron microscope image of pollen grains from a variety of common plants: sunflower (Helianthus annuus), morning glory (Ipomoea purpurea), prairie hollyhock (Sidalcea malviflora), oriental lily (Lilium auratum), evening primrose (Oenothera fruticosa), and castor bean (Gerenious communis).

European honey bee carrying pollen in a pollen basket back to the hive

Pollen sticking to a bee. Insects involuntarily transporting pollen from flower to flower play an important role in many plants' reproductive cycles.

Marmalade fly sitting on a grey-haired rockrose, its face and legs covered in pollen.

Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes (sperm cells). Pollen grains have a hard coat that protects the sperm cells during the process of their movement between the stamens to the pistil of flowering plants or from the male cone to the female cone of coniferous plants. When pollen lands on a compatible pistil of flowering plants, it germinates and produces a pollen tube that transfers the sperm to the ovule of a receptive ovary. The individual pollen grains are small enough to require magnification to see detail.

[edit] The structure of pollen

Pollen itself is not the male gamete,^[1] but each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and a generative (reproductive) cell containing two nuclei: a tube nucleus (that produces the pollen tube) and a generative nucleus (that divides to form the two sperm cells). The group of cells is surrounded by a cellulose-rich cell wall called the intine, and a resistant outer wall composed largely of sporopollenin called the exine.

Pollen is produced in the microsporangium (contained in the anther of an angiosperm flower, male cone of a coniferous plant, or male cone of other seed plants). Pollen grains come in a wide variety of shapes (most often spherical), sizes, and surface markings characteristic of the species (see electron micrograph at top right). Pollen grains of pines, firs, and spruces are winged. The smallest pollen grain, that of the Forget-me-not (Myosotis spp.), is around 6 –m (0.006  mm) in diameter. Wind-borne pollen grains can be as large as about 90-100 –m.^[2] The study of pollen is called palynology and is highly useful in paleoecology, paleontology, archeology, and forensics.

In angiosperms, during flower development the anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. As the flower develops, four groups of sporogenous cells form with in the anther, the fertile sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac, some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporogenous cells. Four haploid microspores are produced from each diploid sporogenous cell called a microsporocyte, after meiotic division. After the formation of the four microspores, which are contained by callose walls, the development of the pollen grain walls begins. The callose wall is broken down by an enzyme called callase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. The exine is what is preserved in the fossil record.

The pollen wall protects the sperm nucleus while the pollen grain is moving from the anther to the stigma, it protects the vital genetic material from drying out and solar radiation. The pollen grain surface is covered with waxes and proteins, which are held in place by structures called sculpture elements on the surface of the grain. The outer pollen wall prevents the pollen grain from shrinking and crushing the genetic material during desiccation and it is composed of two layers. These two layers are the tectum and the foot layer, which is just above the intine. The tectum and foot layer are separated by a region called the columella, which is composed of strengthening rods. The outer wall is constructed with a resistant biopolymer called sporopollenin. The pollen tube passes through the wall by way of structures called apertures.^[3]

Pollen apertures are any modification of the wall of the pollen grain. These modifications include thinning, ridges and pores, they serve as an exit for the pollen contents and allow shrinking and swelling of the grain caused by changes in moisture content. The elongated apertues/ furrows in the pollen grain are called colpi (s. colpus) which along with pores, are a chief criteria for the identifying pollen classes.^[4]

Pollen grains may have furrows, the orientation of which (relative to the original tetrad of microspores) classify the pollen as colpate or sulcate. The number of furrows or pores helps classify the flowering plants, with eudicots having three colpi (tricolpate), and other groups having one sulcus.^[5][6]

Except in the case of some submerged aquatic plants, the mature pollen-grain has a double wall, a thin delicate wall of unaltered cellulose (the endospore or intine) and a tough outer cuticularized exospore or exine. The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual. In some flowering plants, germination of the pollen grain often begins before it leaves the microsporangium, with the generative cell forming the two sperm cells.

[edit] Pollination

The transfer of pollen grains to the female reproductive structure (pistil in angiosperms) is called pollination. This transfer can be mediated by the wind, in which case the plant is described as anemophilous (literally wind-loving). Anemophilous plants typically produce great quantities of very lightweight pollen grains, sometimes with air-sacs. Non-flowering seed plants (e.g. pine trees) are characteristically anemophilous. Anemophilous flowering plants generally have inconspicuous flowers. Entomophilous (literally insect-loving) plants produce pollen that is relatively heavy, sticky and protein-rich, for dispersal by insect pollinators attracted to their flowers. Many insects and some mites are specialized to feed on pollen, and are called palynivores.

In non-flowering seed plants, pollen germinates in the pollen chamber, located beneath and inside the micropyle. A pollen tube is produced, which grows into the nucellus to provide nutrients for the developing sperm cells. Sperm cells of Pinophyta and Gnetophyta are without flagella, and are carried by the pollen tube, while those of Cycadophyta and Ginkgophyta have many flagella.

When placed on the stigma of a flowering plant, under favorable circumstances, a pollen grain puts forth a pollen tube which grows down the tissue of the style to the ovary, and makes its way along the placenta, guided by projections or hairs, to the micropyle of an ovule. The nucleus of the tube cell has meanwhile passed into the tube, as does also the generative nucleus which divides (if it hasn't already) to form two sperm cells. The sperm cells are carried to their destination in the tip of the pollen-tube.

[edit] Pollen as a carrier of ecological information in plants

A Russian theoretical biologist has suggested that the quantity of pollen reaching a pistillate flower can transmit ecological information and also regulate evolutionary plasticity in cross-pollinating plants. Plentiful pollen indicates optimum environmental conditions (for example a plant that is situated at the center of its natural range, in ideal growing conditions, with a large number of male plants nearby, and favorable weather conditions), whereas a small amount of pollen indicates extreme conditions (at the borders of its range, with a deficiency of male plants, and adverse weather conditions). Geodakian believes that the quantity of pollen reaching a pistillate flower defines the sex ratio, dispersion and sexual dimorphism of a plant population. High pollen quantity leads to a reduction of these characteristics and stabilization of a population. Small quantity leads to their increase and destabilization of a population.^[7]

Dependence of the secondary sex ratio on the amount of fertilizing pollen was confirmed on four dioecious plant species from three families ' Rumex acetosa (Polygonaceae),^[8][9] Melandrium album (Cariophyllaceae),^[10][11] Cannabis sativa^[12] and Humulus japonicus (Cannabinaceae).^[13] (see summary of all these data in review article^[14]).

Dependence of offspring phenotype variety on amount of pollen was observed by Ter-Avanesyan in 1949. All three studied species of plants (cotton plant, black-eyed pea, and wheat) showed dependence in the direction forecast by the theory ' fertilization with a small amount of pollen resulted in an increase in the diversity of the offspring. Ter-Avanesian writes that as a result of a limited pollination 'instead of homogenous sorts we get populations'.^[15][16]

[edit] Pollen in the fossil record

Pollen's sporopollenin outer sheath affords it some resistance to the rigours of the fossilisation process that destroy weaker objects; it is also produced in huge quantities. As such, there is an extensive fossil record of pollen grains, often disassociated from their parent plant. The discipline of palynology is devoted to the study of pollen, which can be used both for biostratigraphy and to gain information about the abundance and variety of plants alive ' which can itself yield important information about paleoclimates. Pollen is first found in the fossil record in the late Devonian period^{[verification needed]} and increases in abundance until the present day.

[edit] Hay fever

The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page.

Allergy to pollen is called hay fever. Generally pollens that cause allergies are those of anemophilous plants (pollen is dispersed by air currents.) Such plants produce large quantities of lightweight pollen (because wind dispersal is random and the likelihood of one pollen grain landing on another flower is small) which can be carried for great distances and are easily inhaled, bringing it into contact with the sensitive nasal passages.

In the US, people often mistakenly blame the conspicuous goldenrod flower for allergies. Since this plant is entomophilous (its pollen is dispersed by animals), its heavy, sticky pollen does not become independently airborne. Most late summer and fall pollen allergies are probably caused by ragweed, a widespread anemophilous plant.

Arizona was once regarded as a haven for people with pollen allergies, although several ragweed species grow in the desert. However, as suburbs grew and people began establishing irrigated lawns and gardens, more irritating species of ragweed gained a foothold and Arizona lost its claim of freedom from hay fever.

Anemophilous spring blooming plants such as oak, birch, hickory, pecan, and early summer grasses may also induce pollen allergies. Most cultivated plants with showy flowers are entomophilous and do not cause pollen allergies. In the US, oak pollen starts to cause problems for sufferers in February and is gone by the end of April.^[17]

The percentage of people in the United States affected by hay fever varies between 10% and 20%, and such allergy has proven to be the most frequent allergic response in the nation. There are certain evidential suggestions pointing out hay fever and similar allergies to be of hereditary origin. Individuals who suffer from eczema or are asthmatic tend to be more susceptible to developing long-term hay fever.^[18]

The most efficient way to handle a pollen allergy is by preventing contact with the material. Individuals carrying the ailment may at first believe that they have a simple summer cold, but hay fever becomes more evident when the apparent cold does not disappear. The confirmation of hay fever can be obtained after examination by a general physician.^[19]

[edit] Treatment

Antihistamines are effective at treating mild cases of hay fever, this type of non-prescribed drugs includes loratadine, benadryl and chlorphenamine. They do not prevent the discharge of histamine, but it has been proven that they do prevent a part of the chain reaction activated by this biogenic amine, which considerably lowers hay fever symptoms. A side effect of antihistamines is somnolence, and it is therefore recommended not to take these drugs while driving an automobile or during the consumption of alcoholic beverages. However, the side effects of these medications can vary from person to person.^[20]

Decongestants can be administered in different ways such as tablets and nasal sprays. Decongestants such as sudafed and drixoral can be acquired as over-the-counter medications. Since oral decongestant drugs raise blood pressure levels, individuals with hypertension are advised to avoid them. The oral decongestant type can aggravate the symptoms of an enlarged prostate, making the process of urinating more complicated.^[21]

[edit] Nutrition

Most major classes of predatory and parasitic arthropods contain species that eat pollen. Spiders are normally considered carnivores but pollen is an important source of food for several species, particularly for spiderlings which catch pollen on their webs. It is not clear how spiderling manage to eat pollen however, since their mouths are not large enough to consume pollen grains. Some predatory mites also feed on pollen, with some species being able to subsist solely on pollen. Euseius tularensis can survive by feeding on the pollen of dozens of plant species. Only 36 of 40000 species of ground beetles have been shown to eat pollen, but this is thought to be a severe underestimate as the feeding habits are only known for 1000 species. Ladybirds mainly eat insects, but many species also eat pollen; some are thought to only eat pollen, since they live at too high an altitude for insects. Heteroptera are mostly omnivores but pollen feeding has only been well studied in the Anthocorinae. All adult Syrphidae feed on pollen, and three UK species feed strictly on pollen. Ants also eat pollen, forming part of the diet of many ant larvae. Flies cannot eat pollen directly, but can consume pollen contents that are dissolved in a fluid.^[22]

[edit] In humans

A variety of producers have started selling pollen for human consumption, often marketed as a food (rather than a dietary supplement). The largest constituent is carbohydrates, with protein content ranging from 7 to 35 percent depending on the plant species collected by bees.^[23]

The U.S. Food and Drug Administration (FDA) has not found any harmful effects of pollen consumption, except from the usual allergies. However, FDA does not allow pollen marketers in the United States to make health claims about their produce, as no scientific basis for these has ever been proven. Furthermore, there are possible dangers not only from allergic reactions but also from contaminants such as pesticides and from fungi and bacteria growth related to poor storage procedures. A manufacturers's claim that pollen collecting helps the bee colonies is also controversial.^[24]

[edit] Forensic palynology

In forensic biology, pollen can tell a lot about where a person or object has been, because regions of the world, or even more particular locations such a certain set of bushes, will have a distinctive collection of pollen species.^[25] Pollen evidence can also reveal the season in which a particular object picked up the pollen.^[26] Pollen has been used to trace activity at mass graves in Bosnia,^[27] catch a burglar who brushed against a Hypericum bush during a crime,^[28] and has even been proposed as an additive for bullets to enable tracking them.^[29]

[edit] See also

Wikimedia Commons has media related to: Pollen

• European Pollen Database
• Evolution of sex
• Pollen count
• Pollen source
• Polyphenol antioxidant

[edit] References

[edit] External links

Clumps of yellow pollen on a flower head.

• Pollen and Spore Identification Literature
• Paleobotany and Palynology at the Florida Museum of Natural History
• Palynology at the University of Arizona
• Palynology at French Institute of Pondicherry
• Palynology at the University of Sheffield
• SEM coloured and confocal microscope images of pollen
• Pollen Biology and Pollen Allergy (Radboud University Nijmegen)

 This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). Encyclopædia Britannica (Eleventh ed.). Cambridge University Press. 

• Palynology in Utrecht, the Netherlands
• The flight of the pollen cloud Outcrossing from transgenic maize and quantifying outcrossing rates
• The Evolutionary Theory of Sex: Mechanisms of Regulation of the Population Parameters
• PalDat (database comprising palynological data from a variety of plant families)
• Very large pollen accumulation in the sea
• Pollen movement dynamics Co-extra research project on coexistence and traceability of GM an non-GM supply chains
• Pollen SEM-image gallery

v ' d ' e Botany Subdisciplines of botany Ethnobotany – Paleobotany – Plant anatomy – Plant ecology – Plant evo-devo – Plant morphology – Plant physiology Plants Evolutionary history of plants – Algae – Bryophyte – Pteridophyte – Gymnosperm – Angiosperm Plant parts Flower – Fruit – Leaf – Meristem – Root – Stem – Stoma – Vascular tissue – Wood Plant cells Cell wall – Chlorophyll – Chloroplast – Photosynthesis – Plant hormone – Plastid – Transpiration Plant reproduction Alternation of generations – Gametophyte – Plant sexuality – Pollen – Pollination – Seed – Spore – Sporophyte Plant taxonomy Botanical name – Botanical nomenclature – Herbarium – IAPT – ICBN – Species Plantarum Glossaries Glossary of botanical terms – Glossary of plant morphology terms Category – Portal