Pollen is a fine to coarse powder containing the
microgametophytesIn plants and algae that undergo alternation of generations, a gametophyte is the multicellular structure, or phase, that is haploid, containing a single set of chromosomes:...
of seed plants, which produce the male
gameteA gamete is a cell that fuses with another gamete during fertilization in organisms that reproduce sexually...
s (sperm cells). Pollen grains have a hard coat that protects the sperm cells during the process of their movement between the
stamenThe stamen is the male organ of a flower. Each stamen generally has a stalk called the filament , and, on top of the filament, an anther , and pollen sacs,...
s to the
pistilThe gynoecium is a term with several meanings in botanical usage. In reference to mosses; liverworts; and hornworts, gynoecium refers to a cluster of archegonia and any associated modified leaves or stems present on a gametophyte shoot...
of flowering plants or from the male
coneA cone is an organ on plants in the division Pinophyta that contains the reproductive structures. The familiar woody cone is the female cone, which produces seeds. The male cones, which produce pollen, are usually herbaceous and much less conspicuous even at full maturity...
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
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
to the ovule of a receptive ovary. The Individual pollen grains are small enough to require magnification to see detail.
The structure of pollen
Pollen itself is not the male gamete, 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 tubeThe pollen tube of most seed plants acts as a conduit to transport sperm cells from the pollen grain, either from the stigma to the ovules at the base of the pistil, or directly through ovule tissue in some gymnosperms...
) and a generative nucleus (that divides to form the two sperm cells). The group of cells is surrounded by a
celluloseCellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
-rich cell wall called the intine, and a resistant outer wall composed largely of
sporopolleninthumb|right|270px|[[SEM]] image of pollen grainsSporopollenin is a major component of the tough outer walls of spores and pollen grains. It is chemically very stable and is usually well preserved in soils and sediments...
called the exine.
Pollen is produced in the
microsporangium (contained in the
antherThe stamen is the male organ of a flower. Each stamen generally has a stalk called the filament , and, on top of the filament, an anther , and pollen sacs,...
of an
angiospermThe flowering plants or angiosperms are the most diverse group of land plants. The flowering plants and the gymnosperms are the only extant groups of seed plants...
flowerA flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to mediate the union of male sperm with female ovum in order to produce seeds...
, male
coneA cone is an organ on plants in the division Pinophyta that contains the reproductive structures. The familiar woody cone is the female cone, which produces seeds. The male cones, which produce pollen, are usually herbaceous and much less conspicuous even at full maturity...
of a
coniferousThe conifers, division Pinophyta, also known as division Coniferophyta or Coniferae, are one of 13 or 14 division level taxa within the Kingdom Plantae. Pinophytes are gymnosperms. They are cone-bearing seed plants with vascular tissue; all extant conifers are woody plants, the great majority...
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
pinePines are coniferous trees in the genus Pinus , in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species.-Distribution:...
s,
firFirs are a genus of between 48-55 species of evergreen conifers in the family Pinaceae. All are trees, reaching heights of 10-80 m tall and trunk diameters of 0.5-4 m when mature...
s, and
spruceA spruce is a tree of the genus Picea , a genus of about 35 species of coniferous evergreen trees in the Family Pinaceae, found in the northern temperate and boreal regions of the earth. Spruces are large trees, from 20–60 m tall when mature, and can be distinguished by their whorled branches and...
s are winged. The smallest pollen grain, that of the
Forget-me-notMyosotis is a genus of flowering plants in the family Boraginaceae that are commonly called Forget-me-nots. Its common name was calqued from the French, "ne m'oubliez pas" and first used in English in c.1532...
(
Myosotis spp.), is around 6
µmA micrometre or micron is one millionth of a metre,or equivalently one thousandth of a millimetre....
(0.006 mm) in diameter. Wind-borne pollen grains can be as large as about 90-100 µm. The study of pollen is called
palynologyPalynology is the science that studies contemporary and fossil palynomorphs, including pollen, spores, orbicules, dinoflagellate cysts, acritarchs, chitinozoans and scolecodonts, together with particulate organic matter and kerogen found in sedimentary rocks and sediments...
and is highly useful in paleoecology,
paleontologyPaleontology
[from Greek: παλαιός "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought"] is the study of prehistoric life, including organisms' evolution and interactions with each other and their environments...
, 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.
Pollen
apertureApertures are very small spots on the walls of a pollen, where the wall is thinner and/or softer. For germination it is necessary that the pollen tube can reach out from the inner of the pollen and transport the chromosomes to the egg deep down in the pistil...
s 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. Furrows in the pollen grain are called colpi, which along with pores, are a chief criteria for the identifying pollen classes.
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 plantThe flowering plants or angiosperms are the most diverse group of land plants. The flowering plants and the gymnosperms are the only extant groups of seed plants...
s, with
eudicotsEudicots and Eudicotyledons are terms introduced by Doyle & Hotton to refer to a group of flowering plants that had been called "tricolpates" or "non-Magnoliid dicots" by previous authors. The term means, literally, "true dicotyledons" as it contains the majority of plants that have been...
having three colpi (
tricolpate), and other groups having one sulcus.
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,
germinationGermination is the process in which a seed or spore emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However, the growth of a sporeling from a spore, for example the growth of hyphae from fungal...
of the pollen grain often begins before it leaves the microsporangium, with the generative cell forming the two sperm cells.
Pollination
The transfer of pollen grains to the female reproductive structure (
pistil in angiosperms) is called
pollinationPollination is the process by which pollen is transferred in plants, thereby enabling fertilization and sexual reproduction. Pollen grains, which contain the male gametes to where the female gamete are contained within the carpel; in gymnosperms the pollen is directly applied to the ovule itself...
. This transfer can be mediated by the wind, in which case the plant is described as
anemophilousAnemophily or wind pollination is a form of pollination whereby pollen is distributed by wind. Anemophilous plants may be either gymnosperms or angiosperms ....
(literally wind-loving). Anemophilous plants typically produce great quantities of very lightweight pollen grains, sometimes with air-sacs. Non-flowering seed plants (e.g.
pinePines are coniferous trees in the genus Pinus , in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species.-Distribution:...
trees) are characteristically anemophilous. Anemophilous flowering plants generally have inconspicuous flowers.
EntomophilousEntomophily is a form of pollination whereby pollen is distributed by insects, particularly bees, Lepidoptera , flies and beetles. Entomophilous species frequently evolve mechanisms to make themselves more appealing to insects, e.g., brightly-colored or scented flowers, nectar, or appealing shapes...
(literally insect-loving) plants produce pollen that is relatively heavy, sticky and
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
-rich, for dispersal by
insectInsects are arthropods, having a hard exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae. They are the most diverse group of animals on the planet and include more than a million species that are already described. Insects represent more than half of all...
pollinatorA pollinator is the biotic agent that moves pollen from the male anthers of a flower to the female stigma of a flower to accomplish fertilization or syngamy of the female gamete in the ovule of the flower by the male gamete from the pollen grain...
s attracted to their
flowerA flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to mediate the union of male sperm with female ovum in order to produce seeds...
s. Many insects and some
miteMites, along with ticks, belong to the subclass Acarina and the class Arachnida. Mites are among the most diverse and successful of all the invertebrate groups. They have exploited an incredible array of habitats, and because of their small size most go totally unnoticed...
s are specialized to feed on pollen, and are called
palynivoreIn zoology, a palynivore is an herbivorous animal which selectively eats the nutrient-rich pollen produced by angiosperms and gymnosperms. Most true palynivores are insects or mites...
s.
In non-flowering seed plants, pollen germinates in the pollen chamber, located beneath and inside the micropyle. A
pollen tubeThe pollen tube of most seed plants acts as a conduit to transport sperm cells from the pollen grain, either from the stigma to the ovules at the base of the pistil, or directly through ovule tissue in some gymnosperms...
is produced, which grows into the nucellus to provide nutrients for the developing sperm cells. Sperm cells of
PinophytaThe conifers, division Pinophyta, also known as division Coniferophyta or Coniferae, are one of 13 or 14 division level taxa within the Kingdom Plantae. Pinophytes are gymnosperms. They are cone-bearing seed plants with vascular tissue; all extant conifers are woody plants, the great majority...
and
GnetophytaThe plant division Gnetophyta or gnetophytes comprise three related families of woody plants grouped in the gymnosperms. The gnetophytes differ from other gymnosperms in having vessel elements as in the flowering plants....
are without flagella, and are carried by the pollen tube, while those of
CycadCycads are a group of seed plants characterized by a large crown of compound leaves and a stout trunk. They are evergreen, gymnospermous, dioecious plants having large pinnately compound leaves...
ophyta and Ginkgophyta have many flagella.
When placed on the stigma of a flowering plant, under favorable circumstances, a pollen grain puts forth a
pollen tubeThe pollen tube of most seed plants acts as a conduit to transport sperm cells from the pollen grain, either from the stigma to the ovules at the base of the pistil, or directly through ovule tissue in some gymnosperms...
which grows down the tissue of the style to the
ovaryIn the flowering plants, an ovary is a part of the female reproductive organ of the flower or gynoecium. Specifically, it is the part of the carpel which holds the ovule and is located above or below or at the point of connection with the base of the petals and sepals...
, and makes its way along the
placentaThe placenta is an organ unique to mammals that connects the developing fetus to the uterine wall. The placenta supplies the fetus with oxygen and food, and allows fetal waste to be disposed of via the maternal kidneys...
, guided by projections or hairs, to the micropyle of an
ovuleOvule literally means "small egg." In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integuments forming its outer layer, the nucellus , and the megaspore-derived female gametophyte in its center...
. 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
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
cells. The sperm cells are carried to their destination in the tip of the pollen-tube.
Pollen as a carrier of ecological information in plants
A Russian theoretical biologist, Vigen Geodakyan (Geodakian), has suggested that the quantity of pollen reaching a pistillate flower can transmit
ecologicalEcology is the interdisciplinary scientific study of the interactions between organisms and the interactions of these organisms with their environment....
information and also regulate
evolutionary plasticityPhenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment. Such plasticity in some cases expresses as several highly morphologically distinct results; in other cases, a continuous norm of reaction describes the functional interrelationship...
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 ratioSex ratio is the ratio of males to females in a population. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth, and tertiary sex ratio is the ratio of mature organisms....
, dispersion and
sexual dimorphismSexual dimorphism is the systematic difference in form between individuals of different sex in the same species. Examples include colour , size, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers or tusks.-Examples:In...
of a plant population. High pollen quantity leads to a reduction of these characteristics and stabilization of a
populationIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings. Individuals within a population share a factor may be reduced by statistical means, but such a generalization may be too vague to imply anything...
. Small quantity leads to their increase and destabilization of a population.
Dependence of the secondary
sex ratioSex ratio is the ratio of males to females in a population. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth, and tertiary sex ratio is the ratio of mature organisms....
on the amount of fertilizing pollen was confirmed on four dioecious plant species from three families — Rumex acetosa (Polygonaceae), Melandrium album (Cariophyllaceae), Cannabis sativa and Humulus japonicus (Cannabinaceae). (see summary of all these data in review article).
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”.
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
palynologyPalynology is the science that studies contemporary and fossil palynomorphs, including pollen, spores, orbicules, dinoflagellate cysts, acritarchs, chitinozoans and scolecodonts, together with particulate organic matter and kerogen found in sedimentary rocks and sediments...
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
DevonianThe Devonian is a geologic period and system of the Paleozoic era spanning from . It is named after Devon, England, where rocks from this period were first studied....
period and increases in abundance until the present day.
Hay fever
AllergyAllergy is a disorder of the immune system often also referred to as atopy. Allergic reactions occur to normally harmless environmental substances known as allergens; these reactions are acquired, predictable, and rapid. Strictly, allergy is one of four forms of hypersensitivity and is called type...
to pollen is called
hay feverHay Fever is a comic play written by Noël Coward in 1924 and first produced in 1925 with Marie Tempest as the first Judith Bliss.Laura Hope Crewes played the role in New York...
. 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
goldenrodThe goldenrod is a yellow flowering plant in the Family Asteraceae.-Description:About 100 perennial species make up the genus Solidago, most being found in the meadows and pastures, along roads, ditches and waste areas in North America. There are a handful of species from each of Mexico, South...
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
ragweedRagweeds , also called bitterweeds and bloodweeds, are a genus of flowering plants from the sunflower family ....
, a widespread anemophilous plant.
ArizonaThe State of Arizona is a state located in the southwestern region of the United States. The capital and largest city is Phoenix. The second largest city is Tucson, followed in size by the four Phoenix metropolitan area cities of Mesa, Glendale, Chandler, and Scottsdale.Arizona was the 48th and...
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 gardensIrrigation is an artificial application of water to the soil. It is usually used to assist in growing crops in dry areas and during periods of inadequate rainfall...
, more irritating species of ragweed gained a foothold and Arizona lost its claim of freedom from hay fever.
Anemophilous spring blooming plants such as
oakAn oak is a tree or shrub in the genus Quercus , of which about 400 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus...
,
birchBirch is the name of any tree of the genus Betula , in the family Betulaceae, closely related to the beech/oak family, Fagaceae.-Description:...
,
hickoryTrees in the genus Carya are commonly known as Hickory. The genus includes 17–19 species of deciduous trees with pinnately compound leaves and large nuts...
,
pecanThe pecan is a species of hickory, native to south-central North America, in Mexico from Coahuila south to Jalisco and Veracruz, in the United States from southern Iowa, Illinois and Indiana east to western Kentucky, North Carolina, South Carolina and western Tennessee, south through Georgia,...
, and early summer
grassGrasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns...
es 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.
Pollen in human diets
A variety of producers have started selling pollen for human consumption, often marketed as a
foodFood is any substance, usually composed of carbohydrates, fats, proteins and water, that can be eaten or drunk by an animal, including humans, for nutrition or pleasure. Items considered food may be sourced from plants, animals or other categories such as fungus or fermented products like alcohol...
(rather than a
dietary supplementA dietary supplement, also known as food supplement or nutritional supplement, is a preparation intended to provide nutrients, such as vitamins, minerals, fiber, fatty acids or amino acids, that are missing or are not consumed in sufficient quantity in a person's diet...
). The largest constituent is
carbohydrateCarbohydrates
[Means "hydrates of carbon"] or saccharides
[The word comes from the Greek σάκχαρον, sákcharon, meaning "sugar").] are the most abundant of the four major classes of biomolecules...
s, with protein content ranging from 7 to 35 percent depending on the plant species collected by bees.
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 proved. 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.
Forensic palynology
In
forensic biologyForensic biology is the application of biology to law enforcement.It includes the subdisciplines of Forensic anthropology, Forensic botany, Forensic entomology, Forensic odontology and various DNA or protein based techniques.- Applications :...
, 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. Pollen evidence can also reveal the season in which a particular object picked up the pollen. Pollen has been used to trace activity at mass graves in Bosnia, catch a burglar who brushed against a
HypericumHypericum is a genus of about 400 species of flowering plants in the family Clusiaceae, subfamily Hypericoideae ....
bush during a crime, and has even been proposed as an additive for bullets to enable tracking them.
See also
- European Pollen Database
The European Pollen Database is a freely available database of pollen frequencies, past and present, in the larger European area. The database is hosted by the "".-External links:*...
- Evolution of sex
Scientists currently have developed several competing hypotheses to explain the evolution of sexual reproduction. Many groups of organisms, notably the majority of animals and plants, reproduce sexually. The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance...
- Palynivore
In zoology, a palynivore is an herbivorous animal which selectively eats the nutrient-rich pollen produced by angiosperms and gymnosperms. Most true palynivores are insects or mites...
- Pollen count
Pollen count is the measurement of the number of grains of pollen in a cubic meter of air. The higher the number, the more people will suffer if they are allergic to a particular pollen . Usually, the counts are announced for specific plants such as grass, ash or olive...
- Pollen source
The term pollen source is often used in the context of beekeeping and refers to flowering plants as a source of pollen for bees or other insects. Bees collect pollen as a protein source to raise their brood. For the plant, the pollinizer, this can be an important mechanism for sexual reproduction,...
- Polyphenol antioxidant
A polyphenol antioxidant is a type of antioxidant containing a polyphenolic substructure. In human health these compounds, numbering over 4,000 distinct species, are thought to be instrumental in combating oxidative stress, a process associated with some neurodegenerative diseases and some...
- Sex ratio
Sex ratio is the ratio of males to females in a population. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth, and tertiary sex ratio is the ratio of mature organisms....
External links