Pollination is an agent’s transferring pollen from a gymnosperm’s
sporophyll to an ovule’s micropyle or an agent’s transferring
pollen from an angiosperm’s anther to a carpel’s stigma .
Pollinating agents are animals, water, and wind and even plants
themselves when self-pollination occurs within a closed flower.
Pollination often occurs within a species. When pollination occurs
between species it can produce hybrid offspring in nature and in
Pollination is a major obligate process in seed
In angiosperms, after the pollen grain has landed on the stigma, it
develops a pollen tube which grows down the style until it reaches an
ovary. Sperm cells from the pollen grain then move along the pollen
tube, enter an ovum cell through the micropyle and fertilise it,
resulting in the production of a seed.
A successful angiosperm pollen grain (gametophyte) containing the male
gametes is 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 ovule
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.
Two main modes of fertilization are found in gymnosperms. Cycads and
Ginkgo have motile sperm that swim directly to the egg inside the
ovule, whereas conifers and gnetophytes have sperm that are unable to
swim but are conveyed to the egg along a pollen tube.
The study of pollination brings together many disciplines, such as
botany, horticulture, entomology, and ecology. The pollination process
as an interaction between flower and pollen vector was first addressed
in the 18th century by Christian Konrad Sprengel. It is important in
horticulture and agriculture, because fruiting is dependent on
fertilization: the result of pollination. The study of pollination by
insects is known as anthecology.
4 Evolution of plant/pollinator interactions
5 In agriculture
5.1 Improving pollination in areas with suboptimal bee densities
6 Environmental impacts
7 The structure of plant–pollinator networks
8 See also
11 External links
Pollen germination has three stages; hydration, activation and pollen
tube emergence. The pollen grain is severely dehydrated so that its
mass is reduced enabling it to be more easily transported from flower
Germination only takes place after rehydration, ensuring
that premature germination does not take place in the anther.
Hydration allows the plasma membrane of the pollen grain to reform
into its normal bilayer organization providing an effective osmotic
membrane. Activation involves the development of actin filaments
throughout the cytoplasm of the cell, which eventually become
concentrated at the point from which the pollen tube will emerge.
Hydration and activation continue as the pollen tube begins to
In conifers, the reproductive structures are borne on cones. The cones
are either pollen cones (male) or ovulate cones (female), but some
species are monoecious and others dioecious. A pollen cone contains
hundreds of microsporangia carried on (or borne on) reproductive
structures called sporophylls.
Spore mother cells in the
microsporangia divide by meiosis to form haploid microspores that
develop further by two mitotic divisions into immature male
gametophytes (pollen grains). The four resulting cells consist of a
large tube cell that forms the pollen tube, a generative cell that
will produce two sperm by mitosis, and two prothallial cells that
degenerate. These cells comprise a very reduced microgametophyte, that
is contained within the resistant wall of the pollen grain.
The pollen grains are dispersed by the wind to the female, ovulate
cone that is made up of many overlapping scales (sporophylls, and thus
megasporophylls), each protecting two ovules, each of which consists
of a megasporangium (the nucellus) wrapped in two layers of tissue,
the integument and the cupule, that were derived from highly modified
branches of ancestral gymnosperms. When a pollen grain lands close
enough to the tip of an ovule, it is drawn in through the micropyle (
a pore in the integuments covering the tip of the ovule) often by
means of a drop of liquid known as a pollination drop. The pollen
enters a pollen chamber close to the nucellus, and there it may wait
for a year before it germinates and forms a pollen tube that grows
through the wall of the megasporangium (=nucellus) where fertilisation
takes place. During this time, the megaspore mother cell divides by
meiosis to form four haploid cells, three of which degenerate. The
surviving one develops as a megaspore and divides repeatedly to form
an immature female gametophyte (egg sac). Two or three archegonia
containing an egg then develop inside the gametophyte. Meanwhile, in
the spring of the second year two sperm cells are produced by mitosis
of the body cell of the male gametophyte. The pollen tube elongates
and pierces and grows through the megasporangium wall and delivers the
sperm cells to the female gametophyte inside. Fertilisation takes
place when the nucleus of one of the sperm cells enters the egg cell
in the megagametophyte’s archegonium.
In flowering plants, the anthers of the flower produce microspores by
meiosis. These undergo mitosis to form male gametophytes, each of
which contains two haploid cells. Meanwhile, the ovules produce
megaspores by meiosis, further division of these form the female
gametophytes, which are very strongly reduced, each consisting only of
a few cells, one of which is the egg. When a pollen grain adheres to
the stigma of a carpel it germinates, developing a pollen tube that
grows through the tissues of the style, entering the ovule through the
micropyle. When the tube reaches the egg sac, two sperm cells pass
through it into the female gametophyte and fertilisation takes
Depending on the source of pollen, pollination can be classified into
2 types -
Self-pollination and Cross
Pollination (Xenogamy). Self
Pollination is further divided into
Autogamy and Geitonogamy.
Depending on agent of Pollination, pollination can be classified into
abiotic pollination and biotic pollination.
Pollination is the type of
Pollination in which pollen grains are
transferred from anther to the stigma of the same flower (Autogamy) or
pollen grains are transferred from anther to the stigma of different
flower of the same plant (Geitonogamy).
Xenogamy is the type of pollination in which
pollen grains are transferred from anther to the stigma of a different
Abiotic pollination refers to situations where pollination is mediated
without the involvement of other organisms. The most common form of
abiotic pollination, anemophily, is pollination by wind. Wind
pollination is very imprecise, with a minute proportion of pollen
grains landing by chance on a suitable receptive stigma, the rest
being wasted in the environment. This form of pollination is used by
grasses, most conifers, and many deciduous trees.
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. Of the abiotically pollinated species of plant,
98% are anemophilous and 2% hydrophilous, their pollen being
transported by water.
It is thought that among angiosperms, entomophily is the primitive
state; this is indicated by the vestigial nectaries in the
Urtica and other plants, and the presence of
fragrances in some of these plants. Of the angiosperms, grasses,
sedges, rushes and catkin-bearing plants are in general wind
pollinated. Other flowering plants are mostly biotic, the pollen being
carried by animal vectors. However a number of plants in multiple
families have secondarily adopted wind pollination in contrast to
other members of their groups. Some plants are intermediate between
the two pollination methods. common heather is regularly pollinated by
insects, but produce clouds of pollen and some wind pollination is
inevitable, and the hoary plantain is primarily wind pollinated, but
is also visited by insects which pollinate it.
Melissodes desponsus covered in pollen
Hummingbirds typically feed on red flowers
Main article: Pollinator
More commonly, the process of pollination requires pollinators:
organisms that carry or move the pollen grains from the anther of one
flower to the receptive part of the carpel or pistil (stigma) of
another. 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. At least
100,000 species of animal, and possibly as many as 200,000, act as
pollinators of the estimated 250,000 species of flowering plants in
the world. The majority of these pollinators are insects, but about
1,500 species of birds and mammals have been reported to visit flowers
and may transfer pollen between them. Besides birds and bats which are
the most frequent visitors, these include monkeys, lemurs, squirrels,
rodents and possums.
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
In zoophily, pollination is performed by vertebrates such as birds and
bats, particularly, hummingbirds, sunbirds, spiderhunters,
honeyeaters, and fruit bats.
Ornithophily or bird pollination is the
pollination of flowering plants by birds.
Chiropterophily or bat
pollination is the pollination of flowering plants by bats. Plants
adapted to use bats or moths as pollinators typically have white
petals, strong scent and flower at night, whereas plants that use
birds as pollinators tend to produce copious nectar and have red
Insect pollinators such as honey bees (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
European honey bee
European honey bee collects nectar, while pollen collects on its
Africanized honey bees immersed in Yellow
Opuntia engelmannii Cactus
Pollination can be accomplished by cross-pollination or by
Cross-pollination, also called allogamy, occurs when pollen is
delivered from the stamen of one flower to the stigma of a flower on
another plant of the same species. Plants adapted for
cross-pollination have several mechanisms to prevent self-pollination;
the reproductive organs may be arranged in such a way that
self-fertilisation is unlikely, or the stamens and carpels may mature
at different times.
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 is transferred 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 are
by necessity found on self-compatible or self-fertile plants.
Although certain orchids and grasses are entirely cleistogamous, other
plants resort to this strategy under adverse conditions. Often there
may be a mixture of both cleistogamous and chasmogamous flowers,
sometimes on different parts of the plant and sometimes in mixed
inflorescences. The ground bean produces cleistogamous flowers below
ground, and mixed cleistogamous and chasmogamous flowers above.
Geranium incanum, like most geraniums and pelargoniums, sheds its
anthers, sometimes its stamens as well, as a barrier to
self-pollination. This young flower is about to open its anthers, but
has not yet fully developed its pistil.
These Geranium incanum flowers have opened their anthers, but not yet
their stigmas. Note the change of colour that signals to pollinators
that it is ready for visits.
This Geranium incanum flower has shed its stamens, and deployed the
tips of its pistil without accepting pollen from its own anthers. (It
might of course still receive pollen from younger flowers on the same
An estimated 48.7% of plant species are either dioecious or
self-incompatible obligate out-crossers. It is also estimated that
about 42% of flowering plants exhibit a mixed mating system in
nature. In the most common kind of mixed mating system, individual
plants produce a single type of flower and fruits may contain
self-pollinated, out-crossed or a mixture of progeny types.
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-compatible (self-fertile) and
can pollinate and fertilize themselves. 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.
Mischocyttarus rotundicollis transporting pollen grains of
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
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 favor 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. Some
pollinator species can show huge variation in pollination
effectiveness because their ability to carry pollen is impacted by
some morphological trait. This is the case in the white-lined sphinx
moth, in which short-tongued morphs collect pollen on their heads but
long-tongued morphs do not carry any pollen. Some flowers have
specialized mechanisms to trap pollinators to increase
effectiveness. Other flowers will attract pollinators by odor. For
example, bee species such as
Euglossa cordata are attracted to orchids
this way, and it has been suggested that the bees will become
intoxicated during these visits to the orchid flowers, which last up
to 90 minutes. 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
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
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
Melittosphex burmensis, the oldest bee fossil, from the Cretaceous
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.
Bees provide a good example of the mutualism that exists between
hymenopterans and angiosperms. Flowers provide bees with nectar (an
energy source) and pollen (a source of protein). When bees go from
flower to flower collecting pollen they are also depositing pollen
grains onto the flowers, thus pollinating them. While pollen and
nectar, in most cases, are the most notable reward attained from
flowers, bees also visit flowers for other resources such as oil,
fragrance, resin and even waxes. It has been estimated that bees
originated with the origin or diversification of angiosperms. In
addition, cases of coevolution between bee species and flowering
plants have been illustrated by specialized adaptations. For example,
long legs are selected for in Rediviva neliana, a bee that collects
oil from Diascia capsularis, which have long spur lengths that are
selected for in order to deposit pollen on the oil-collecting bee,
which in turn selects for even longer legs in R. neliana and again
longer spur length in D. capsularis is selected for, thus, continually
driving each other's evolution.
Main article: List of crop plants pollinated by bees
Andrena bee collects pollen among the stamens of a rose. The female
carpel structure appears rough and globular to the left. The bee's
stash of pollen is on its hind leg.
Bombus ignitus, a popular commercial pollinator in Japan and China
Blueberries being pollinated by bumblebees.
Bumblebee hives need to be
bought each year as the queens must hibernate (unlike honey bees).
They are used nonetheless as they offer advantages with certain fruits
as blueberries (such as the fact that they are active even at colder
outdoor ambient temperature).
Well-pollinated blackberry blossom begins to develop fruit. Each
incipient drupelet has its own stigma and good pollination requires
the delivery of many grains of pollen to the flower so that all
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
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. Farmers can also raise
native crops in order to promote native bee pollinator species as
shown with L. vierecki in Delaware and L. leucozonium in southwest
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.
In some situations, farmers or horticulturists may aim to restrict
natural pollination to only permit breeding with the preferred
individuals plants. This may be achieved through the use of
Improving pollination in areas with suboptimal bee densities
In some instances growers’ demand for beehives far exceeds the
available supply. The number of managed beehives in the US has
steadily declined from close to 6 million after WWII, to less than 2.5
million today. In contrast, the area dedicated to growing
bee-pollinated crops has grown over 300% in the same time period.
Additionally, in the past five years there has been a decline in
winter managed beehives, which has reached an unprecedented rate of
colony losses at near 30%. At present, there is an
enormous demand for beehive rentals that cannot always be met. There
is a clear need across the agricultural industry for a management tool
to draw pollinators into cultivations and encourage them to
preferentially visit and pollinate the flowering crop. By attracting
pollinators like honey bees and increasing their foraging behavior,
particularly in the center of large plots, we can increase grower
returns and optimize yield from their plantings. ISCA
Technologies, from Riverside California, created a semiochemical
formulation called SPLAT Bloom, that modifies the behavior of honey
bees, inciting them to visit flowers in every portion of the field.
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, climate change and
others, and many researchers believe it is the synergistic effects of
these factors which are ultimately detrimental to pollinator
The structure of plant–pollinator networks
Wild pollinators often visit a large number of plant species and
plants are visited by a large number of pollinator species. All these
relations together form a network of interactions between plants and
pollinators. Surprising similarities were found in the structure of
networks consisting out of the interactions between plants and
pollinators. This structure was found to be similar in very different
ecosystems on different continents, consisting of entirely different
The structure of plant-pollinator networks may have large consequences
for the way in which pollinator communities respond to increasingly
harsh conditions. Mathematical models, examining the consequences of
this network structure for the stability of pollinator communities
suggest that the specific way in which plant-pollinator networks are
organized minimizes competition between pollinators and may even
lead to strong indirect facilitation between pollinators when
conditions are harsh. This means that pollinator species together
can survive under harsh conditions. But it also means that pollinator
species collapse simultaneously when conditions pass a critical point.
This simultaneous collapse occurs, because pollinator species depend
on each other when surviving under difficult conditions.
Such a community-wide collapse, involving many pollinator species, can
occur suddenly when increasingly harsh conditions pass a critical
point and recovery from such a collapse might not be easy. The
improvement in conditions needed for pollinators to recover, could be
substantially larger than the improvement needed to return to
conditions at which the pollinator community collapsed.
Fruit tree pollination
Hermann Müller (botanist)
Paul Knuth (botanist)
Plant reproductive morphology
Pollen DNA barcoding
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Look up pollination in Wiktionary, the free dictionary.
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Resources on Pollinators from the National Academies
Pollination Home page
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Pollination syndromes images at bioimages.vanderbilt.edu
"Pollination". Encyclopædia Britannica (11th ed.). 1911.
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