Pollination is the process of transfer of pollen grains from anther to stigma.
The two types of pollination found in flowering plants are:
that occurs within the same plant.
that occurs between two flowers of two different plants but of the same kind.
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.
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.
Pollination may be biotic or abiotic. Biotic pollination relies on living pollinators to move the pollen from
one flower to another. Abiotic pollination relies on wind, water or even rain. About 80% of angiosperms
rely on biotic pollination.
Abiotic pollination uses nonliving methods such as wind and water to move pollen from one flower to
another. This allows the plant to spend energy directly on pollen rather than on attracting pollinators
with flowers and nectar.
Some 98% of abiotic pollination is anemophily, pollination by wind. This probably arose from insect
pollination, most likely due to changes in the environment or the availability of pollinators. The transfer
of pollen is more efficient than previously thought; wind pollinated plants have developed to have
specific heights, in addition to specific floral, stamen and stigma positions that promote effective pollen
dispersal and transfer.
Pollination by water, hydrophily, uses water to transport pollen, sometimes as whole anthers; these can
travel across the surface of the water to carry dry pollen from one flower to another. In Vallisneria
spiralis, an unopened male flower floats to the surface of the water, and, upon reaching the surface,
opens up and the fertile anthers project forward. The female flower, also floating, has its stigma
protected from the water, while its sepals are slightly depressed into the water, allowing the male
flowers to tumble in.
Rain pollination is used by a small percentage of plants. Heavy rain discourages insect pollination and
damages unprotected flowers, but can itself disperse pollen of suitably adapted plants, such as
Ranunculus flammula, Narthecium ossifragum, and Caltha palustris.In these plants, excess rain drains
allowing the floating pollen to come in contact with the stigma.In some orchids ombrophily occurs, and
rain water splashes cause the anther cap to be removed, allowing for the pollen to be exposed. After
exposure, raindrops causes the pollen to be shot upward, when the stipe pulls them back, and then fall
into the cavity of the stigma. Thus, for the orchid Acampe rigida, this allows the plant to self-pollinate,
which is useful when biotic pollinators in the environment have decreased.
More commonly, pollination involves pollinators (also called pollen vectors): 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. Between 100,000 and 200,000 species of animal act as pollinators of the world’s 250,000
species of flowering plant.The majority of these pollinators are insects, but about 1,500 species of birds
and mammals 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 dinosaur era.
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 petals.
Insect pollinators such as honey bees (Apis spp.),bumblebees (Bombus spp.), and butterflies (e.g.,
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.
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 too short.
POLLINATION BY AGRNTS:
Pollinators range from physical agents, especially the wind (wind pollination is called anemophily), or
biotic agents such as insects, birds, bats and other animals (pollination by insects is called entomophily,
by birds ornithophily, by bats chiropterophily).