POLLINATION
Introduction:
The majority of flowering plants reproduce sexually i.e.,
through seed formation. We know sexual reproduction is
incomplete without fertilization. The male and female
gametes have to meet for fertilization and further
development. Have you ever wondered how plants
ensure their continuity on earth despite their immobile
nature? Let us answer the same by having a brief
discussion on a process called pollination.
Pollination:
Pollination is defined as the pre-fertilization event or
process, where pollen grains from anther are transferred
to the stigma of a flower.
Plants are immobile. They can’t copulate with each other
by themselves. They need a vector for this. Pollination is
the process that helps to unite the male and female
gametes and thus helps in fertilization. It can be broadly
classified into two, cross-pollination and self-pollination
and this is achieved with the help of a variety of
vectors/agents. For successful pollinations, it must occur
between the same species.
Types of pollinations:
Pollinations can occur either within a flower or between
flowers of the same plant or flowers of different plants.
Depending on this, pollinations are of three types, namely:
1. Autogamy
2. Geitonogamy
3. Xenogamy
Autogamy:
It is a type of self-pollination where the transfer of pollen
grains from the anther to the stigma takes place within the
same flower. Coordinated opening, maturation and
exposure of the anther and stigma are necessary for
autogamy
There are two conditions for autogamy to take place:
1. Anther-stigma synchronization; when the pollen
is released, stigma should be ready to receive it.
2. The position of or distance between anther and
stigma. Both should be close enough for
pollination.
In chasmogamous flowers, anther and stigma are exposed.
The exposed reproductive parts give a chance of cross-
pollination in chasmogamous flowers. While in
cleistogamous flowers anther and stigma are not exposed
but lie close enough for transfer. Thus, the chances of
cross-pollination in cleistogamous flowers are almost none.
In addition, they barely require a pollinating agent.
Geitonogamy:
Geitonogamy is the type of self-pollination where the
transfer of pollen grains from the anther to the stigma takes
place between different flowers in the same plant. Though
it seems like cross-pollination and takes place with the help
of pollinators, both the gametes have the same plant as their
origin.
Xenogamy:
Xenogamy is the cross-pollination where the pollen grain
transfer occurs across flowers of two different plants. In
other words, the transfer of pollen from the anther of one
plant to the stigma of another plant.
Pollinating Agents:
The agents, which are involved in transferring the pollen
grains from one flower to another flower are called
pollinating agents
Plants utilize both biotic and abiotic agents for pollination
Biotic agents – Animals, insects, butterflies, etc.
Abiotic agents – Wind and water
Wind pollination:
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
Water pollination:
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.[13] 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
Insect pollination:
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,
beetles, moths and butterflies, and flies. The existence of
insect pollination dates back to the dinosaur era
Vertebrate pollination:
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
Artificial pollination:
Artificial pollination is carried out from ancient times when
people themselves used to transfer pollen grains from
anthers to stigma.
The ancient civilizations of Babylonia dusted the pollen
from male flowers on the female flowers.
This technique has been advanced by the scientist these
days to develop new and improved varieties of plant.
Plant breeders remove the anthers before maturation and
cover these flowers with plastic bags. When the stigma
matures it is artificially pollinated from the pollen of
desired variety. This process is called as emasculation
Advantages of artificial pollination:
There are so many advantages of artificial pollination.
Some of them have been listed below:
Artificial pollination can increase the fruit size
and seed numbers
It has the ability of converting flowers to export
fruits.
It is more suitable because it does not depend on
any chance factors
Through this type of pollination, we can generate
a large variety of hybrid plants
