CHAPTER 2 SEXUAL REPRODUCTION IN FLOWERING PLANTS
Pollination
(1) The male and female gametes in flowering
plants are produced in the pollen grain
and embryo sac, respectively.
(2) As both
types of gametes are non-motile,
they have to be brought together for fertilisation to occur. ( Pollination is the mechanism to achieve this objective ).
(3) Transfer
of pollen grains (shed from the anther) to the stigma of a pistil is termed pollination.
(4) Flowering
plants have evolved an amazing array of adaptations
to achieve pollination.
(5) They make
use of external agents to achieve
pollination.
Kinds of Pollination
Depending
on the source
of pollen, pollination can be divided into three types.
(1) Autogamy
(1) In this
type, pollination is achieved within the same flower. ( Transfer of pollen grains
from the anther to the stigma of the same flower ).
(2) In a
normal flower which opens and exposes the anthers and the stigma, complete
autogamy is rather rare.
(3) Autogamy
in such flowers requires synchrony in pollen release and stigma
receptivity and also, the anthers and the stigma should lie close to each other
so that self-pollination can occur.
(4) Some
plants such as Viola ( common pansy ) , Oxalis , and Commelina produce two types of flowers – chasmogamous flowers which are similar to flowers of other species with
exposed anthers and stigma, and cleistogamous
flowers which do not open at all. In such flowers,
the anthers and stigma lie close to each other. When anthers dehisce in the flower
buds, pollen grains come in contact with the stigma to effect pollination.
(5) Cleistogamous
flowers are invariably autogamous as there is no chance of cross-pollen landing
on the stigma. Cleistogamous flowers produce assured seed-set even in the
absence of pollinators.
(2) Geitonogamy
(1) Transfer
of pollen grains from the anther to
the stigma of another flower of the same plant.
(2) Although
geitonogamy is functionally cross-pollination
involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant.
(3) Xenogamy
(1) Transfer
of pollen grains from anther to the stigma of a different plant.
(2) This is
the only type of pollination which during pollination brings genetically different types of pollen
grains to the stigma.
Agents of Pollination
(1) Plants use
two abiotic (wind and water)
and one biotic (animals) agents
to achieve pollination.
(2) Majority
of plants use biotic agents for pollination. Only a small proportion of plants
use abiotic agents.
(3) Pollen
grains coming in contact with the stigma is a chance factor in both wind and
water pollination. To compensate for this uncertainties and associated loss of
pollen grains, the flowers produce enormous amount of pollen when compared to
the number of ovules available for pollination.
KINDS OF POLLINATION
(A) WIND POLLINATION
(1) Pollination
by wind is more common amongst abiotic pollinations.
(2) Wind
pollination also requires that the pollen
grains are light and non-sticky so that they can be transported in wind
currents.
(3) They often
possess well-exposed stamens ( so
that the pollens are easily dispersed into wind currents ) and large often-feathery stigma to easily
trap air-borne pollen grains.
(4) Wind pollinated
flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence; a familiar example
is the corn cob – the tassels you see are nothing but the stigma and style
which wave in the wind to trap pollen grains.
(5) Wind-pollination
is quite common in grasses.
(B) WATER POLLINATION
(1) Pollination
by water is quite rare in flowering
plants and is limited to about 30 genera,
mostly monocotyledons.
(2) Water is a
regular mode of transport for the male
gametes among the lower plant groups
such as algae, bryophytes and
pteridophytes.
(3) It is
believed, particularly for some bryophytes and pteridophytes, that their
distribution is limited because of the need for water for the transport
of
male gametes and fertilisation.
(4) Some examples
of water pollinated plants are Vallisneria and Hydrilla which grow in fresh water and several marine sea-grasses such as Zostera.
(5) Not all
aquatic plants use water for pollination. In a majority of aquatic plants such
as water hyacinth and water lily, the flowers emerge above
the level of water and are pollinated by insects
or wind as in most of the land plants.
(6) In Vallisneria, the female flower
reach the surface of water by the
long
stalk and the male flowers or pollen grains are released on to the surface of
water. They are carried passively by water currents; some of them eventually
reach the female flowers and the stigma.
(7) In another
group of water pollinated plants such as sea
grasses, female flowers remain submerged in water and the pollen grains are
released inside the water. Pollen grains in many such species are long, ribbon like and they are carried
passively inside the water; some of them reach the stigma and achieve
pollination.
(8) In most of
the water-pollinated species, pollen grains are protected from wetting by a mucilaginous covering.
(9) Both wind
and water pollinated flowers are not
very colourful and do not produce
nectar.
(C) POLLINATION BY ANIMALS
(1) Majority
of flowering plants use a range of animals as pollinating agents. Bees,
butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and humming
birds) and bats are the common pollinating agents.
(2) Among the animals,
insects, particularly bees are the dominant biotic pollinating agents. Even
larger animals such as some primates (lemurs), arboreal (tree-dwelling)
rodents, or even reptiles (gecko lizard and garden lizard) have also been
reported as pollinators in some species.
(3) Often
flowers of animal pollinated plants are specifically adapted for a particular
species of animal.
(4) Majority
of insect-pollinated flowers are large, colourful, fragrant and rich in nectar.
When the flowers are small, a number of flowers are clustered into an
inflorescence to make them conspicuous. Animals are attracted to flowers by
colour and/or fragrance.
(5) The
flowers pollinated by flies and beetles secrete foul odours to attract these animals.
(6) To sustain
animal visits, the flowers have to provide rewards to the animals. Nectar and
pollen grains are the usual floral rewards.
(7) For
harvesting the reward(s) from the flower the animal visitor comes in contact
with the anthers and the stigma.
(8) The body
of the animal gets a coating of pollen grains, which are generally sticky in
animal pollinated flowers. When the animal carrying pollen on its body comes in
contact with the stigma, it brings about pollination.
(9) In some
species floral rewards are in providing safe places to lay eggs; an example is
that of the tallest flower of Amorphophallus
(the flower itself is about 6 feet in height).
(10) A similar
relationship exists between a species of moth and the plant Yucca where
both species – moth and the plant – cannot complete their life cycles without
each other. The moth deposits its eggs in the locule of the ovary and the
flower, in turn, gets pollinated by the moth. The larvae of the moth come out
of the eggs as the seeds start developing.
(11)
Many insects may consume pollen or the nectar without bringing about
pollination. Such floral visitors are referred to as pollen/nectar robbers.
