2. SEXUAL REPRODUCTION IN FLOWERING PLANTS
POLLINATION
It
is the transfer of pollen grains from the anther to the stigma of a pistil.
Based
on the source of pollen, pollination is 3 types:
a. Autogamy (self-pollination):
It is the transfer of pollen grains from the anther to stigma of the same
flower.
In
flowers with exposed anthers & stigma, complete autogamy is rare. Autogamy
in such flowers requires synchrony in pollen release and stigma receptivity. Also,
anthers & stigma should be close to each other.
Plants
like Viola (common pansy), Oxalis & Commelina produce
2 types of flowers:
· Chasmogamous
flowers: They are similar to flowers of other species with
exposed anthers and stigma.
·
Cleistogamous
flowers: They do not open at all. Anthers & stigma lie close to each other.
They are autogamous. When anthers dehisce in the flower buds, pollen grains
come in contact with stigma for pollination. Cleistogamous flowers produce
assured seed-set even in the absence of pollinators.
Cleistogamy leads to inbreeding depression.
b. Geitonogamy: It
is the transfer of pollen grains from the anther to the stigma of another
flower of the same plant. It is
functionally cross-pollination involving a pollinating agent. But it is
genetically similar to autogamy since the pollen grains come from the same
plant.
c. Xenogamy:
It
is the transfer of pollen grains from anther to the stigma of a different
plant. It brings genetically different pollen grains to the stigma.
Agents of
Pollination
1. Abiotic agents (wind & water)
Pollination
by wind (anemophily):
-
More common abiotic agent.
- Wind pollinated flowers often have a single ovule in each
ovary and numerous flowers packed into an inflorescence.
-
E.g. Corncob – the tassels are the stigma
and style which wave in the wind to trap pollen grains. Wind-pollination is
quite common in grasses.
-
Ways for effective pollination:
o
The flowers produce enormous amount of
pollen.
o
Pollen grains are light and non-sticky.
o
They often possess well-exposed stamens (for
easy dispersion of pollens into wind currents).
o
Large, feathery stigma to trap air-borne
pollen grains.
Pollination
by water (hydrophily):
- It is quite rare. It is limited to
about 30 genera, mostly monocotyledons. E.g. Vallisneria & Hydrilla
(fresh water), Zostera (marine sea-grasses) etc.
-
But in lower plants, water is a regular
mode of transport for the male gametes. Distribution of some bryophytes &
pteridophytes is limited because they need water for the transport of male
gametes and fertilisation.
-
In Vallisneria, the female flower
reaches 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 by water
currents and reach the female flowers.
- In
sea grasses, female flowers remain submerged in water. Pollen grains are long
and ribbon like. They are carried inside the water and reach the stigma.
- The
pollen grains of most of the water-pollinated species have a mucilaginous
covering to protect from wetting.
-
Not all aquatic plants use hydrophily. In most
of aquatic plants (water hyacinth, water lily etc.), the flowers emerge above
the level of water for entomophily or anemophily.
-
Wind and water pollinated flowers are not
very colourful and do not produce nectar.
2. Biotic agents (animals)
- Majority of flowering plants use animals
as pollinating agents. E.g. Bees, butterflies, flies, beetles, wasps, ants,
moths, birds (sunbirds & humming birds) bats, primates (lemurs), arboreal
(tree-dwelling) rodents, reptiles (gecko lizard & garden lizard) etc.
-
Pollination by insects (Entomophily),
particularly bees is more common.
-
Often flowers of animal pollinated plants
are specifically adapted for a particular species of animal.
-
Features
of insect-pollinated flowers:
o
Large, colourful, fragrant and rich in
nectar. Nectar & pollen grains are the floral rewards for pollination.
o
Small flowers form inflorescence to make
them visible.
o
The flowers pollinated by flies and
beetles secrete foul odours to attract these animals.
o
The pollen grains are generally sticky.
- When the animal comes in contact with the anthers and the
stigma, its body gets pollen grains. When it comes in contact with the stigma,
it results in pollination.
- Some plants provide safe places as floral reward to lay
eggs.
E.g. Amorphophallus (It has the tallest flower of
6 feet).
A
moth species and the plant Yucca cannot complete their life cycles
without each other. The moth deposits its eggs in the locule of ovary. The flower
gets pollinated by moth. The larvae come out of the eggs as seeds start
developing.
- Many insects consume pollen or nectar without bringing
about pollination. They are called pollen/nectar robbers.
Outbreeding Devices
Hermaphrodite
flowers can undergo self-pollination. Continued self-pollination results in
inbreeding depression.
To
avoid self-pollination (autogamy) and encourage cross-pollination,
there are some devices in plants:
a.
Avoiding synchronization: Here,
the pollen is released before the stigma becomes receptive or stigma becomes
receptive before the release of pollen.
b. Arrangement of anther & stigma at different
positions.
c. Self-incompatibility: It
is a genetic mechanism to prevent self-pollen (from same flower or other
flowers of the same plant) from fertilization by inhibiting pollen germination
or pollen tube growth in the pistil.
d. Production
of unisexual flowers: If male & female flowers are present
on the same plant (i.e., monoecious, e.g. castor & maize), it prevents
autogamy but not geitonogamy. In dioecious plants (e.g. papaya), male and
female flowers are present on different plants (dioecy). This prevents
both autogamy and geitonogamy.
Pollen-pistil
Interaction
-
It
is a process in which pistil recognizes compatible or incompatible pollen through
the chemical components produced by them.
- Pistil
accepts compatible pollen and promotes post-pollination events.
-
It rejects incompatible pollen by
preventing pollen germination or pollen tube growth.
- Pollen grain germinates on the stigma to produce a pollen
tube through one of the germ pores. The contents of pollen grain move into
pollen tube. Pollen tube grows through the tissues of stigma and style and
reaches the ovary.
-
In plants which shed pollen grains at 2-celled
condition (a vegetative cell & a generative cell), the generative
cell divides into two male gametes during pollen tube growth.
-
In plants which shed pollen in 3-celled
condition, pollen tubes carry 2 male gametes from the beginning.
-
Pollen tube → ovary → micropyle
→ ovule → enters one of the synergids through filiform
apparatus. Filiform apparatus guides the entry of pollen tube.
-
A plant breeder can manipulate
pollen-pistil interaction, even in incompatible pollinations, to get desired
hybrids.
Artificial
hybridisation
It
is a crop improvement programme in which desired pollen grains are used for
pollination.
Steps:
o
Emasculation: Removal
of anthers from the bisexual flower bud of female parent before the anther
dehisces.
o
Bagging: Here,
emasculated flowers are covered with a bag (butter paper) to prevent
contamination of its stigma with unwanted pollen.
o
Pollination: When
stigma attains receptivity, pollen grains collected from male parent are dusted
on the stigma.
o
Rebagging the
flowers. It is allowed to develop the fruits.
For unisexual flowers,
there is no need for emasculation. Female flower buds are bagged before the
flowers open.
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