1. Pollen grains being
non-motile, for the transfer of pollen grains agencies or agents are required
are known as pollinating agents.
2. pollinating agents are two types
-
a)abiotic agents (wind,
water).b) biotic agents (birds, insects, snails, Bats,
animals.).
Types of pollination
1)Self-pollination- The transfer of
pollen grains from the anther to the stigma of the same flower or different
flower of same plants of same species
The transfer of pollen
grains from the anther of one flower to the stigma of another flower of
different plants of same species
Pollination can be
further divided into three types based on the source of Pollination-
1)It is transfer of
pollen grains from the anther to the stigma of the same flower
only. It is possible only in bisexual flowers.
2)Offsprings produced
are genetically identical to their parents e.g. pea.
3)When a flower opens to
expose its sex organs, it is called Chasmogamous
i)Bisexuality- Male and female reproductive structures
are present in the same flower.
ii) Homogamy- Stigma and Anther of a flower mature at the same
time.
iii) Cleistogamy- Self-pollinated takes place before the
opening of the flower. Underground flowers in some plants show
Cleistogamy.
e.g. Commelina plant produces both chasmogamous and cleistogamous flowers on the same plant
It is the transfer
of pollen grains from the anther to the stigma of a different flower of the
same plant.
1)It involves
pollinating agents.
2)It cannot bring about
genetic variations and is only of ecological significance. e.g. Cucurbita
maxima
3)It is similar to autogamy as pollen grains come from the same plant.
Transfer of pollen grains from the anther to the stigma of another flower of a different plant of the same species with the help of pollinating agency.
It generates
genetically varied offsprings.e.g. - Papaya.
Pollinating agents are of two types:-
A. Abiotic Agents:
1)Pollination by wind
(Anemophily)
2)Pollination by water
[Hydrophily:]
are two type
i)Hypohydrophily: ii)Epihydrophily:
B. Biotic Agents:
1)Pollination by
insects (Entomophily)
3)Pollination by Bats (Chiropteryphily)
A. Abiotic Agents:
These are non-living
agents which include wind and water.
1) Pollination by wind (Anemophily)=
pollination takes
place by a wind is called wind pollination
Most of the important
crop plants are wind-pollinated.
These includes wheat,
rice, corn, rye, barley, and oats. Palms are also wind-pollinated.
Adaptations in
anemophilous flowers-
i)The flowers are small,
inconspicuous, colorless, without nectar and fragrance (odor).
ii)The pollen grains are
lightweight, dry, and produced in large numbers to increase chances to waste.
iii)Stigma is feathery
to trap pollens carried by the wind.
iii)Stamens are exserted
with long filaments and versatile anthers.
iv) Stamens and stigmas are exposed to air currents.
i)Hypohydrophily-Pollination occurs below the surface of the water Here the pollen grains have the same specific gravity as water so they can swim in the water. e.g. In Zostera (seagrass) the pollen grains are long, ribbon-like and without exine .
ii)Epihydrophily:The pollen grains float on the water surface and reach the stigma of the female flowers. e.g. Vallisneria is a submerged dioecious, in which female flowers reach the water surface temporarily to ensure pollination and male flowers float on the surface of the water.Specific the gravity of pollen grain is equal to that water. why they float on the surface of water Anemophilous plant e.g. Potamogeton, Halogaris, etc. entomophilous plant e.g. Lotus, Water hyacinth, Waterlily, etc
Adaptations in
hydrophilous flowers-Flowers are small and
inconspicuous.
Perianth and other
floral parts are unwettable.
Pollen grains is unwettable, Nectar and fragrance are lacking in flowers.
1. Pollination by insects
(Entornophily]: It occurs in Rose, Jasmine, Cestrum, etc.
Adaptations in entomophilous flowers:
i)They are large, showy,
and often brightly colored.
ii)The flowers produce a
sweet odor (smell) and have nectar glands.
iii)The stigma is rough
due to the presence of hair or is sticky due to mucilaginous secretion.
iv)The pollen grains are
spiny and sticky.
v)Some plants have special adaptations for the insect visitor to help in cross pollination, e.g. Jasmine, Rose, Salvia, (estrum, etc.
Only a few types of
birds are specialized for pollination.
e.g. Sunbirds and
hummingbirds
e.g. - Bombax,
Callistemon (Bottle Brush), Butea, Bignonia,
etc.
Adaptations in ornithophilous
flowers:
i)Flowers are usually brightly colored, large, and
showy.
ii)They secrete profuse,
dilute nectar.
iii)Pollen grains are
sticky and spiny.
iv)Flowers are generally without fragrance, as birds have a poor sense of smell.
Bats can
transport pollens over a long distance, some times several kilometers.
Adaptations in Chiropterphilous flowers:
i)Flowers are
dull-colored with a strong fragrance?
ii)Flowers are large and
strong.
iii)Flowers open at
night time.
iv)Flower produce
abundant nectar.
v)Flowers produce a
large number of edible pollen grains.
vi)e.g. Autocephalous (Kadam tree), Adansonia (Baobab tree), Kigelia (Sausage tree).
These are the mechanisms
which prevent self-pollination and promote cross-pollination.
To increase genetic
diversity, plants have evolved a wide variety of sexual
strategies. Continuous self-pollination results in the
inbreeding depression and organism become weaker.
1. Unisexuality:-
Some plants produce
unisexual flowers.
Monoecious:- Male and female flowers are produced separately but on the
same plant. It prevents only autogamy.e.g. - Castor, Cucurbita, Maize, etc.
Dioecious:- Male and female flowers are produced on different
plants. It prevents both autogamy as well as geitonogamy. e.g.
- Papaya, Mulberry, etc.
2. Dichogamy: In bisexual flower anthers, and stigmas
mature at different times to prevent self-pollination.
It is two types: a. Protandry: Androecium
matures earlier than the gynoecium, e.g. sunflower.
b. Protogyny: In this
type, gynoecium matures earlier than the androecium, e.g. Gloriosa
3. Prepotency:
Pollen grains of other
flowers germinate rapidly over the stigma than the pollen grains from the same
flower, e.g. Apple.
4. Prepotency:
Pollen grains of another
flower germinate rapidly over the stigma than the same flower, e.g. Apple.
These devices to prevent
self-pollination in a bisexual flower.
In plants, a natural
physical barrier is present between two sex organs and avoid contact of pollen
with the stigma of the same flower.
e.g.
Calotropis-pentangular stigma is positioned above the level of anthers
(pollinia).
6. Self-incompatibility (self-sterility):
This is a genetic
mechanism due to which the germination of pollen on the stigma of the same
flower is inhibited. e.g. Tobacco, Thea.
Pollen - Pistil Interaction:
It is the
interaction of pollen grain with stigma. It begins with pollination and ends
with
fertilization.
i)All the events from
the deposition of pollen grain on stigma to the entry of pollen tube in the
ovule (synergid) are referred to as pollen - pistil interaction.
ii) Pollination does not
guarantee the transfer of the right type of pollen, often the wrong type also
lands on stigma.
iii)Stigma have the
capacity to recognize the pollen and it allows only the right type of pollens
iv) It is a very dynamic
process involving pollen recognition followed by inhibition or promotion of
pollen germination.
v)Cross pollination can
transfer right as well as the wrong type of pollens on the stigma.
vi)The right type of
pollens i.e. compatible with the same species allow to germinate and forms
pollen tubes.
viii)The wrong type or
incompatible pollen i.e. from other species or self-incompatible pollen does
not allow to germinate by stigma.
ix)The ability of
pistil to recognize the
pollen is because of certain chemical components present in the pollen grain,
which interacts with stigma.
x)Pollen grain can also
be induced to germinate in a synthetic medium.
xi)Sucrose induces
pollen germination and tube growth in vitro. The addition of boric acid
facilitates and accelerates pollen germination.
Artificial hybridization: It is one of the
major approaches used
in crop improvement