XII BIOLOGY
CH-1 Reproduction in lower and higher plants
PART- Pollination
Pollination-
Transfer of pollen grain from anther to the
stigma of flower same plant or different
plant is known as pollination.
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-
The transfer of pollen grains from the antherto 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 divided into three types on the basis
of source of Pollination –
a)Autogamy
(self
Pollination)
b)
Geitonogamy
c)Xenogamy
(cross Pollination)
a)Autogamy(self Pollination)
It is a type of pollination in which bisexual flower is pollinated by its own pollen grains is called autogamy .
1)It
is transfer of pollen grains from anther to the stigma of same flower
only.
It
is possible only in bisexual flowers.
2)Offsprings
produced are genetically identical to their parents e.g. pea.
3)When
flower opens to expose its sex organs, it is called Chasmogamous
i)Bisexuality-
Male
and female reproductive structures are present in same flower.
ii)Homogamy-
Anther and stigma of a flower become mature at
the same time.
iii)
Cleistogamy-
Self-pollinated
takes place before the opening of flower. Underground flowers in some plants
shows Cleistogamy. e.g. Commelina benghalensis.
Plants
like Viala, Commelina can produce both chasmogamous and cleistogamous
flowers on the same plant.
b)Geitonogamy(Geiton=neighbour)-
It
is transfer of pollen grains from anther to the stigma of a different flower of
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 same plant.
c)Xenogamy-
(cross
Pollination / out breeding)-
Transfer
of pollen grains from anther to the stigma of another flower of a different
plant of same species with the help of pollinating agency.
It generates genetically varied offsprings.
e.g. - Papaya.
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Process of Pollination
The process of
pollination begins when the pollen grains from the respective flowers lands on
the stigma and form a pollen tube with the style length, which connects both
the stigma and ovary. After the completion of the pollen tube, the pollen grain
starts transmitting sperm cells from the grain to the ovary.
Types of Pollination
All plants having
flowers completely rely on pollination method for reproduction. There are 2
types of pollination
Self Pollination
It is referred to as
the primary type of pollination as it includes a single flower.
Self-pollination occurs when pollen grains fall directly from anther into the
stigma of the flower. This process is quite simple and fast, which leads to a
reduction in genetic diversity as the sperm and egg cells of the flower share
some genetic information.
Advantages
and Disadvantages of Self-pollination
Self- pollination ensures that recessive characters are
eliminated.
The wastage of the pollen grain is
very less compared to cross-pollination
In the process of self- pollination,
the purity of the race is maintained, as there is no diversity in the genes
In self- pollination, there is no
involvement of external factors like wind, water, and other pollinating agents.
Self-pollination ensures that even a
smaller quantity of produced pollen grains from plants have a good success rate
in pollination.
Disadvantages
The vigour and vitality of the race are reduced
The immunity to diseases is reduced in
the resultant offsprings.
Cross-Pollination
It refers to a complex type of pollination that
allows the transfer of pollen grains from the anther of the flower into the stigma
of another flower. This method leads to an increase in genetic diversity as
different flowers will share and combine their genetic information to create
unique offspring.
Types of Cross-Pollination
Pollination
by Wind- Anemophily
There are only a
few flowers that use wind pollination and their features are greenish, small
and odourless flowers. As these flowers do not attract the pollinators, their
energy is not used for making colourful petals. This type of pollination
usually occurs when plants lack flowers with nectar and other features
including inconspicuous. The male parts of the Anemophilous flowers tend
to produce very large quantities of pollen and the stigma, the female
reproductive part of a flower are very large, sticky and feathery to extend
completely outside the flower. Thus the pollen is more likely to reach them.
Coconut, palm,
maize, grasses and all gymnosperms are the best examples of
wind-pollinated plants.
You must have
seen that your car is fully covered with yellow film in the spring, it is
actually the pollen that uses wind for pollination process.
Pollination
by Animals – Zoophily
Animals play an
important role in plant reproduction. They help in seed dispersal. When an
animal eats the fruit of a plant, they move to a different location. This
movement helps in spreading the seeds and with this, we get new plants to new
locations.
Artificial Pollination – Anthropophily
Artificial
pollination is done by human beings. This process is also called as
the Anthropophily. If there are any difficulties in the pollination
process through abiotic
or biotic agents, the artificial method of pollination is
performed by spreading pollen grains over the female flowers. Hybridization
techniques are also used in this process.
Advantages and Disadvantages Of Cross-pollination
Advantages
1.The produced seeds are good in
vigour and vitality.
2.All unisexual plants can reproduce
through the process of Cross-pollination.
3.The recessive characters in the
lineage are eliminated as a result of genetic recombination.
4.This process improves the immunity
of the offsprings towards the diseases and other environmental factors.
5.Cross-pollination introduces new
genes into a sequence of species and this is mainly due to the
fertilization between genetically different gametes.
Disadvantages
1.In this process, there is a great wastage of pollen grains.
2.Due to genetic
recombination during meiosis, there are chances of eliminations of good qualities
and additions of unwanted characteristics in offspring.