POLLINATIONName : Sheryl Bhatnagar

Roll no : 2047

Sub : Reproductive biology

Course : Botany 2nd yr

POLLINATION

Definition:

The transfer of pollen grains

from the anther to the stigma of

a flower.

Pollen grains are immobile. They

cannot reach the stigma by

themselves. An external agent is

required for this. It can be wind,

water, animal, gravity or growth

contact.

POLLINATION

Two types of pollination

Self-pollination

Cross-pollination

SELF POLLINATION

It is the transfer of pollen grains from the anther of a

flower to the stigma of either the same or

genetically similar flower.

Accordingly, self pollination is of two types,

autogamy and geitonogamy.

AUTOGAMY

It is a type of self pollination in which an intersexual

or perfect flower is pollinated by its own pollen.

Autogamy occurs by 2 methods :

- Homogamy :The anthers and stigmas of

chasmogamous or open flowers are brought

together by growth, bending or folding.

- Cleistogamy : In cleistogamous flowers, the

anthers dehisce inside closed flowers. Growth of

style brings the pollen grains in contact with stigma.

Pollination and seed set are assured. Pollinators

are not required.

GEITONOGAMY

It is a type of pollination in which pollen grains of

one flower are transferred to the stigma of

another flower belonging to either the same

plant or genetically similar plant. In geitonogamy,

the flowers often show modifications similar to ones

found in xenogamy or cross pollination.

ADVANTAGES & DISADVANTAGES

Advantages of self Pollination:

1. It maintains the parental characters

or purity of the race indefinitely.

2. Self pollination is used to maintain

pure lines for hybridisation

experiments.

3. The plant does not need to

produce large number of pollen

grains.

4. Flowers do not develop devices for

attracting insect pollinators.

Disadvantages of Self Pollination:

1. New useful characters are seldom

introduced.

2. Vigour and vitality of the race

decreases with prolonged self

pollination.

3. Immunity to diseases decreases.

4. adaptability to changed environment

are reduced.

CROSS POLLINATION

(XENOGAMY, ALLOGAMY):

Cross pollination is the transfer of pollen grains

from the anther of one flower to the stigma of a

genetically different flower.

Cross pollination is performed with the help of an

external agency.

ANEMOPHILY

It is a mode of cross pollination or transfer of pollen grains through the agency of wind.

e.g., Coconut Palm, Date Palm, Maize, many grasses, Cannabis.

Characterstics :

The flowers are colourless, odourless and nectarless.

Pollen grains are light, small and winged or dusty, dry smooth, nonsticky and unwettable.

stigma is hairy, feathery or branched to catch the wind-borne pollen grains.

Pollen grains are produced in very large number.

HYDROPHILY

It is the mode of pollination or transfer

of pollen grains through the agency of

water.

Eg. Zoostera , vallisneria.

Characters:

Flowers are small and inconspicuous.

Nectar and odour are absent.

Pollen grains are light and unwettable

due to presence of mucliage cover.

Stigma is long, sticky but unwettable.

EPIHYDROPHILY HYPOHYDROPHILY

ENTOMOPHILY

the pollen grains are transferred to a mature through the agency of insects like moths, butterflies, wasps, bees, beetles, etc.

Example:

Characters :

They are showy or brightly coloured.

Most insect pollinated flowers have a landing platform.

The pollen grains are spiny, heavy and surrounded by a yellow oily sticky substance called pollenkit.

Stigmas are often inserted and sticky.

Some flowers provide safe place to insects for laying eggs, e.g., Yucca.

ORNITHOPHILY

It is the mode of allogamy performed by

birds. Only a few types of birds are

specialised for this. They usually have

small size and long beaks.

Example :

Characters :

Ornithophilous flowers are usually brightly

coloured— red, orange, yellow or blue.

The floral parts are commonly leathery.

The ornithophilous flowers secrete

abundant watery nectar or have edible

parts.

Scent is often absent.

METHODS TO ENSURE CROSS POLLINATION

Dichogamy :

Anthers and stigmas mature at different

times in a bisexual flower so as to prevent

self pollination.

- Protandry Anthers mature earlier than

stigma of the same flower. Their pollen

grains become available to stigmas of the

older flowers, e.g., Sunflower, Salvia,

- Protogyny

Stigmas mature earlier so that they get

pollinated before the anthers of the same

flower develop pollen grains, e.g., Mirabilis

jalapa (Four O’Clock), Gloriosa, Plantago.

Herkogamy :

In some bisexual flowers the

structure of male & female sex

organs itself prove a barrier to self

pollination.

Self Sterility (Self

Incompatibility):

Pollen grains of a flower do not

germinate on the stigma of the

same flower due to presence of

similar self sterile gene (S,S3 in

pistil and S1 or S3 in pollen grain),

e.g., Tobacco, Potato, Crucifers.

Heterostyly :

There are 2 or 3 types of flowers with different heights of styles and

stamens.

(a) Diheterostyly (Dimorphic Heterostyly):

There are two types of flowers

- Pin eyed (long style and short stamens)

- Thrum eyed (short style and long stamens), e.g., Primula

(Primrose),

(b) Triheterostyly (Trimorphic Heterostyly or tristyly):

There are three types of flowers with different heights of styles.

e.g., Lythrum. Pollination occurs between anthers and stigmas of the

same height present in different flowers

ARTIFICIAL POLLINATION

This technique is used for the purpose of

Hybridization.

It involves pollination of desired female parents

with pollen from the desired male parent, taking

all precautions to prevent contamination of stigma

with undesired pollen.

In unisexual plants the procedure is simple but its

complicated in bisexual ones.

2 processes that takes place in bisexual flower :

- Emasculation

- Bagging

IMPORTANCE OF CROSS POLLINATION:

ADVANTAGES

Cross pollination introduces genetic re-combinations and hence variations in the progeny.

Cross pollination increases the adaptability of the offspring towards changes in the environment.

It makes the organisms better fitted in the struggle for existence.

The plants produced through cross pollination are more resistant to diseases.

DISADVANTAGES

•A factor of chance is always

involved in cross .pollination.

•It is less economical.

•Some undesirable characters

may creep in the race.

•The very good characters of

the race are likely to be

spoiled.

COEVOLUTION OF FLOWER AND ITS POLLINATOR

SPECIES:

Coevolution is the evolution in two species that interact

extensively with one another so that each acts as a major

force of natural selection on the other.

When one evolves a new feature or modifies itself, the

other evolves new adaptations in response of it.

This constant mutual feed back modification between the two

species is known as coevolution.

The coevolution of the flower and its pollinator species are

tightly linked with one another.

Disa nivea is a rare orchid found only in a few places in South

Africa, and until Johnson came to study it, no one knew how

it was pollinated. After a lot of patient orchid-watching, he and

his colleagues discovered that it is visited exclusively by the

fly shown in the picture. Its proboscis is well-matched to the

length of the orchid, and the orchid grows pollen in just the

right place so that they get stuck to the fly. You can see them

in this picture–the two dangling yellow packets on the fly’s

snout. This fly is named as Orchid fly.