What is Pollination and its Types-

What is Pollination and its Types-

What is Pollination -

It is a process of transfer of pollen grains from the male reproductive part (anther) to the female reproductive part to the stigma of (ovary).It genarally done by means of air(Anemophily), water(hydrophily),insects(ornithophily) or bats(chiropterophily).

Apis mellifera taking a pollen from anther for pollination

POLLEN GRAINS- 
It is produced by the male reproductive part , stamen is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium. Pollen is a powdery substance consisting of pollen grains which are male microgametophytes of seed plants, which produce male gametes (sperm cells).

Magnified image of anther having pollen's 

Pollen grains have a hard coat made of sporopollenin (it is a very hard substance which provides strength to the pollen and make them survive in very harsh conditions) it protects the gametophytes during the process of their movement from the stamens to the pistil of flowering plants.

In a typical angiosperm anther is bilobbed and dithecous(having two compartments) except in malvaceae family family anther is monothecous.

Each pollen chamber represent a microsporongia thus an anther have 4 microsporongia, the anther wall is composed of following layers-

1-EPIDERMIS
It is the outermost layer and meant for protection.

2-ENDOTHECIUM-
It is characterised by the thickenings of alpha cellulose and the region where it is absent is called stomium. it have some hydroscopic cells which help in anther dehisscense (bursting of anther) so that pollens can be released in atmosphere.

3-MIDDLE LAYER-
It is 2-3 layer thick and meant for protection.

4-TAPETUM
It is the innermost layer, multinucleated and exhibit polyploidy
features -

• it provides nutrition to the developing pollen grains.
• Secretory tapetum produces pro ubish bodies which help in synthesis of sporopollenin.
• tapetum secretes a pollen kitt which makes the pollen grain sticky

Within the tapetum a mass of homogenous tissue of parenchyma cells is present which helps in storage of  food for developing pollen.

If pollen lands suitable stigma, it germinates, producing a pollen tube that transfers the pollen to the ovule containing the female gametophyte (megasporophyte). Individual pollen grains are small enough to require magnification to see detail. The study of pollen grains is called palynology, scientists determine the age of fossils, forensics by analysing the exine layer of pollens. 

Pollen in plants is used for transferring haploid male genetic material from the anther of a single flower to the stigma of another in cross-pollination.In a case of self-pollination, this process takes place from the anther of a flower to the stigma of the same flower(self pollination). 

Apis mellifera flying with a pollen

Cross pollination- If pollen from one flower is transferred to the stigma of other flower . it produces a genetically different offspring.

AUTOGAMY-
transfer of pollen from anther to stigma within the same flower.
it is genetically and functionally self pollination.
eg-Pea

GEITONOGAMY-
Transfer of pollen grains from anther to stigma of different flower of the same plant.
Genetically it is self pollination but functionally cross pollination.
eg- maize and cucumber

XENOGAMY-
Transfer of pollen grains from anther to stigma of different flower of different plant.
It is both genetically and functionally cross pollination.