MECHANISM OF FERTILIZATION

The process of fertilization can be studied by the following steps are

1- Germination of pollen grains on stigma 

The pollen grains fall on the stigma by the process of pollination .

The stigma has the capability to recognize the pollen grains and only the right type or compatible pollen grains can germinate by absorbing nutrients from stigmatic surface .

At the time of germination the pollen grains slightly increase in size and their cytoplasm become active .

Pollen-pistil interaction helps in the pollen germination helps in the pollen germination .

2- Growth of pollen-tube 

After germination the pollen tube is formed and it always comes out through the germ pore.

Growth of the pollen tube is carried out by some enzymes present on the surface of the stigma .It grows down wards to the ovule .

The pollen tube contains a vegetative nucleus or tube nucleus and 2 male gametes.

By the time the tube reaces the ovule the vegetative nucleus degenerates .

3- Path taken by the pollen -tube

The pollen tube forms its way through the tissues of stigma style and ovary wall .

It moves through the solid part of stigma and style by secreting pectinases and hydrolytic enzyme.

 Pollen tube travel intercellularly and chemotropically along the concentration gradient of calcium boron inosital sugar complex 

 The style may be hollow or solid .Hollow style has a canal lined by special large cells where as the solid style has conducting tissue or transmitting tissue .

 In hallow style pollen tube travels along the canal lining and absorbs nourishment .From its living cells.

 In solid style the pollen tube grows through the conducting tissue by separating their cells by secreting pectinases .The food is obsorbed by  diffusion .

4- Entry of pollen tube into embryo sac

The pollen tube enters into embryo sac of the ovule in three

a- Porogamy -The entry of pollen tube through microphyle is called porogamy 

Ex- lily

b-Mesogamy - It is the entry of pollen tube through integument or meddle of ovule .

Ex- cucurbita

c- Chalazogamy - It is the entry of pollen tube through chalaza .

Ex- casurina , betavulgaris

5- Path of male gametes

The male gametes are non-motile so they are carried by pollen tube towards the female gametes .This process is called as siphonogamy .

The pollen tube is attracted by chemothropically active substances secreted from the filliform apparatus of synergid.

Usually the pollen tube containing 2 male gametes enters into embryo sac through any one of the synergids and then discharges their male gametes .This act destroys the synergid .

6- Fusion of gametes 

When the 2 mole gametes are discharged into the embryo sac the fusion of gametes takes place.

Out of these two male gametes one fuses with the haploid egg to form a