Fertilization
Fusion of male gamete (egg) to form a diploid zygote within the embryosac(or female gametophyte) is called fertilization.
In angiosperms, the male gametes are carried to the egg by pollen tube , This process is called siphonogamy.
Growth of The Pollen Tube
The two called pollen grains reach the receptive stigma of the carpel by the act of pollination. Pollen grains, after getting attached to the stigma , absorb water and swell. They release the wall-held proteins and other components so that recognition factors from the pollen and stigma come close to each other. Subsequent to mutual recognition and acceptance of pollen grains, the pollen grains germinates to produce a pollen tube which grows into stigma towards the ovarian cavity. Initial growth on pollen tube occurs at the expense of energy and food materials stored in the pollen tube.
The tube secreted exogenous pectinases and other hydrolytic enzymes to create a passage for its entry. The style is hollow in most of the plants.
Entry of The Pollen Tube Into The Ovule
After reaching the ovary, the pollen tube enters the Ovule, either through micropylar end , chalazal end or laterally. Porogamy is most common, chalazogamy is less common and mesogamy is rare. In (micropylar end, ) porogamy, tip of the pollen tube enters the micropyle, pushes through the nucellar tissue and finally pierces the egg apparatus end of the embryosac.
The mature monosporic Polygonum type of embrosac consists of three antipodal cells, one secondary nucleus, two synergids and an egg. The two synergids and an egg make the egg apparatus which is located towards the micropylar end of the embryosac. The synergids direct the growth of pollen tube by secreting some chemical substance. The tip of pollen tube enters into one synergids. The penetrate synergids start degenerating. After penetrating, the tip of pollen tube enlarge and ruptures , releasing most of its content including the male gamete and the vegetative nucleus into synergids.
