FERTILIZATION IN PLANT

 STD-12      UNIT-6     CHA-2

REPRODUCTION IN FLOWERING PLANT

FERTILIZATION

MODE OF ENTRY OF POLLEN TUBE IN THE OVULE.

FERTILIZATION

• The fusion of male and female gametes is called fertilization . 
• In seed plants the male gametes are brought to the egg containing female gametophyte by a pollen tube ( Strasburger , 1884 ) . The phenomenon is called siphonogamy . 
• A large number of pollen grains ( several times more than the number of ovules ) come to germinate over the stigma . A pollen grain does not pass down the stigma . Only its pollen tube does so . 
• The pollen tube eats its way through the solid part of the stigma and style by secreting pectinases and hydrolytic enzymes . 
• Pollen tube travels intercellularly and chemotropically along the concentration gradient of calcium boron inositol sugar complex . 
• A small copper containing protein chemocyanin is also involved in some cases . Initial growth of the pollen tube takes places on expenditure of food present in the pollen grain . 
• Pollen tube formation seems to be stimulated by factors present in the stigmatic secretion . 
• For further growth the pollen tube obtains its nourishment from the interior of stigma and style . 
• The contents of the pollen grains shift into pollen tube with the tube or vegetative nucleus moving to its tip followed by the two gametes . Further growth of the pollen tube occurs only towards its tip . 
• Depending upon the length of the style and passage inside the ovary , the pollen tube may reach a length of a few millimetres to 45 cm in Maize . 
• Style may be hollow or solid . 
• Hollow style has a canal lined by special large cells . 
• Solid style has a special tissue of pectinised thick walls known as transmitting or conducting tissue . 
• The pollen tube travels along the lining of canal in hollow style drawing nourishment from its living cells . 
• In solid style the pollen tube grows through transmitting tissue by separating their cells through secretion of pectinases . 
• The food is absorbed by diffusion .

Mode of Entry of Pollen Tube in the Ovule

• In the ovary the growth of the pollen tube is directed by another tissue called obturator . 
• The pollen tube enters the ovule , either through its micropyle ( porogamy , e.g . Lily ) , 
• chalaza ( chalazogamy , e.g .Casuarina , Juglans ) 
• The sides after piercing through the integu ments or funicle ( mesogamy , e.g. Cucurbita , Populus ) . 
• Porogamy is the most common . 

• In the ovule the pollen tube is attracted by secretions of syner gids . Usually the pollen tube enters the embryo sac by passing into one of the two synergids 
• The impact destroys that synergid . The pollen tube also breaks open to release its contents . Out of the two male gametes one fuses with egg or oosphere to perform generative fertilization .
• Generative fertilization is also called syngamy or truefertilization . 
• It gives rise to a diploid zygote or cospore . Soon after , the vacuole and plasmodesmal connections of the egg degenerate . 
• It now becomes ready to produce the embryo . 
• The nucleus of the second male gamete fuses with the two haploid polar nuclei or diploid secondary nucleus of the central cell to form a triploid primary endosperm nucleus ( PEN ) . 
• The central cell is now called primary endosperm cell ( PEC ) . 
• This second fertilization is called vegetative fertilization since as a consequence of it a vegetative or nutritive tissue is formed to nourish the embryo . 
• Vegetative fertilization is also called triple fusion since three nuclei get fused , two polar nuclei and one male gamete . 

• In angiosperms or flowering plants two acts of fertilization occur in the same embryo sac , one generative and other vegetative . The phenomenon is called double fertilization . Double Fertilization

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