STRUCTURE OF EMBRYO SAC

             

STD-12      UNIT-6     CHA-2

REPRODUCTION IN FLOWERING PLANT

STRUCTURE OF EMBRYO SAC

STRUCTURE OF EMBRYO SAC ( FEMALE GAMETOPHYTE)

• In angiosperms , the female ga metophyte is called embryo sac . 
• Embryo sac is oval multicellular haploid structure which is embedded in the nucellus towards micropylar half of the ovule . 
• It is covered over by a thin membrane derived from the parent megaspore wall . 
• The typical and most common type embryo sac , found in 80 % flowering plants Polygonum type. 

• It contains 8 nuclei but 7 cells- 3 micropylar , 3 chalazal and one central . 
• It is formed by one meiosis ( formation of 4 megaspores from one MMC ) and three mitosis (inside functional megaspore ) . 
• The three micro pylar cells are collectively known as egg apparatus ( equivalent to one archegonium ) . 
• They epyriform in outline and are arranged in a triangular fashion .
• The three cells of egg appa ratus have conspicuous common walls towards micropylar hall . 
• They separate and become thin towards the central cell . 
• One middle cell is larger and is called egg or cosphere . 
• It has a central or micropylar vacuole and a nucleus towards the chalazal end . 
• A filiform apparatus may or may not be present . 
• The remaining two cells are called synergids , cooperative cells or help cells Each of them bears a filiform apparatus in the micropylar region , a lateral hook , chalazal vacuole and a central nucleus . 
• A filiform apparatus is a mass of finger like projections of the wall into the cytoplasm . 
• In embryo sac , one synergid degenerates at the time of entry of pollen tube into the embryo sac , whereas , the second one degenerates shortly after the embryo sac has received the pollen tube discharge . 
• All the three cells of the egg apparatus communicate with one another and to the central cell by plasmodesmata . 
• The egg or cosphere represents the single female gamete of the embryo sac . 
• The synergids help in obtaining nourishment from the outer nucellar cells , guide the path of pollen tube by their secretion and function as shock absorbers during the penetration of pollen tube into the embryo sac . 
• The three chalazal cells of the embryo sac are called antipodal cells . 
• They are the vegetative cells of the embryo sac which may degenerate soon or take part in absorbing nourishment from the surrounding nucellar cells . 
• Internally they are connected with the central cell by means of plasmodesmata . 
• The central cell is the largest cell of the embryo sac . 
• It has a highly vacuolate cytoplasm which is rich in reserve food and Golgi bodies . 
• In the middle , the cell contains two polar nuclei which have large nucleoli 
• The polar nuclei often fuse to form a single diploid secondary or fusion or definitive nucleus . 
• Thus all the cells of the embryo sac are haploid except the central cell which is first binucleate and then becomes diploid due to fusion of polar nuclei .

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