question_answer 6)

With a neat diagram, explain the 7-celled, 8-nucleate mature female gametophyte.

Answer:

Embryo sac is an oval multicellular structure. It is covered over by a thin membrane derived from the parent megaspore wall. The typical or Polygonum type of embryo sac contains 8 nuclei but 7 cell ? 3 micropylar, 3 chalazal and one central. The three micropylar cells are collectively known as egg apparatus. One cell is larger and is called egg or oosphere. It bears a central or micropylar vacuole and a nucleus towards the chalazal end. The remaining two cells are called synergids or help cells. Each of them bears a filiform apparatus in the micropylar region, a lateral hook, chalazal vacuole and a central nucleus. The egg or oosphere 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. The central cell is the largest cell of the embryo sac. It has a highly vacuolated cytoplasm which is rich in reserve food and Golgi bodies. In the middle, the cell contains two polar nuclei which often fuse to form a single diploid secondary or fusion nucleus. Thus all the cells of the embryo sac are haploid except the central cell which becomes diploid due to fusion of polar nuclei.