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.
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