Essential Parts of Flower: Gynoecium - Development of Female Gametophyte or Embryo Sac

Development of Female Gametophyte or Embryo Sac:

• The nucleus of the megaspore undergoes divisions and gives rise to the embryo sac or female gametophyte, which is known as mega gametogenesis.   
• Out of the four haploid megaspores, upper 3 (towards micropylar end) degenerate and one at the base remains functional. It is the 1^st cell of the female gametophyte.
• The megaspore elongates along the micropylar-chalazal axis.
• The functional megaspore enlarges and undergoes mitotic nuclear division to produce two nuclei.
• These nuclei, migrate to opposite poles of the megaspore.
• At each pole, the nucleus divides twice to form 4 nuclei, 4 at each pole. Nuclear divisions are not followed immediately by cell wall formation.
• The functional megaspore enlarges gradually and becomes an 8-nucleate embryo sac (female gametophyte).
  

  2, 4, and 8-nucleate stages of embryo sac and a mature embryo sac

• After the last nuclear division, the cell undergoes appreciable elongation, assuming a sac-like appearance. This is followed by the cellular organization of the embryo sac.
• Of the four nuclei at the micropylar end of the embryo sac, three organize into an egg apparatus, and the fourth one is left free in the cytoplasm of the central cell as the upper polar nucleus.
• Three nuclei of the chalazal end form three antipodal cells whereas the fourth one functions as the lower polar nucleus.
• Depending on the plant the 2 polar nuclei may remain free or may fuse to form a secondary nucleus (central cell). The egg apparatus is made up of a central egg cell and two synergids, one on each side of the egg cell.
• Synergids secrete chemotropic substances that help to attract the pollen tube.
• The special cellular thickening called the filiform apparatus of synergids helps in the absorption and conduction of nutrients from the nucellus to embryo sac. It also guides the pollen tube into the egg. Thus, a 7 celled with 8 nuclei embryo sac is formed.
  

  Structure of mature embryo sac