Write the events of development of female gametophyte in correct order

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Types of embryo Sac Development and their classification depends on:

1.The number of 9 spore nuclei entering into the formation of embryo sac Thus embryo sac may be monosporic, bisporic or tetrasporic type
2.The arrangement, and chromosome number of the nuclei in the mature embryo sac.
3.The total number of nuclear divisions occurring during megasporogenesis and development of female gametophyte.
4.Monosporic, or Polygonum Type

This embryo sac has 4 well-defined megaspores. 1 of  which gives rise to the embryo sac. The active megaspore enlarges. Its nucleus divides. A big vacuole is formed between the nuclei. Due to which the daughter nuclei move to the micropylar and chalazal poles of the embryo sac. Each nucleus divides 2 times. Then 4 nuclei are formed at each pole. 1 nucleus from each pole migrates to the centre of the embryo sac. The 2 nuclei fuse to form a diploid secondary nucleus. 3 nuclei at micropylar end are encompassed by membranes. They form egg apparatus. The central cell enlarged and become egg cell. The other 2 cells becomes synergid. After this whole process embryo sac is formed containing  eight-nucleoli and then seven-celled during its development.
2.Bisperic Type
This type of embryo sac is found in Allium. It is generally present in monocot and dicot families. 2 diad cells are formed during first meiotic division.
3. Tetrasporie Type
In this type of embryo sac wall is not found after the meiotic nuclear division. All 4  haploid megaspore nuclei take part in the formation of the embryo sac.

The primary endosperm nucleus will divide again and again  . It forms polyploidy nutritive tissue called endosperm.
There are 2 types of seeds for storage of food:
a)   Endospermic or albuminous seed
b)   Non-endospermic or ex-albuminous seeds.

Formation of Endosperm
Endosperm is formed from the primary endosperm nucleus. Its evolution starts before the evolution of embryo. Primary endosperm nucleus is formed  by fusion of monoploid polar nuclei (secondary nucleus) and a monoploid second male gamete.

Types of Endosperm
There are 3 types of endosperm on the basis of mode of development.
  • Nuclear type,
  • Cellular type and
  • Helobial type.

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