Sexual reproduction in flowering plants (New syllabus chapter 1 / old syllabus Chapter 2)

Megasporogenesis|reproduction in flowering plants|short notes| NCERT| standard 12| Biology

Note 7

Megasporogenesis

> It occurs in Ovule.

> Ovule generate (by differentiation) one single Megaspore Mother Cell (MMC).

--> Diploid

--> Large cell

--> Dense cytoplasm

--> prominent nucleus

--> Present inside nucellus

--> Position is in the micropylar end

> MMC undergoes meiotic division and generates 4 megaspores - which is called as megaspore tetrad

> All megaspore are haploid

> Out of 4 megaspores, only 1 megaspore is functional

> Other 3 megaspores undergo degeneration

> The only functional megaspore - develops into female gametophyte / embryo sac

--> functional megaspore divides mitotically - forms 2 nuclei

--> these 2 nuclei move towards opposite ends of the cell - this is called as 2-nucleate embryo sac

--> further 2 sequence of mitotical division occurs - first forms 4-nucleate embryo sac and later 8-nucleate embryo sac is formed

--> till the stage of 8-nucleate embryo sac, the divison is free-nuclear division

--> free nuclear division is a type of division where nucleus divides and the cell walls do not start to develop immediately after nuclear division.

--> after 8-nucleate stage of embryo sac, cell wall start to develop - this will create a typical female gametophyte/ embryo sac.

--> the formation of an embryo sac from one megaspore is called as monosporic development.

--> one mature female gametophyte has 8 nuclei and 7 cells.

--> at the micropylar end - 3 cells are grouped together

----> one egg cell and 2 synergids - together these three cells are called as egg apparatus

-----> synergids - they have special cellular thickening at the micropylar tip - it is called as filiform apparatus

-----> filiform apparatus plays crucial role in guiding pollen tubes into synergid - important part of fertilisation

--> at the chalazal end - 3 cells are situated - they are called as antipodals

--> at the centre - the central cell is seen - contains 2 remaining nuclei - these nuclei are called as polar nuclei

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Manish Mevada

Urvi Bhanushali