Question

Explain the structure ofmaturead embryosac of angisperms

Answer 1

- A single megaspore mother cell of the nucellus undergoes sporic meiosis to produce four haploid megaspore. Out of the four only one is functional while the other three degenerate. The functional megaspore develops into the female gametophyte or embryo sac. Thus embryo sac formation is processeded by meiosis.

The functional megaspore divides mitotically forming 2 - nucleate embryo sac. Two more sequential mitotic nuclear divisions results in the formation of the 4 - nucleate and 8 - nucleate stage, leading to the organisation of the typical female gametophyte or embryo sac. Each cell of the embryo sac is haploid.

Each embryo sac has a three celled egg apparatus -- one egg cell and two synergids, three antipodal cells and two polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary nucleus.

# definition of megasporogenesis ;

ovules generally differentiate a single megaspore mother cell (MMC) in the micropylar region of the nucellus.

~ MMC containing dense cytoplasm and prominent nucellus.

~ The MMC undergoes meiosis and forms a linear tetrad of four haploid megaspores.Out of which only one remain functional ( chalazal end) and other three degenerate (micropylar end)

other important point is that__meiosis in MMC is needed to ensure the formation of a haploid female gamete before fertilization.

The embryo sac formation are three types;:;

1- Monosporic embryo sac / polygonam type -** found in 80 % of flowering plants.

eg:: ( polygonam,oenothera )

2 - Bisporic embryo sac

eg:: (Allium, Endymion )

3 - Tetrasporic embryo sac

eg:: (Adoxa, Plumbago, Drusa, Fritilaria)

Answer 2

Answer:

Explanation:

•In angiosperms the female gametophyte is called as embryosac.

•The most common type of embryosac in angiosperms is seven celled and 8 nucleated "Polygonum type"

•In majority of flowering plants,one of the megaspores is functional and acts as mother cell for the development of female gametophyte (embryosac), the other 3 degenerate.The method of embryosac formation from a single functional megaspore is termed as monosporic development.

•Polygonum type of embryosac shows 3 main parts.They are

1)Egg apparatus

2)central cell

3)antipodals.

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1)EGG APPARATUS-

Three cells are grouped together at the micropylar end and constitute the egg apparatus. There is one egg cell and 2 synergids.

•The synergids have special cellular thickening at the micropylar tip called the filiform apparatus.

•The middle largest cell in egg apparatus is oospore or egg.

Role of synergids-

A) they nourish female gametophyte by absorbing nutrients from surrounding nucellus.

B)The filliform apparatus of synergids secrete chemical substances which help in guiding the pollen tubes into synergids.

___________________________

2) Central cell-

•It is the largest cell of embryosac.

•It has 2 polar nuclei which fuse to form a single diploid secondary nucleus.

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3) Antipodals-

•The 3 cells present at the chalazal end of embryosac are called as antipodals.

•The cells are ephemeral,smallest cells of embryosac and degenerate before or after fertilization.

•They are considered as vegetative cells of embryosac.

___________________________

Hope this helps you.... :)