Structural change in plants female reproductive unit and megasporogenesis?

The gynoecium of a flower is the female reproductive unit. If gynoecium consists of a single pistil, it is called monocarpellary, and if it consists more than one pistil, it is called multi carpellary.

In multi carpellary condition if all the pistils are fused together then called Syncarpous and when they are free, it is called apocarpous.

A pistil has three parts:-

  1. Stigma – A terminal receptive disc-like stigma which serves as a landing platform for pollen grains.
  2. Style – A stalk-like style beneath the stigma.
  3. Ovary – The swollen basal part of the pistil is the ovary. The ovary contains an ovarian cavity, and inside the ovarian cavity, a placenta is present. This placenta gives rise to megasporangia (ovules).

Examples :-

Ovary with one ovule – wheat, paddy, mango and Ovary with many ovules – papaya, watermelon, Orchids.

Megasporangium (ovule)

The ovule is an integumented megasporangium found in spermatophytes which develop into a seed after fertilization.

Inside ovary, each ovule is attached to the placenta through a stalk called funicle.The region where ovule and funicle fuse together is called hilum.

Each ovule is protected by one or two multicellulars envelop called integuments. 

The outer and inner integuments encircle the ovule except a small opening at the apex of integuments known as micropyle.

The mass of thin-walled parenchymatous cells enclosed within the integuments is called nucellus.

Nucellus cells have abundant reserve food materials. Inside nucellus female gametophyte or embryo sac is present, and this sac bears the embryo later on.Each ovule has one embryo sac.

Megasporogenesis

The method of formation of haploid megaspores from diploid megaspore mother cell by the meiotic division called megasporogenesis.

In the young ovules, a hypodermal archesporial cell is differentiated into archesporium of the ovule. The archesporial cell divides by a periclinal division and form:-

  1.  primary parietal cell (outer) – creates nucellus
  2. primary sporogenous cell (inner) – differentiates into megaspore mother cells

 In the micropylar region of the nucellus, ovules differentiate a single megaspore mother cell (MMC).MMC is a large cell with a prominent nucleus and dense cytoplasm.The MMC undergo meiosis and form four megaspore.

In a majority of flowering plant, out of four megaspores, only one is functional while other three degenerate. This functional megaspore is the first cell of female gametophyte or embryo sac. 

This method in which the embryo sac is formed from a functional megaspore is termed as monosporic development.

The nucleus of functional megaspore divide (free nuclear division) into two nuclei which move to opposite pole and form 2-nucleated embryo sac.

These two nuclei further divide in four and finally eight daughter nuclei. After the 8-nucleate stage, cell wall laid down.

This cell wall formation leads to the organization of the typical female gametophyte or embryo sac. Out of eight nuclei, six nuclei are surrounded by cell walls and organized into cells while one nucleus from each of the pole migrates to the center to for polar nuclei. Polar nuclei are situated below the egg apparatus in the large central cell.

The egg apparatus is present near the micropylar end and consists of two synergids or help cells and one egg cell or oospore. The egg cell is present in between the two synergids cells.

The synergids cell have special cellular thickenings at the micropylar tip which guide the pollen tube into synergids is called Filiform apparatus. 

The filiform apparatus is also useful for the absorption and transportation of materials from the nucellus to the embryo sac.

Three cells present at the base of embryo sac or chalazal end are called Antipodals.

Thus a typical embryo sac contains eight nuclei, but seven cells – 3 micropylar, three chalazal and one central and this is known as Polygonum type of embryo sac because it is discovered in Polygonum by Strasburger.

After fertilization,

  • The egg cell on Fertilization form zygote.

 

  • The synergids get disorganized soon after fertilization.

 

  • The antipodal cell sooner or later also get disorganized.