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Thursday, 30 September 2021

The Pistil, Megasporangium (ovule) and Embryo sac l Notes l NCERT based l NEET

The Pistil, Megasporangium (ovule) and Megasporogenesis

 

The Pistil

·         The gynoecium represents the female reproductive part of the flower.

·         The gynoecium may consist of

 

 

·         a single pistil (monocarpellary) or

 

·         more than one pistil (multicarpellary)

 

When there are more than one, the pistils may be

 

·         fused together (syncarpous) or

 

·          may be free (apocarpous)

 

·         Each pistil has three parts

 

1.       Stigma            (landing platform for pollen grains)

 

2.        Style              (elongated slender part beneath the stigma)

 

3.       Ovary             (The basal bulged part of the pistil)

 

      Inside the ovary is the ovarian cavity (locule).

      The placenta is located inside the ovarian cavity.

      Arising from the placenta are the megasporangia, commonly called ovules.

 

      The number of ovules in an ovary may be

 

      one

·         wheat

·          paddy

·         mango

                                                             or

       many

·         papaya

·         water melon

·          orchids

 

The Megasporangium  (Ovule) :

 

·         The ovule

      is a small structure

      attached to the placenta (parenchymatous cushion)

      by means of a

              stalk called funicle. (or funiculus)

 

·         The body of the ovule fuses with funicle in the region called hilum.

 

      Thus, hilum represents the junction between ovule and funicle.

 

·         Typical ovule is inverted or anatropus. Here funiculus is fused lengthwise with ovule beyond the hilum. A ridge is formed, which is called raphe. Funiculus is vascular to supply nourishment.

 

·          Each ovule has one or two protective envelopes called integuments.

 

·         Integuments encircle the ovule except at the tip where a small opening called the micropyle is organised.

 

·         Opposite the micropylar end, is the chalaza, representing the basal part of the ovule.

 

·          Enclosed within the integuments is a mass of cells called the nucellus.

 

·         Cells of the nucellus have abundant reserve food materials.

 

·         Located in the nucellus is the embryo sac or female gametophyte (1N).

 

·         An ovule

      generally has a single embryo sac

 

       formed from a megaspore

 

                      through reduction division.(meiosis)

 

 

Megasporogenesis :

 

 

·         The process of

      formation of megaspores

      from the megaspore mother cell

                                                                       is called megasporogenesis.

 

·         One of the cells of nucellus  differentiate

      as a single megaspore mother cell (MMC) (2N)

      in the micropylar region of the nucellus.

 

·         Megaspore mother cell (MMC)

 

      is a large cell containing

      dense cytoplasm and

      a prominent nucleus.

 

·         The MMC undergoes meiotic division.

 

·          Meiosis results in the production of four (haploid) megaspores .

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