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Sunday, 26 September 2021

Stamen, Microsporangium and Microsporogenesis l Sexual reproduction in flowering plants l NEET l Class XII l NOTES

 

Stamen, Microsporangium and Microsporogenesis 

Stamen:

 

·         There are two parts of a typical stamen –

 

      the long and slender stalk called the filament, and

 

      the terminal generally bilobed structure called the anther.

 

·         The proximal end of the filament is attached to the thalamus or the petal of the flower.

 

·         Anther & filament are attached together with help of small region,  called connective.

 

·         A typical angiosperm anther

 

      is bilobed

--- with each lobe having two theca,

                           i.e., they are dithecous.

·         Often a longitudinal groove runs lengthwise separating the theca.

 

·         The bilobed nature of an anther is very distinct in the transverse section of the anther.

·         The anther

 

      is a four-sided (tetragonal) structure

      consisting of four microsporangia (tetrasporangiate)

 

Ø  located at the corners,

Ø  two in each lobe.

 

·          The microsporangia

      develop further and

 

      become pollen sacs.

Ø  They extend longitudinally all through the length of an anther and

Ø   are packed with pollen grains.

 

Structure of microsporangium:

 

·         In a transverse section, a typical microsporangium appears near circular in outline.

 

·         It is generally surrounded by four wall layers

 

1.       Epidermis,

2.       Endothecium,

3.       Middle layers

4.       The tapetum

 

·         The outer three wall layers

 

      perform the function of protection and

      help in dehiscence of anther to release the pollen.

 

·         The innermost wall layer is known as tapetum.

 

      It  nourishes the developing pollen grains.

 

       Cells of the tapetum possess dense cytoplasm

 

      It generally have more than one nucleus.

 

·         Tapetal cells  become bi-nucleated by endoploidy.

 

 

·         When the anther is young,

 

      a group of compactly arranged homogenous cells called the sporogenous tissue

 

                                                       occupies the centre of each microsporangium.

 

Microsporogenesis :

 

·         As the anther develops,

 

      the cells of the sporogenous tissue

---  undergo meiotic divisions


  

                      ---- to form microspore tetrads.

 

·         Cells of the tetrad are haploid.

 

·          As each cell of the sporogenous tissue

 

      is capable of giving rise to a microspore tetrad.

      Each one is a potential pollen or microspore mother cell (PMC).

 

·          The process of

 

      formation of microspores from a pollen mother cell

 

F  through meiosis

               is called microsporogenesis.

 

·         The microspores, as they are formed, are arranged in a cluster of four cells–the microspore tetrad .

 

·         As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains .

 

·         Inside each microsporangium

 

      several thousands of microspores or pollen grains are formed

 

F  that are released with the dehiscence of anther

 

 

 

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