Biology Today - May 2018

Microsporogenesis

• The process of formation of pollen grains through meiosis in pollen mother cells is termed as microsporogenesis.


• Sporogenous tissue within the anther grow and transform into pollen
  mother cell (PMC) which in turn produce tetrads of haploid microspores
  or pollen grains.


• The pollen grains of a tetrad grow and separate from one another.


• Usually the arrangement of microspores in a tetrad is tetrahedral or
  isobilateral. However, decussate, linear and T-shaped tetrads are also found.

Structure of pollen grain

• Mature anther has two cavities, therefore, it is called dithecous.


• Pollen grain is commonly globular in outline though several other shapes
  are found.


• Covering of pollen grain is called sporoderm consisting of two layers, outer
  exine and inner intine.


• Intine is pecto-cellulosic in nature while exine is made of a highly
  resistant fatty substance called sporopollenin. Because of sporopollenin, pollen grains are well preserved as microfossils as
  sporopollenin is not affected by any enzyme, high temperature, strong acid or strong alkali.


• Pollen grain is uninucleate in the beginning but at the time of liberation from anther it is 2-3 celled.

Structure and development of male gametophyte

• Development of male gametophyte is precocious, i.e., it begins inside the microsporangium or pollen sac.


• Young pollen grain has a centrally placed nucleus embedded in dense cytoplasm covered by plasma membrane.


• It grows in size with the inflow of nutrients and eventually protoplast divides mitotically to form two unequal cells-small
  generative cell and large tube or vegetative cell.


• The generative cell is spindle shaped to spherical in outline with thin dense cytoplasm surrounding a prominent nucleus.


• The tube cell has a vacuolate cytoplasm rich in the food reserves and cell organelles. Its nucleus is large and irregular.


• In some species the generative cell divides into two nonmotile male gametes prior to the dehiscence of anther and release
  of the pollen grains.


• Therefore, at the time of pollination, the pollen grain is either 2-celled (tube cell + generative cell) or 3-celled (tube cell + two
  male gametes).


• On the stigma the compatible pollen grain absorbs water and nutrients from the stigmatic secretion through its germ pores.


• The tube or vegetative cell enlarges and comes out of the pollen grain through one of the germ pores to form a pollen tube
  covered over by intine.


• The tube nucleus along with generative cell descend to the tip of the pollen tube.


• The generative cell soon divides into two nonmotile male gametes if it is not already divided.


• Each male gamete has a large nucleus surrounded by a thin sheath of cytoplasm and is considered to be one cell.


• The tube nucleus may degenerate completely.


• A pollen grain with pollen tube carrying male gametes represent mature male gametophyte and is 3 celled (1 tube cell and 2
  male gametes) and 3 nucleated structure.

PISTIL - THE FEMALE REPRODUCTIVE ORGAN

• Gynoecium represents the female part of a flower.


• The free unit of gynoecium is called pistil.


• A pistil has three parts-stigma, style and ovary.


• Stigma is the terminal receptive part of the pistil which functions as landing platform for the pollen grains. The style is the
  elongated slender part beneath the stigma that connects stigma with the ovary. The basal bulged part of the pistil is the ovary.


• Inside the ovary lies the ovarian cavity (locule). The placenta is located inside the locule.


• From the placenta megasporangia arises, commonly called ovules.


• The number of ovules in an ovary may be one (wheat, paddy, mango) to many (papaya, watermelon, orchids).

2013

1

2

3

4

5

6

7

8

9

10

2014 2015 2016 2017

AIPMT/NEET

AIIMS

JIPMER

Analysis of various PMTs from 2013-2017