Explain the process of microsporogenesis in angiosperms.
or
Describe the sequence the events that lead to the development of a 3-celled pollen grain from microspore mother cell in angiosperms.
(I) When the anther develops, each cells of
sporogenous tissue undergo melutic divisions to
form microspore tetrads.
(ii) Each one is a microspore mother cell (MMC).
(iii) The process of formnation of microspores from a
pollen mother cell (PMC) through meiosis is called
microsporogenesis.
(iv) The microspores get arranged in a cluster of four
cells and hence are microspore tetrad.
(y) As the anthers mature and dehydrate, the
microspores dissociate from each ether and develop
into pollen grains.
(vi) From each microsporangium, thousands of pollen
grains are formed and released with the dehiscence
of anther.
Pollen grain (male gametuphyte)
(i) The pollen grains represent the male gametophytes.
(ii) A pollen grain has a two-layered wall namely,
(a) Exine
(b) Intine
Exine:
It is the hard outer layer which is made up of
sporopollenin.
Intine:
(I) It is the inner, thin and continuous layer which is
made up uf cellulose and pectin.
(ii) A matured pollen grain contains two cells namely,
© Vegetative cell
(d) Generative cell
Vegetative cell:
It is bigger cell having abundant food reserve and a
large irregularly shaped nucleus.
Generative cell:
(j) It is small cell that floats in the cytoplasm of the
vegetative cell.
(ii) It is spindle shaped with dense cytoplasm and a
nucleus.
• Thi pollenn grains are shed at the 2-celled stage in
flowering plants.
• In other plants, the generative cell divides
mitotically to give rise to the two male gametes
before pollen grains are shed in 3-celled stage.
• Once they are shed, pollen grains have to land on
the stigma before they lose viability.