Page 1 - LN-CH-2[2]
P. 1
Pre fertilization: structure and events:
Hormonal and structural changes in plants leads to development of flower,
Androecium consists of a whorl of stamens represents male sex organ.
Gynoecium represents the female reproductive organ.
Stamen, Microsporangium and Pollen grain:
Typical stamen consists of two parts, long and slender stalk called filament and terminal bilobed structure
calledanther.
Atypical angiosperm anther is bilobed.
Each lobe have two theca i.e. dithecous.
Each anther contains four microsporangia located at the corners, two in each lobe.
Microsporangia become pollen sacs and are packed with pollen grains.
Structure of microsporangium:
Each microsporangium surrounded by four wall layers
o Epidermis
o Endothecium
o Middle layer.
o Tapetum.
The innermost layer is tapetum which is multinucleated, with dense cytoplasm; it nourishes the developing
pollen grain.
The centers of each microsporangium contain homogenous cells called sporogenous tissues.
Microsporogenesis:
The process of formation of microspores from pollen mother cell through meiosis is
called microsporogenesis.
The sporogenous tissue of microsporangium differentiated into microspore mother cell or pollen mother
cell.
Each microspore mother cell undergoes meiosis and gives rise to haploid microspore tetrad.
On dehydration microspore tetrad dissociated to form four microspores.
Each microspore developed into a pollen grain.
Pollen grain:
Pollen grain represents the male gametophytes.
It is spherical and measuring about 25-50 micrometer in diameter.
It is covered by two layers.
The hard outer layer called the exine is made up of sporopollenin, which is one of the most resistant
organic materials known. It can withstand high temperature and strong acids and alkali. No enzyme can
degrades sporopollenin is so far known.
The exine has prominent apertures called germ pore where sporopollenin is absent.
The inner wall of pollen grain is called intine. It is thin and continuous layer made of cellulose and pectin.
