50 MTBIOLOGY TODAY | JULY ‘15• The presence of fibrous bands, differential expansion
of inner and outer tangential walls and hygroscopic
nature of endothecial cells play an important role in the
dehiscence of anthers.- Middle layers 1-4 in number.
- Tapetum, the innermost parietal layer with cells having
dense cytoplasm and large nuclei.
• Sporogenous tissue fills the whole interior of a
microsporangium.
Functions of tapetum
• Tapetum provides nourishment to developing microspores.
It also secretes callase enzyme which dissolves callose
substances by which 4 pollens of a pollen tetrad are united,
hence separating microspores or pollens of a tetrad.
• Tapetum secretes sporopollenin coated übisch bodies
whose function is not clearly determined yet. Some consider
them to assist in transport and deposition of sporopollenin
at pollen wall while some consider them just to be a
byproduct of sporopollenin.
• Tapetum also secretes pollen kitt, the outermost oily, thick,
sticky, coating of pollen grains, mainly composed of lipids
and carotenoids. It is secreted in case of entomophilous
pollen grains.
microsporogenesis
• The process of formation of microspores from sporogenous
tissue is known as microsporogenesis.
• The sporogenous cells may directly function as microspore
mother cells (also called pollen mother cells or PMCs)
or they may undergo a few mitoses to add up to their
number before entering meiosis.
• Each PMC, by a meiotic division, gives rise to a group of
four haploid microspores. Aggregates of four microspores
are referred to as microspore tetrads.
• In some cases, one microspore is formed from one pollen
mother cell, e.g., Family Cyperaceae.
• Usually the arrangement of microspores in a tetrad is
tetrahedral or isobilateral, however decussate, (e.g.,
Magnolia, Atriplex) linear (e.g., Halophila) and T-shaped
(e.g., Aristolochia) tetrads are also found.
• In members of Asclepiadaceae and Orchidaceae, all the
pollens of an anther lobe are packed in a bag like structure
called pollinium. In certain members of Family Orchidaceae,
the pollinium is less compact and comprises smaller loose
groups of pollen grains termed as massulae.
• When 4 pollens of a tetrad do not separate and remain
attached, they are known as compound pollen grains
(e.g., Drimys, Drosera).
• Polyspory is the occurrence of more than 4 spores in a
pollen tetrad, e.g., Cuscuta reflexa.Dehiscence of anther
• Anther dehisces due to certain changes which take place
in its wall layers at maturity. These changes are affected
by environmental conditions.
• The endothecial layer of the anther wall has fibrous bands
on its inner tangential walls, except along the line of
dehiscence. This region, which is marked by the presence
of thin walled cells, is known as stomium. The hygroscopic
nature and differential expansion of the tangential walls
of the endothecium cells are mainly responsible for anther
deshiscence.
• Mostly, the stomium extends as a narrow strip throughout
the length of the anther and hence the dehiscence is usually
longitudinal. But in certain taxa the anther dehisces through
a terminal valve (e.g., Berberidaceae), apical pore (e.g.,
Solanum, Polygala) or irregularly (e.g., Najas).
• Sometimes the endothecial cells do not possess fibrous
bands (e.g., Cassia). In such cases the anther dehisces due
to unequal thickenings of its radial walls. In cleistogamous
flowers, the endothecium is highly reduced and the pollen
tube comes out through anther wall.