3. INTRODUCTION
• The microspores or the pollen grains represent the
first cell generation of the male gametophyte.
• The further development of the male gametophyte
or microsporogenesis gives rise to a large
vegetative cell and a small generative cell.
• The latter undergoes another mitotic division to
form two gametes (sperms) either in the polllen
grain or pollen tube
• Along with these changes the wall synthesis is
also followed.
4. MICROSPORE
• The newly formed pollen grains have very dense
cytoplasm with distinct nucleus.
• Soon after its release from tetrad, it undergoes
rapid increased in volume.
• The nucleus of the pollen may remain in resting
period varying from a few days to several weeks
depending upon the species.
• The pollen grains have reserve food materials in
the form of carbohydrates, proteins or lipids.
5. Formation of vegetative &
generative cell
• The first division of the microspore gives
rise to two unequal cells:
1. The larger one is vegetative cell, which
develops into pollen tube.
2. The smaller one is the generative cell,
which with the subsequent mitotic
division ftwo morms ale gamets or
sperms.
6. • Geitler (1935) observed that the
metaphase spindle usually shows
pronounced asymmetry, the wall ward
pole being blunt and the free pole acute.
• The cause of asymmetry has been
attributed to differences In the time of
the development of the spindle poles:-
• generative pole develops more slowly
than the vegetative because the smaller
amount of the cytoplasm is associated
with the former.
7.
8. • However, the microspores of the same
tetrad show same stage of development
but a complete synchronization is
observed in massulae and pollinia, where
microspores are united together as in
Mimosaceae, Asclepiadaceae and
Orchidaceae.
9.
10. • The generative cell is either cut off inner side or
on the outer side or on the radial wall or in a
corner.
• The generative cell soon loses contact with the
wall of the microspore and come to lie freely in
the vegetative cell.
• Immediately after detachment, the generative
cell is spherical.
• However, variation in shapes, like elliptical,
lenticular, spindle shaped or sometimes
vermiform have also been observed in different
species.
• The generative cell contains all usual cell
organelles except plastids with a few exceptions.
• The nucleus is smaller but contains higher
amount of DNA.
11. • The generative cell either divides when
the pollens are still within the anther or it
may takes place after the pollen discharge.
• The two male gametes or sperms thus
formed are discrete cells and their
cytoplasmic sheath persists throughout
their course in the pollen tube.
• The tube cell, which enclose the
generative cell, will produce the pollen
tube, a structure essential for sperm
delivery to the egg.
12. Polllen wall
• Pollen grains rather uniform
in their wall architecture:
1. tectate exine
2. Pilate exine
• A mature pollen wall shows
two distinct layers- outer
exine & inner intine.
• The exine is composed of
sporopollelin, a highly
resistant material believed
to be produced by the
oxidative polymerization of
carotenoids pigments.
• The sculptured part of the
exine is made up of radically
oriented rod like baculae
which may remain open or
covered by a roof or tectum.
13. • The exine covers the entire pollen grain except germinal
apertures ( germ pores) where it is absent or highly
reduced.
• In a tectate grain the void between baculae open to the
outside through perforations termed microspores.
• The exine is composed of an outer ektexine (sexine&
nexine) and an inner endexine.
• The endexine is often well developed in dicots and is
absent in monocots.
• The intine is composed of pecto- cellulose materials .
• In some taxa, particularly Poaceae a middle layer is rich
in pectic polysaccharides, termed as Z- layer, is
distinguishable.
• This layer is thickened at germ pore and is termed
"Zwischenkorper".
