Stamen, Microsporangium and Microsporogenesis
· There are two parts of a typical stamen –
→ the long and slender stalk called the filament, and
→ the terminal generally bilobed structure called the anther.
· The proximal end of the filament is attached to the thalamus or the petal of the flower.
· Anther & filament are attached together with help of small region, called connective.
· A typical angiosperm anther
→ is bilobed
--- with each lobe having two theca,
i.e., they are dithecous.
· Often a longitudinal groove runs lengthwise separating the theca.
· The bilobed nature of an anther is very distinct in the transverse section of the anther.
· The anther
→ is a four-sided (tetragonal) structure
→ consisting of four microsporangia (tetrasporangiate)
Ø located at the corners,
Ø two in each lobe.
· The microsporangia
→ develop further and
→ become pollen sacs.
Ø They extend longitudinally all through the length of an anther and
Ø are packed with pollen grains.
Structure of microsporangium:
· In a transverse section, a typical microsporangium appears near circular in outline.
· It is generally surrounded by four wall layers–
3. Middle layers
4. The tapetum
· The outer three wall layers
→ perform the function of protection and
→ help in dehiscence of anther to release the pollen.
· The innermost wall layer is known as tapetum.
→ It nourishes the developing pollen grains.
→ Cells of the tapetum possess dense cytoplasm
→ It generally have more than one nucleus.
· Tapetal cells become bi-nucleated by endoploidy.
· When the anther is young,
→ a group of compactly arranged homogenous cells called the sporogenous tissue
occupies the centre of each microsporangium.
· As the anther develops,
→ the cells of the sporogenous tissue
--- undergo meiotic divisions
---- to form microspore tetrads.
· Cells of the tetrad are haploid.
· As each cell of the sporogenous tissue
→ is capable of giving rise to a microspore tetrad.
→ Each one is a potential pollen or microspore mother cell (PMC).
· The process of
→ formation of microspores from a pollen mother cell
F through meiosis
is called microsporogenesis.
· The microspores, as they are formed, are arranged in a cluster of four cells–the microspore tetrad .
· As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains .
· Inside each microsporangium
→ several thousands of microspores or pollen grains are formed
F that are released with the dehiscence of anther