The formation and differentiation of microspores is called microsporogenesis. In the cavity of microsporangium, the microspore mother cells (MMC) divide meiotically to produce microspore tetrads.
Cytokinesis may occur after each meiotic division (successive type), thus isobilateral tetrad of microspores is formed e.g., monocots or it occurs after complete meiotic (I and II) division (simultaneous type), thus tetrahedral tetrad of microspores is formed e.g., dicots. Successive type of cytokinesis is advanced type.
A-D. Various stages during successive type of division
during microsporogenesis, (A-B) dividing mother cell., (C). dividing dyad, (D). Tetrad
A-F. Various stages during simultaneous type
of division during microsporogenesis
Tetrads are of 5 types - tetrahedral, isobilateral, T-shaped, linear and decussate
Types of microspore tetrads
A. Tetrahedral, B. Isobilateral, C. T-shaped, D. Linear, E. Decussate
In Aristolochia elegans all five types of tetrads are present.
Some exceptions to remember :
(a) After formation the microspores are separated from tetrad. But in Elodea, Drosera, Typha, the microspores do not separate from each other, thus developing into compound pollen grains.
(b) In family Asclepiadaceae (Calotropis) and Orchidaceae, all the microspores in a sporangium adhere together in a single mass known as pollinium.
(c) In Calotropis, the pollina of adjacent anthers of different stamens are attached by thread like caudicies to a sticky disc called corpusculum. The whole structure is called transiator.
A pair of pollinia
(d) In family Cyperaceae, out of 4 microspores formed, 3 degenerate, so ultimately one MMC (2N) produces only one microspore or pollen grain.
(e) Sometimes more than four microspores are produced from one microspore mother cell. It is called polyspory e.g., Cuscuta reflexa.