`star` Microsporogenesis
`star` Pollen grains
A) Structure
B) Significance
C) Viability


● As the `color{Violet}"anther develops"`, the cells of the `color{Violet}"sporogenous tissue"` undergo `color{Violet}"meiotic"` divisions to form `color{Violet}"microspore tetrads"`.

● As each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad, each one is a `color{Violet}"potential pollen"` or `color{Violet}"microspore mother cell (PMC)"`.

● The process of formation of `color{Violet}"microspores from a pollen mother cell"` through meiosis is called `color{Violet}"microsporogenesis"`.

● The microspores, as they are formed, are arranged in a `color{Violet}"cluster"` of four cells–the `color{Violet}"microspore tetrad"`.

● As the anthers `color{Violet}"mature and dehydrate"`, the microspores `color{Violet}"dissociate"` from each other and develop into `color{Violet}"pollen grains"`.

● Inside each microsporangium `color{Violet}"several thousands"` of microspores or `color{Violet}"pollen grains"` are formed that are released with the `color{Violet}"dehiscence of anther"`.


● They are seen as a deposition of `color{Violet}"yellowish powdery substance"` on the opened anthers of Hibiscus and other flowers.

● The pollen grains from `color{Violet}"different species"` show a `color{Violet}"variety of architecture"` – sizes, shapes, colours, designs.

● Pollen grains are generally `color{Violet}"spherical"` measuring about `color{Violet}"25-50 micrometers"` in diameter.

● It has a prominent `color{Violet}"two-layered wall"`.

`star` `color{Brown}"Exine"`:

● The `color{Violet}"hard"` outer layer called the `color{Violet}"exine"` is made up of `color{Violet}"Sporopollenin"` which is one of the most `color{Violet}"resistant organic material"` known.

● It can withstand `color{Violet}"high temperatures"` and strong `color{Violet}"acids and alkali"`.

● `color{Violet}"No enzyme"` that degrades sporopollenin is so far known.

● Pollen grain exine has `color{Violet}"prominent apertures"` called `color{Violet}"germ pores"` where sporopollenin is absent.

● Pollen grains are well preserved as `color{Violet}"fossils"` because of the presence of `color{Violet}"sporopollenin"`.

● The exine exhibits a fascinating array of `color{Violet}"patterns and designs"`.

`star` `color{Brown}"Intine"`:

● The `color{Violet}"inner wall"` of the pollen grain is called the `color{Violet}"intine"`.

● The cytoplasm of pollen grain is surrounded by a `color{Violet}"plasma membrane"`.

`star` `color{Brown}"Cells within Pollen Grains"`:

● When the pollen grain is `color{Violet}"mature"` it contains two cells, the `color{Violet}"vegetative cell"` and `color{Violet}"generative cell"`.

● The vegetative cell is `color{Violet}"bigger"`, has `color{Violet}"abundant food reserve"` and a large `color{Violet}"irregularly shaped"` nucleus.

● The generative cell is `color{Violet}"small and floats"` in the cytoplasm of the vegetative cell.

● It is `color{Violet}"spindle shaped"` with `color{Violet}"dense cytoplasm"` and a nucleus.

● In over `color{Violet}"60 per cent"` of angiosperms, pollen grains are shed at this `color{Violet}"2-celled stage"`.

● In the remaining species, the generative cell `color{Violet}"divides mitotically"` to give rise to the two male gametes before pollen grains are shed (`color{Violet}"3-celled stage"`).


● Pollen grains of many species cause severe `color{Violet}"allergies and bronchial afflictions"` in some people often leading to `color{Violet}"chronic respiratory disorders"`– asthma, bronchitis, etc.

● E.g. `color{Violet}"Parthenium"` or `color{Violet}"carrot grass"` that came into India as a contaminant with imported wheat,
has become ubiquitous in occurrence and causes `color{Violet}"pollen allergy"`.

● Pollen grains are rich in `color{Violet}"nutrients"`.

● In recent years, pollen tablets are being used as `color{Violet}"food supplements"`.

● In western countries, a large number of `color{Violet}"pollen products"` in the form of `color{Violet}"tablets and syrups"` are available in the market.

● Pollen consumption has been claimed to increase the performance of `color{Violet}"athletes and race horses"`.


● When once they are `color{Violet}"shed"`, pollen grains have to `color{Violet}"land on the stigma"` before they `color{Violet}"lose viability"` if they have to bring about fertilization

● The period for which pollen grains remain viable is `color{Violet}"highly variable"`.

● To some extent, it depends on the prevailing `color{Violet}"temperature and humidity"`.

● `color{Brown}"E.g."`: In some cereals such as `color{Violet}"rice and wheat"`, pollen grains lose viability within 30 minutes of their release.
● `color{Brown}"E.g."`: In some members of `color{Violet}"Rosaceae"`, `color{Violet}"Leguminoseae"` and `color{Violet}"Solanaceae"`, they maintain viability for months.

● It is possible to `color{Violet}"store pollen grains"` of a large number of species for years in `color{Violet}"liquid nitrogen"` (-196 degree celsius).

● Such stored pollen can be used as `color{Violet}"pollen banks"`, similar to seed banks, in `color{Violet}"crop breeding programmes"`.