Question

How many types of pollen or microspore tetrads are formed? Explain with the help of well-labelled diagrams.

Answer 1

In Angiosperms, five different types of pollen tetrads can be formed. These are:
1. Tetrahedral
2. Isobilateral
3. Decussate
4. T-shaped
5. Linear
1. Tetrahedral: Only three microspores are observed when we view from one side and the fourth microspore is at the backside.
Example: Most dicotyledonous plants.
2. Isobilateral: In this, all four microspores are arranged in one plane of the tetrad.
Example: Monocot plants.
3. Decussate: In this two + two microspores are arranged perpendicular in such a way that upper two microspores are visible and only one from lower tier is visible.
Example: Magnolia.
4. T-shaped: In this microspores are arranged in a tetrad in such a way that two microspores are arranged in transverse and two in a longitudinal plane.
Example: Aristolochia.
5. Linear: All microspores are arranged in a single linear fashion.
The wall between the microspores is made up of callose. This wall is degraded by enzyme callase as a result of which the microspores separate from each other and become spherical. At this stage, these are seen freely distributed in the pollen chamber and are called pollen grains.
In some plants, pollen grains or microspores of a sporangium cohere (stick) in a single mass called pollinium.
Example: Calotropis and in some orchids.
Most commonly the microspores soon separate from one another but sometimes they adhere in tetrads to from compound pollen grains.
Example: Drimys, Anona, Drosera, Elodea, Typha.