In this article we will discuss about the symmetry of pollen and spore wall of plants.
The term symmetry with relation to a pollen grain implies similarity of halves on either side of a median line or plane so far as aperture and ornamentation etc., are concerned.
Most pollen and spores are symmetrical. As a rule, symmetric pollen and spores are either radio-symmetric (Fig. 4.34) or bilateral (Fig. 4.35). Radially symmetrical isopolar pollen and spores exhibit two or more vertical planes and one horizontal plane of symmetry (ex. Centaurea).
All the planes are of equal length. In radially symmetrical heteropolar spores there is no horizontal plane of symmetry; there exist two or more vertical planes of symmetry (ex. Osmunda regalis, Ophioglossum vulgatum and Pteridium aquilinum etc.).
Bilateral isopolar spores exhibit two vertical planes and one horizontal plane of symmetry. All the planes are not of equal length (ex. Rungia grandis). In bilateral heteropolar spores, there exist two vertical planes of symmetry. The planes are unequal in length and intersect each other at right angles (ex. Ephemerum serratum, Picea abies and Cycas revoluta etc.).
There exists no plane of symmetry in asymmetrical pollen and spores (ex. Berberis darwinii, Papaver argemone and Myriophyllum alterniflorum etc.). Asymmetric pollen and spores may be with fixed shape, termed fixiform or may be without any definite shape called non-fixiform. Asymmetric pollen and spores are rare.
