In this article we will discuss about the size of pollen and spore walls.
There exist much variations in sizes of pollen and spores. The size may be 5 µ × 2.4 µ as observed in pollen of forget-me-not (Myosotis) or as high as 200 µ or more as is noted in certain genera of Cucurbitaceae and Nyctaginaceae etc. The diameter of megaspores of Selaginella exaltata is about 1.5 mm, i.e. 1500 µ.
In radially symmetrical pollen grains the size is expressed with regards to the length of polar axis (PA) and equatorial diameter (ED). The axes are measured from the equatorial view of pollen. In bilateral pollen and spores also the size is expressed with regards to length of polar axis and equatorial diameter.
But it is a rule to measure the equatorial breadth (= equatorial diameter) of bilateral pollen and spores from the polar view also. In polar view the diameter of amb is measured. Amb is defined as the outline of a spore or pollen when viewed from directly above with one of the poles. In isopolar, not equatorially constricted spore and pollen, the diameter of amb is same as that of equator.
Erdtman in 1945 and 1969, suggested the following pollen and spore-size classes. The measurements were based on the length of the longest pollen and spore axis:
1. Very small grains or spores (pollina perminuta; sporae perminutae) … < 10 µ.
Lycoperdon, Myosotis palustris.
2. Small grains and spores (pollina minuta; sporae minutae) … 10 µ-25 µ.
3. Medium size grains or spores (pollina media; sporae mediae) … 25 µ-50 µ.
4. Large grains or spores (pollina magna; sporae permagnae) … 50 µ-100 µ.
5. Very large grains or spores (pollina permagna; sporae magnae) … 100 µ-200 µ.
6. Gigantic grains or spores (pollina gigantea; sporae giganteae) … > 200 µ.
The size of a pollen grain is taxonomically significant. It is frequently used in the formulation of pollen keys that are used to differentiate the sporomorphs.
As for example Melandrium noctiflorum (diameter greater than 37 µ) is differentiated from M. rubnim (diameter 37 µ or less), Dianthus deltoides with ± 40 µ in diameter is differentiated from Stellaria media that has diameter less than 40 µ, Heliotropium indicum (PA × ED = 45.0 µ × 30.0 µ) is differentiated from Euphorbia hirta (PA × ED = ± 28.0 µ x 17.0 µ) and Cestrum noctumum (PA × ED = ± 39.0 µ. x 32.0 µ) is differentiated from Dalbergia sissoo (PA × ED = ± 28.0 µ × 24.0 µ) etc. Banik et al, [PA = polar axis; ED = equatorial diameter].
It is to note that in the general dimensions of pollen and spores the sculptural elements like spines etc. should not be included. The sculptural elements are to be dealt with independently. Mycologists also adopt this view.