A general comparative study of the spores of living pteridophyta has been attempted by Knox (1938), to provide "students of fossil spores with some basis for comparison with recent types". She described spores of 39 genera of ferns and 7 of fern allies.
This was followed by a detailed and well-illustrated account of the spore morphology of the Lycopsida by Knox (1950) in which spores of over 480 sp. of Lycopodium, Phylloglossum drummondii, Isoetes and Selaginella are described. Erdtman (1957) published detailed illustrations of spores of over 100 genera of pteridophytes.
Over 300 sp. of Indian ferns belonging to Aspidiaceae, Aspleniaceae, Blechnaceae, Polypodiaceae Pteridaceae and Grammitidaceae have been palynologically investigated by Nayar and Santha Devi (1963).
Over 75 % of the pteridophytes are tropical plants, many of them with a restricted geographical range and often growing in inaccessible tropical jungles. The difficulties of procuring material, and of proper identification of the sample, may well discourage an amateur worker. The external morphology of the pteridophyte spores, unlike that of pollen grains, often undergoes marked changes immediately before the time of natural shedding.
In addition to acetolyzed preparations, it is advantageous to study fresh spores as well, as acetolysis sometimes alters spore-morphology (mainly the spore size) affecting differently the various species (Nayar and Santha Devi 1964).
There are some examples of acetolysis altering the general morphology of the spore coat. Thus, in Plagiogyria, the verrucae on the exine in many species are deciduous and are nearly always shed on acetolysis, leaving a smooth surface. Majority of the verrucae on the exine of some sp. of Pyrrosia and the Grammitid ferns are often shed on acetolysis. In Pyrrosia nummularifolia a thick ornamental layer of the exine is completely dissolved, and removed without a trace during acetolysis (Figs. 7.1 a, b).
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