Tetrad markings of pteridophytic spores and their evolutionary significance

Abstract

A comparative study of the morphology of the spores of ferns and fern allies was carried out during which, among others, representatives of 277 genera of ferns were investigated. In 29% of all cases the spores are exclusively trilete, in 49%, exclusively monolete, and 22% are mixed or exhibit transitional apertures. Trilete and monolete spores together with all types of transitional apertures were observed sometimes within the same species. Zonate spores were found in 4% of the investigated cases. Trilete spores are found since Silurian times zonate and monolete spores since the Middle Devonian. The progression from trilete to zonate and/or to monolete can be traced through the evolution of all orders of the pteridophytes. A better food supply for the immature spore developing from the spore mother cell may have been the survival advantage. Progression from zonate spores may have led to the monosaccate spores of the coniferous line. Progression from monolete spores to presulcate and/or to sulcate pollen grains may have led to the pteridospermous line of reduction of the microgametic generations.