The evolution of angiospermid pollen characteristics: conjectures and queries

Abstract

The origin and evolution of different exine layers of palynofossils is analysed in the light of accumulating ultrastructural data. Semi-diagramatic illustrations based on the published TEM results of various exine types representing a cross section of extinct and extant plant groups are given for easy reference and comprehension. Some of the important palynological questions and issues discussed in the present work are imprecise use to describe the infra-tectum of pollen, of the flexible term "granular" that often leads to erroneous derivations and conclusions; ontogenetic differences between the apparently similar complexly alveolate columellate sexine types of gymnosperms and angiosperms respectively; independent evolution of columellar complexity in unrelated taxa: role of ubiquitous white lines in the exines of extinct and extant spores and pollen and adaptive rather than phylogenetic significance of sacci in progymnosperms, gymnosperms and angiosperms. Despite recognition of angiospermid pollen characteristics especially in tectally reticulate and columellate pollen of Triassic (Cornet, 1979, 1985, 1989; Pocock & Vasanthy, 1988; Pocock, Vasanthy & Venkatachala. 1988) the pre-Cretaceous origin of angiosperm still remains an open question.