Contributions of Electron Microscopy to Angiosperm Phylogeny and Systematics: Significance of Ultrastructural Characters in Delimiting Higher Taxa

Abstract

Electron microscopy up to the present time has made significant contributions toward a better understanding of the phylogeny and systematics of the flowering plants at the higher taxonomic levels in two areas, namely in the study of angiosperm sieve-element plastids and the study of pollen morphology. Sieve-element plastids are of three main kinds: S-plastids or starch-containing plastids, which contain starch grains only; P-plastids or protein-containing plastids, which contain protein inclusions (and may have starch grains as well); and N-plastids or null plastids, which lack both starch grains and protein inclusions. P-plastids in turn may be classified, according to the morphology of their protein inclusions,in a number of different types and subtypes whose taxonomic occurrence has great systematic significance. After classifying and providing a synoptical key to sieve-element plastids as well as summarizing their taxonomic distribution, I discuss my interpretation of their evolution. I conclude among other things that S-plastids are the basic, primitive kind of angiosperm sieve-element plastid from which the various types and subtypes of P-plastids have been derived. With regard to pollen morphology, I note that the main contribution of electron microscopy to angiosperm systematics and phylogeny has been in the study of the nature of the pollen wall itself, and more particularly in the detailed examination of pollen wall sculpturing, structure, and stratification, and that a major use of electron microscopy in the future will undoubtedly be to study fossil angiosperm pollen grains.