An SEM study of the oospore of some Nitella species (Charales, Chlorophyta) with descriptions of wall ornamentation and an assessment of its taxonomic importance

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Abstract
The structure and ornamentation on the outer organic wall (ectosporotine) of the oospores of 22 species of Nitella are described based on scanning electron microscope (SEM) observations of type or other critical collections. Our findings are compared with those of light microscope investigations and reveal that the nature of the oospore wall has been often misinterpreted. Some features are described for the first time and provide characters which assist in defining the studied taxa. These new characters and other details of the outer wall allow the following basic oospore types to be recognized: smooth or slightly roughened, with surface projections of various kinds; smooth and devoid of any projections; smooth or slightly roughened, regularly dimpled, sometimes the wall between the depressions raised to form a network of angled lines joined to give a regular reticulate pattern; smooth or slightly roughened, wrinkled or ridged, the ridges simple or branched and often undulate, or else raised to form an irregular reticulum; network of anastomosing or branched fibrils, or else with small irregularly-arranged dimples and numerous pits giving the surface a sponge-like appearance. These different oospore wall types are not necessarily mutually exclusive and the distinction between them is not always clear. Taxonomic problems concerning many of the species investigated are discussed in the light of these new findings.

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