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Abstract

Although Pinnularia is one of the older and apparently readily recognised diatom genera, on closer investigation, not only are there problems of specific delimitation, but the boundaries of the genus itself are blurred, particularly in relation to Caloneis. Separation from the latter rests largely on the position, number and width of the inner alveoli openings, which are not always easily seen with light microscopy, while interspecific separation often rests on size, outline, hyaline area shapes, raphe form, striae orientation and density, as well as features of the alveoli. Earlier studies revealed variation in chloroplast morphology within Pinnularia and prompted the suggestion that this was potentially useful for the taxonomy of the genus. Here that work is extended with the description of other chloroplast types in relation to valve morphology. Attempts have been made to discover the range of variation of both chloroplast and valve features, within and between Pinnularia species, and in relation to other genera. The taxonomic implications will be discussed.