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Diatoms of the United States is now known as Diatoms of North America.
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An opening, or perforation, through the central area of the valve that is distinct in structure from the areolae composing the striae. The external opening is typically round, or nearly round, while the internal opening has the shape of a slit, or is complex. Plural form is stigmata.

A stigma is present in many genera, including Cymbella, Didymosphenia, Geissleria, Gomphoneis, Gomphonema, and Luticola, and may not be homologous across these taxa.

Alternative Term

isolated punctum

While "stigma" indicates a structure with different morphology from an areola, and "isolated punctum" indicates an opening with the same morphology as an areola, the two terms may be used interchangeably by some authors.


Round, F.E., Crawford, R.M. and Mann, D.G. (1990). The Diatoms. Biology and Morphology of the Genera. Cambridge University Press, Cambridge, 747 pp.