Stigma

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 Terms

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.
Gomphoneis Geitleri Ex
Image Credit: Marina Potapova
SEM image of an external view of the stigma of Gomphoneis geitleri. The external opening of the stigma is round and is positioned in the central area.
Gomphoneis Int
Image Credit: Sarah Spaulding
SEM image of an internal view of the stigma of Gomphoneis herculeana. The internal opening of the stigma has the shape of a slit.
Cymbella Affinis Int
Image Credit: Marina Potapova
SEM image of an internal view of two stigmata of Cymbella affinis. The internal openings are complex.