Stigma

A stigma is a pore 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 it may be a more complex structure. The plural 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

stigmoid: Stigmoid is an alternative term which is gaining increased use by some authors.
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.

Citations

Cox, E.J. (2012) Ontogeny, homology, and terminology - wall morphogenesis as an aid to character recognition and character state definition for pennate diatom systematics Journal of Phycology 48: 1–31

Publication Link: 10.1111/j.1529-8817.2011.01081.x
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.

Gomphoneis Geitleri Ex — 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 — 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 — SEM image of an internal view of two stigmata of *Cymbella affinis*. The internal openings are complex.