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An alveolus is an elongated chamber extending from the axial or central part of the valve toward the margin and forming all or part of a stria. Openings vary widely among genera. SEM is needed to distinguish most features. From Latin for small cavity. Plural is alveoli.
Examples:
- Alveolus forms the whole stria (Pinnularia, Caloneis)
- Alveolus forms part of the stria (some Mastogloia, Cyclostephanos dubius in this flora)
- Internal openings are foramina (Pinnularia, Ardissonea, Cyclostephanos dubius in this flora)
- Internal openings are smaller pores (some Mastogloia)
- External openings are single pores (some Mastogloia, Ardissonea)
- External openings are plates of fine pores (Pinnularia)
This adapted version of Ross et al. 1979 p. 527 is used because so much has yet to be learned about this structure. Authors use variations of this definition so it might be best to consult their papers when using this term.
Alveoli may be difficult to distinguish from loculi. A loculus and an alveolus are both chambered openings in the valve wall but typically only the alveolus is transversely, or transapically, elongated. Images can be a guide.
As with other glossary terms, alveolus describes structures that are physically similar, but may not be homologous (derived from a common ancestor).
See also areola, loculus, and stria.