Sternum

A longitudinal element of the valve of pennate diatoms. Usually thickened and hyaline. The sternum is usually positioned along the apical axis as in Navicula, but is sometimes positioned along the valve margin as in Eunotia. In many raphid diatoms, the sternum contains the raphe. Plural form is sterna.

The sternum is the ontogenic center of pennate valves, that is, it is the first silica deposited in valve formation following cell division.

Sternum
Image Credit: Marina Potapova
The arrow in this SEM micrograph indicates the sternum in Cavinula sp. It is the thickened, longitudinal silica along the apical axis. In this taxon, the sternum contains the raphe and is synonymous with the axial area.