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.
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.