In raphid diatom ontogeny, the side of the valve formed by fusion of silica branches extending from the center and the poles of the
sternum. The secondary side forms after the
The secondary side can be identified by Voigt discontinuities and by the characteristic where both distal or both proximal raphe ends turn toward the same side of the valve, or are unilaterally deflected.
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.
Image Credit: Adapted from Round et al. (1990).
This series of drawings, from top to bottom, represents the pattern of silica deposition as a valve is formed. First, a narrow rib of silica, the sternum, is deposited. Next, at the poles and central nodule, the sternum is joined after forming the raphe slit. The primary side of the valve is the side based on the first part of the sternum, sometimes termed the primary rib. The secondary side of the valve is the side based on the joined sternum and may include Voigt dicontinuities.
Image Credit: Steve Main
A light micrograph of a species of Neidium from Iowa. The two arrows indicate the Voigt discontinuities, formed by the incomplete joining of the secondary rib during valve formation. The Voigt discontinuities are indicative of the secondary side of the valve.