The secondary side of a valve is the side that is formed following on the side of joining the raphe sternum, in the ontogeny of raphid diatoms. If terminal raphe fissures are turned towards the same side of the valve (unilaterally deflected), they are usually turned to the secondary side. Voight discontinuities are always found on the secondary side of a valve, although they are not always present.
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).
The 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.