Secondary side

The side of the valve formed by fusion of silica branches extending from the center and the poles of the raphe sternum, during raphid diatom ontogeny. The secondary side forms after the primary side.

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.

Valve Morphogenesis 1
Image Credit: Adapted from Round et al. (1990).
This series of drawings represents the pattern of silica deposition as a raphid valve forms. At top, a narrow rib of silica, the primary side of the sternum, forms first. Next, at the poles and central nodule the primary side extends to form the secondary side of the sternum, creating the raphe slit. Finally, both sides complete the valve formation. This secondary side includes a Voigt discontinuity.
Neidium Sp2  L 9 59 1
Image Credit: Steve Main
Light micrograph of a species of Neidium from Iowa. The two arrows indicate the Voigt discontinuities, formed by the incomplete joining of the secondary side during valve formation.