The Voigt discontinuities may be found on the secondary side of raphid diatoms. The Voigt discontinuities are interruptions in the striae that indicate the point where the silica of the valve was initially deposited. The Voigt discontiunity marks the point of fusion of the sternum.
Mann, D.G. (1981). A note on valve formation and homology in the diatom genus Cymbella. Annals of Botany 47: 267-269.
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: Teo Nakov
Light micrograph of a valve of Neidium, with two arrows indicating the Voigt discontinuities. These structures are common in Neidium, less often visible in other genera of raphid diatoms.
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: Marina Potapova
The secondary side, and Voigt discontinuities are evident in this light micrograph of Navicula germainii.