In raphid diatoms, the thickly silicified area located between the proximal raphe slits. The central nodule is the site where the first silica is deposited in by the silica deposition vesicle in valve formation. The central nodule is often thickened, both in valve and girdle views.
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: Sarah Spaulding
The central nodule in Diploneis. It is unornamented (lacks areolae) and is often thicker is silica than other parts of the valve.
Image Credit: Marina Potapova
Scanning electron micrograph of the internal view of a valve of Hippodonta. The central nodule is the area between the proximal raphe ends.
Image Credit: Gene Stoermer
The central nodule is shown here in Caloneis amphisbaena. Note that in the light microscope the thicker silica of the central nodule is distinct, between the proximal raphe ends.