Morphology
The diagram of frustule, shows valves and bands. Diatom cell walls (frustules) are essentially bipartite structures, with an older and a
younger half (valve), each with a series of linking bands (girdle bands or cingula), often likened to a petri dish or pill-box. Once formed,
the siliceous components are rigid and cell expansion is only possible by the addition of extra girdle bands associated with the younger
valve. Valves vary in shape and symmetry, as well as in the type and arrangement of pores, slits and other processes of the silica wall, and
these features have formed the basis of their identification and classification. Diatom cells are often approximated to geometric shapes and
described in relation to their major axes and planes, radial, apical, transapical and valvar. Because they are three-dimensional structures,
shape will differ depending on the aspect presented to the viewer, either valve or girdle view. Occasionally cells are seen end-on, but this is
unlikely in fixed or cleaned material, since cells tend to lie with their largest face on the slide.