What are diatoms? One celled plants belonging into the plant class Bacilariophyceae of the division or phylum Bacilariophyta. Diatoms are either solitary and free, attached to a substratum by gelatinous extrusions or joined to each other in chains of varying length. Some species are capable of active movement but others are merely free floating and depend on currents for transport. Individual diatoms range in size from 2 microns to several millimeters, although there only very species that are larger than 200 microns. The actual number of extinct and extant diatom species may well be over 50.000.

The only entity that is truly specific to diatoms is the cell wall, a laminated component of a siliceous core enveloped by layers of organic matter. The silica shell is secreted and deposited into a vesicle before it is moved to the cell surface.

Model of arrangement of layers in the cell wall of diatoms. Layer of polysaccharids with Gl (glucose), M (mannose), Fu (fucose), and X (xylose). Layer of template protein for polycondensation of Si(OH)₄ with Ser (serine), Thr (threonine), Asp (aspartic acid) and Gly (glycine). From Hecky et al. (1973).
From Dr. Werner, 1977: Silicate metabolism. in The biology of diatoms (edt. D. Werner). Botanical Monographs, Vol. 13, University of California Press, 110-149.

Schematic drawing of valve development: (a-c) devopment of the base layer, (c-f) development of the outer layer, (g-i) development of the organic coat, , organic layer and outer layer.
From A.-M. Schmid 1986: Organization and function of cell-structures in diatoms and their morphogenesis. Proceedings of the Eigth International Diatom Symposium, Paris (1984), Koeltz Scientific Books, Koenigstein 1986, 271-314.