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  Diatoms frustules (both centric and pennate) consist of two halves called valves, this structure giving the group its name. The epitheca (upper valve) fits over the hypotheca (lower valve) similar to a lid of a box, but the overlap is minimal. Each valve consists of the valve surface and the cingulum, or valve side. The two cingula form the girdle.

Valve view of a pennate diatom illustrating valve margins (labeled 'valve') and girdle bands.  Photograph from Kelly et al. (2005), River Diatoms:  A multiaccess key, posted online.

• The view observed depends on how the diatom rests on the microscope slide. Usually the cell falls on its widest side, with either the valve (valve view) or the girdle (girdle view) apparent.

See further details and a valve face SEM at Mann (1985); Round, Crawford and Mann (1990); and online.

Valve view of a pennate diatom illustrating costae on either side and raphe along axis in center of clear area, interrupted in the middle by a lighter-colored silica nodule.  Excellent photomicrograph by Charles Krebs posted online, Issaquah, WA USA.


Cyclotella choctawhatcheeana: SEM view of a centric diatom 'exploded' (partially dissembled), illustrating outer valve face on epitheca (upper valve) and inner face of hypotheca (lower valve), as well as at least two girdle bands.  Photograph by discoverer Dr. A. K. S. K. Prasad of the Florida State Uuniversity Department of Biology, posted online.


Biochemical determination of frustule morphology

The longest polyamine chains known are involved in the species specific nanomorphology of silica frustules.  Polyamines cause rapid precipitation of silica at ambient temperature and pressure (Kröger et al. 2000).



Graham, L. and L. Wilcos 2000. Algae. Prentice-Hall

Kelly, M.G., H. Bennion, E.J. Cox, B. Goldsmith, J. Jamieson, S. Juggins D.G. Mann & R.J. Telford  2005.  Common freshwater diatoms of Britain and Ireland: an interactive key. Environment Agency, Bristol.

Kröger, N., R. Deutzmann, C. Bergsdorf and M. Sumper 2000.  Species-specific polyamines from diatoms control silica morphology.  Proceedings of the National Acadmy of Sciences of the United States of America (PNAS) 97(25):14133-14138.

Mann, D.G. (1989). The diatom genus Sellaphora:separation from Navicula . British Phycological Journal 24: 1-20.

Round, F.E., Crawford, R.M. & Mann, D.G. (1990). The diatoms. Biology and morphology of the genera. Cambridge University Press





Home / Diatoms / More on morphology