Much of the high percentage ash content of the diatoms is attributable to the extent of silicifi-cation of the cell walls. Besides their value to taxonomic diagnosis, the ornate and often delicate exoskeletal structures, celebrated in such photographic collections as that of Round et al. (1990), command wonder at the evolutionary trait and at the genetic control of frustule assembly. There are ecological ramifications, too, arising from the relatively high density of the deposited silicon polymers, seemingly quite opposite to the adaptive requirements for a plank-tic existence. Moreover, the amounts of silicon consumed in the development of each individual diatom cell have to be met by uptake of silicon dissolved in the medium. Whatever may have been the situation at the time of their evolution, the present-day concentrations of dissolved
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