Introduction

Abstract

Siliceous structures occur in a variety of organisms but predominate in the more “primitive” (protistan) forms of life, in which amorphous, hydrated silica forms cells walls, tests, scales and other skeletal features. The occurrence of silica in primitive organisms and the essential involvement of silicic acid in biochemical and growth processes in these forms, as well as in other more highly evolved groups, suggest that early life forms may have been dependent upon this element, a dependence that has been carried through evolution to present-day organisms.