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Summary. iChanges are reported in total cellular organic carbon, nucleic acids, proteins, carfbohydrates, lipids and chlorophylls during the course of silicon-starvation synchrony of Navicula pelliculosa. All constituents increased at the same rate, relative to cel1 number, for 30 hours of exponential growth during which silicon was depleted from the medium. Increase in cell number then stopped. but net synthesis of most components continued for a further 5 to 7 hours before ceasing. Deoxyribonucleic acids and lipids accumulated throughout the 14 hour silicon-starvation period. When silicon was resupplied, lipid synthesis ceased and organic carbon anld carbohydrates decreased slightly. Net synthesis remained low during the 4 hour silicon uptake period but was resumed at higher rates as cell num-ber began to rise. In cultures transferred to the dark 1 hour prior to readdition of silicon, total carbon, carbohydrates, and lipids decreased markedly during silicon uptake and cell separation. This was due in,part to con'version of protein which maintained the protein level of the dark cells close to that of cells kept in the light. Mechanisms by which silicon starvation and reintroduction of silicon might affect rates of cellular synthesis are discussed.Silicon is a major component of the cell wall in almost all diatom species and is essential for their growth, but whether it is required only as a wall constituenit or affects other metabolic processes as well is not known. Studies conducted, in this laboratory (4, 17), primarily to investigate the metabolism of silicon in wall formation, have shown that in the fresh water diatom, Navlicula pelliculosa, cell separation ceases once the medium is depleted of silicon.However, cellular development is not arrested until after cytokinesis has taken place and deposition of new walls has ibegun. Thus after 14 hours of silicon starvation more than 70 % of the cell population consists of bi'protoplastic cells, i.e., 2 daughter protoplasts, each surrounded by a new plasmalemma and separated by an intercellular space, contained within the parent frustule. When silicon is resupplied there is a 4 hour period of rapid silicon uptake after which

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