Diatoms usually undergo a decrease in size concomitant with cell division. Because the developmental and physiological faculties of diatoms are dependent on cell size, the ability to manipulate cell size would be valuable for experimental purposes. Although enlargement of cells usually can be obtained by auxospore formation, the latter, as an essentially sexual process (in a diploid organism), alters the genetical make-up of the strain. Methods to carry out the enlargement of the cell vegetatively are therefore required and are in fact applied routinely in the Marburg laboratory. Their use is described and discussed. They are not of general application, however, but are always dependent on some cooperation of the object and on its specific reactions. If there is cooperation, as in Stephanopyxis turris, Bellerochea malleus, Streptotheca, Lithodesmium, 5 species of Biddulphia, Grammatophora marina, Rhabdonema arcuatum, Achnanthes longipes and others, vegetative enlargement can be induced by nutriti...