SUMMARY: The morphogenesis of organized thalli in pure cultures of was found to be dependent upon the sequential differentiation of specialized cells and filaments. In a culture of previously kept in darkness, a brief exposure to light induced development during subsequent incubation in darkness, and resulted in the formation of simple unbranched thalli. Such structures are interpreted as being the product of a single complete turn of the developmental cycle. Continuous illumination produces ramified thalli (dendroid macrostructures) which bend toward the source of light. The various differentiated cell and filament stages of development are oriented within the dendroid thallus. The presence of glucose in culture media prolongs growth at the heterocystous filament stage. The consequent inability to form motile hormogonia leads to the growth of a thallus lacking an ordered macroscopic structure. The term lanose structure is applied to this form to describe its fleece-like appearance.

Hormogone development in cultures of appears to be sensitive to a metabolic product. In cultures containing glucose this product is formed rapidly, hence a lanose macrostructure results. In cultures containing sucrose, the breakdown of organized thalli and formation of a lanose macrostructure takes place only after prolonged incubation. It is suggested that the pattern of organization in other, more complex, cyanophycean colonies may arise similarly from a photo-induced sequence of cellular and filamentous development.


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