SUMMARY: The Allison strain of , when cultivated in complete darkness, differs morphologically from the forms which prevail in the light. When cultivated in the dark with glucose as energy and carbon source, the alga grows very slowly as a mass of large undifferentiated cells (the aseriate stage). Exposure to small amounts of light or the addition of aqueous extracts of light-grown cells enables development of typical nostocacean filaments. The aseriate morphology is a transient stage of development found in all autotrophic or photoheterotrophic cultures. By growing Nostoc in complete darkness and then exposing to light of low intensity, synchronous development occurs. The cyclic sequence of development is described. The inhibition of growth in the dark is interpreted as a lack of morphogenetic substances formed only in the light.


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