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Spores of twenty species of Spirogyra were treated by acetolysis and viewed by scanning electron microscopy. The terms exo-, meso-and endospore should be maintained and based on chemical composition. Exo-and endospore are cellulose and/or pectin containing membranes. The mesospore is acetolysis-resistant and presumably contains sporopollenin.As basic pattern the spore wall in Spirogyra is composed of four layers: one exo-and endospore layer and two mesospore layers. The outer mesospore layer is mostly thin and hyaline, the inner layer thick and brown or yellow coloured and often sculptured. The germination suture is located in the inner mesospore layer. Some variations on this basic pattern were observed in Spirogyra bellis, S. cleveana and S. majuscula. Details of mesospore sculpture were observed and some taxonomic implications are discussed.The observed basic pattern is assumed to be valid for the Zygnemataceae as a whole.

The acetolysis resistant mesospore walls of Spirogyra were investigated by infrared spectrometry and chromic acid degradation. It was demonstrated that the mesospore wall contains chromic acid and hydrofluoric acid resistant sporopollenin. The biological significance could be a protection of the spores against desiccation and fungal attack.

Optimum growth in uni-algal cultures of Tribonema minus (Wille) Hazen, a dominant alga in blanketing algal mats in the early spring, was obtained at temperatures between 15-25°C and light intensities above 181!Einstein m -2 s -I. Uni-algal cultures of Spirogyra singularis Nordstedt, dominant in algal mats during summer, grew well between 2o-25"C and the highest light intensities tested(62 I! Einstein m -2 s -I). In mixed cultures T. minus outgrows S. singularis below 131!Einstein m-2 s-I and 15"c. At higher light intensities and temperatures levels S. singularis dominates, which agreeswith field observations. The growth reduction of T. minus at these higher levels is discussed.