SUMMARY: Zoospores of in a concentration gradient of hydrochloric acid or its chloride salts exhibited acute, repetitive turning movements at a point which corresponded to a critical concentration of cation and not to a critical concentration gradient. The critical concentration was calculated to be 37 μ for H, 628 μM for K, 5040 μ for NH and 6480 μ for Na with Cl as the coionic species. The result of the negative chemotactic reaction was that zoospores swam away from the source of the chemical, regaining normal motility quickly as they returned to lower concentrations of the cation. Zoospores encysted immediately when suspensions were mixed with acids or salts having a concentration in the mixture greater than that which induced the turning reaction. They encysted swiftly at slightly lower concentrations of the cation. Negative chemotaxis appears to be largely an all-or-none response to a threshold concentration, but weaker effects caused by prolonged exposure to sub-threshold concentrations probably also occur. The response of zoospores to the various cations followed the Hoffmeister lyotropic series for cation exchange reactions, and negative chemotaxis can be interpreted as involving surface exchange phenomena.


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