SUMMARY: The changes in volume, measured by a Coulter particle counter (the ‘electronic volume‘), when suspensions of Tetrahymena are diluted with aqueous electrolyte have been examined kinetically. The diluting solution can have an optimal colligative concentration in which there is initially no change of the measurement; in suboptimal concentration there is initial increase followed by a decrease to a value somewhat less than that prevailing at the moment of dilution (95%), whereas in superoptimal concentrations there is initial decrease followed by an increase. If the initial decrease is severe the subsequent increase restores the value to only 80% of the original. The measurements of electronic volume, and of its changes, reflect the corresponding mean cell volume, and changes of it, estimated by photographic methods. The changes can be accounted for by changes of balance between the elasticity of the cell walls and the osmotic pressure across the walls.


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