Transcellular osmosis measurements are combined with measurements of the kinetics of relative length change, when C. corallina cells are transferred from artificial pond water to polyethylene glycol (MW 300-400) of higher osmotic pressure to yield the constant k, which relates relative volume changes .DELTA.V/V to relative length change .DELTA.l/l (.DELTA.V/V = k.DELTA.l/l). The value of k is 3.5 and is temperature independent between 7 and 30.degree. C. Static and kinetic estimates of the bulk modulus, .epsilon., indicate that .epsilon. is temperature independent and has a value between 500-650 bars at high turgor pressures. The value of .epsilon. declines as the turgor pressure declines. It is shown how .epsilon. and k are related to Young''s modulus and to Poisson''s ratio for anisotropic C. corallina cell walls and point out that the previous treatments of the problem are in error.