96 P.E. WEATHERLEY 



in hydrostatic pressure. The eventual technique for estimating the osmotic 

 permeabihty coefficient was to measure the immediate change in flux 

 caused by a change in the osmotic potential of the medium, the hydrostatic 

 pressure remaining constant (Arisz's method). Thus 



f,= k,{Os-Onn) 



/2= kQ{Os-Om?) 

 ^1-/2 = K{Oni2- Omi) 



or 



L = 



where/i and/2 are the water fluxes with external media of osmotic potential 

 Omi and Om2- In this method the duration of the measurements was kept 

 very short so that the osmotic potential of the xylem sap (Og) had no 

 chance to change appreciably. The pressure permeabihty coefficient was 

 measured as the change in flux caused by a change in external pressure, the 

 osmotic potential of the medium being kept constant. Thus from eq. v 



f^=k^{Os-Om) + kpP^ 

 f^=ko{Os-Om) + kpP2 

 f,-f,= kp{P,-P,) 



^^=AP 



where /i and/2 are the water fluxes with appHed pressures of Pj and P^. 

 Under conditions of appHed pressure the osmotic potential of the xylem 

 sap {Os) was small so that any changes in Og could be safely neglected. In 

 effect the osmotic permeabihty coefficient was measured as the change in 

 flux in response to unit change of osmotic potential of the external medium 

 whilst the pressure permeability coefficient was measured as the change in 

 flux in response to unit change of apphed pressure. 



A comparison of measured values of the osmotic and pressure perme- 

 abihty coefficients is shown in Table i. It will be seen that in every experi- 

 ment kp was greater than /?„, the mean value of fep/^o being about 1-3. This 

 imphes that with a pressure gradient of about 2 atm f of the flux was 

 osmotic and | mass flow. 



These two fluxes could also be differentiated by the use of cyanide. As 

 shown in Table 2, three hours after the addition of cyanide k^ suffered an 



