THE PATHWAY OF WATER MOVEMENT 95 



in addition this will only come into operation with a pressure gradient 

 and wiU result in a greater flux in response to pressure than to an equivalent 

 osmotic gradient.* It has been shown (Sabinin, 1925; Arisz et al, 195 1) 

 that root exudation conforms to the equation 



f=ko{Os-Om) (iv) 



where/equals the flux of water across the cortex, Og the osmotic potential 

 of the xylem sap and O^ the osmotic potential of the external medium, 

 j^o is a constant, the osmotic permeability coefficient, reflecting the pernie- 

 abihty of the root cells to the osmotic movement of water. If in addition 

 there is a difference of hydrostatic pressure P across the cortex: 



f^ko{Os-Om) + k^P 



Further, if there is a mass flow component of water movement across the 

 cortex : 



f=ko{Os-Om) + k,P+k'P 



where ^' is a coefficient reflecting the reciprocal of the resistance of the 

 cortex to viscous flow of water. Thus 



f^k,{Os-Om)+{ko+k')P (v) 



For practical purposes it is convenient to lump (^0 + ^) as a single con- 

 stant kp the pressure permeability coefficient. Evidently if there is no mass 

 flow (^ =0) then kp = kQ. Thus the problem resolved itself into measuring 

 feo and kp. 



Experimental 



Tomato plants grown in water culture were used. Detopped plants were 

 placed in a pressure canister through the lid of which the cut stem pro- 

 truded and in which the pressure on the medium surrounding the roots 

 could be raised by compressed air The medium could be rapidly changed 

 and thus the osmotic potential around the roots varied (Mees and 

 Weatherley, 1957). In practice direct measurement of /jq ^^d kp from eqs. 

 iv and v was not possible owing to changes in k^ itself in response both to 

 changes in the osmotic potential of the surrounding medium and to changes 



* The view is held (see Pappenheimer, 1953) that osmosis itself involves mass flow. 

 But if an osmotic flux through a semi-permeable membrane is regarded as a catena 

 involving mass flow through the pores but with a diftusional link in the chain (at 

 the orifice of each pore), then osmotic pressure and hydrostatic pressure will still have 

 identical effects. 



