Grafts et al. 



58 Water in Plants 



at zero concentration at zero. Though illogical, this is convenient; it should be under- 

 stood that it is merely an expedient. . ^ . u n :^„ ;f +« 

 The diffusion pressure of the solvent may be demonstrated by allowmg it to 

 evaporate through a membrane permeable only to liquid when wet; the energy of the 

 molecules is exhibited by the great lifting force shown. Addition of a solute reduces 

 this pressure and the only way to recover it is to put energy back as by evaporating and 

 condensing it again in pure form, or by forcing pure solvent from an osmometer by 

 pressure upon the solution. 



