UPWARD TRANSPORT OF NITROGEN 71 



tioned in the introduction, when it was recognized that 

 plants required certain inorganic salts, that these were 

 necessarily absorbed from the soil, that the soil solution 

 was very dilute, that most land plants absorbed large 

 quantities of water from the soil, and that most of this 

 water evaporated from the leaves; it seemed perfectly 

 logical to conclude that the amount of water lost from the 

 leaves would determine both the amount of solution and 

 therefore salts absorbed, as w^ell as the distribution after 

 absorption. In more recent years, however, it has been 

 recognized that passage through membranes reduces or 

 precludes the possibility of mass flow, and diffusion there- 

 fore predominates over mass flow. Under such conditions 

 each substance, solvent and solute alike, diffuses inde- 

 pendently, and the movement of one through the mem- 

 branes may have no effect on the diffusion of the other. 



A number of investigators have experimentally demon- 

 strated that there is no direct relation between water 

 absorption and salt absorption. Among these are Hassel- 

 bring (1914), Muenscher (1922), Prat (1923), and Hoagland 

 (1923). Experimental evidence is therefore accumulating 

 which shows that there is no direct relation between 

 water absorption and salt absorption. Prat (1923) claims 

 a slight positive effect, but it is interesting to note that 

 in those cultures having high transpiration rates and a 

 slightly increased salt absorption, his data show the external 

 salt solution to have been distinctly increased because of 

 the decreased volume of solution which resulted from the 

 high rate of transpiration. It is highly probable that 

 the observed increase in salt absorption was due to this 

 increased external salt concentration and not due to any 

 effect of transpiration toward directly absorbing the 

 external solution, nor is it necessarily due to an increased 

 transport from the roots to the leaves. If he had kept 

 the external concentration constant this effect would 

 probably have been eliminated. Muenscher (1922) and a 

 number of other investigators have found that an increased 

 external salt contraction will increase salt absorption. 



