Hans Jenny 123 



nal, mixed system differs from the root behavior in the extract, a con- 

 tact effect is strongly suggested. 



The required extraction of the "intermicellar liquid" is often difficult 

 to achieve. Seemingly clear ultrafiltrates, obtained in high-speed cen- 

 trifuges, may reveal numerous fine colloidal particles if viewed in the 

 ultramicroscope. In addition, there are certain aspects connected with 

 Donnan equilibria which must be taken into consideration. The situ- 

 ation is further complicated by carbon dioxide evolution of the roots. 

 Prior to separation the system has to be saturated with carbon dioxide 

 to simulate conditions of root environments. 



The soil solution theory contends that any ion removed from the soil 

 solution by plants is replenished by the solid phase. Some investigators 

 even assume that the composition of the soil solution remains constant. 

 Since rate studies also require constant composition of media, the ex- 

 tracted intermicellar liquid must be offered to the roots in such large 

 amounts that the contents remain unchanged during the absorption 

 process. 



Only two experiments have been published {20, ^5) which satisfy all 

 of these requirements: the sorption of radioactive rubidium as a func- 

 tion of the degree of saturation, and the sorption of radioactive sodium 

 from various clay concentrations. Table II reports the uptake of radio- 

 active rubidium from rubidium-hydrogen clay suspensions and rubid- 

 ium bicarbonate solutions, the latter having the same rubidium con- 



