The Plant as a Metabolic Unit 225 



experimental solution containing potassium bromide* and potas- 

 sium nitrate, with a rapid stream of air passing through the 

 solution for 10 hours. Another set of roots was subjected to a 

 similar solution, but nitrogen gas instead of air was passed 

 through it. A third set of roots served for the study of initial 

 composition. Roots were frozen and thawed, and sap expressed, 

 concentrations of nitrate, potassium, and bromine being then 

 determined, and conductivity of the sap also. 



It is shown in figure i that very large increases in concentra- 

 tions in the sap of the ions named above, accompanied by corre- 

 lated increase in conductivity, occurred when air was passed 

 through the solution in which the roots were immersed, but ac- 

 tual salt accumulation was not significant when nitrogen gas 

 was substituted for air. The nitrogen gas contained a very small 

 amount of oxygen as an impurity, apparendy sufficient to pre- 

 vent, over the brief experimental period, loss of solutes already 

 accumulated by the root cells. 



The conclusion from this and other experiments, in brief, is 

 that metabolic activities of root cells requiring oxygen bring 

 about the very rapid intake of electrolytes, so that there are built 

 up in the cell sap far higher concentrations of the ions concerned 

 than exist in the external solution. As already stated, work is 

 done by the cell in the process of accumulation. Diverse types of 

 living plant cells represented by storage organs, certain aquatic 

 plants, and root systems of higher plants all seem to have the 

 same general requirements for solute accumulation. The mecha- 

 nism by which energy is used in the transport of solutes against 

 concentration gradients is still without an explanation in accord 

 with all experimental evidence. 



* Bromine is a very convenient element for studies on accumulation, and does 

 not differ from the ordinary nutrient elements in its relation to the general 

 physiological processes involved. 



