236 D. R. Hoagland 



tion. At the end of the period, the residual solution was analyzed 

 and compared with the original solution. 



The concentrations of potassium and nitrate were greatly re- 

 duced, potassium almost to zero, while calcium, magnesium, and 

 sulphate ions were present in the residual solutions in slighdy 

 higher concentrations than in the original solution. Obviously, 

 solutes were absorbed much more rapidly than water with re- 

 spect to the first concentrations mentioned. Furthermore, the 

 general balance of ions was completely changed by the absorp- 

 tive processes of the plant. Bicarbonate ions, present at the begin- 

 ning in only negligible concentration, entered or were formed in 

 the culture solution in large concentration as a result of those 

 root activities which brought about a larger intake of anions 

 than of cations. The metabolic activities of the root system, 

 through their relation to the absorption of mineral solutes, thus 

 effectively and continuously disturb the equilibrium conditions 

 of the solid and solution phases of the soil. 



The conclusion has been generally accepted in recent years 

 by plant physiologists that absorption of mineral solutes is inde- 

 pendent of transpiration, but much of the earlier evidence for 

 this conclusion (growing plants in shade and sunlight, and de- 

 termining ash content, and the like) has not permitted the same 

 unambiguous interpretation as may be placed on the results 

 from simpler experiments of the type referred to just above. 

 A satisfactory understanding of the phenomena requires an 

 appreciation of the role of root metabolism in accumulation of 

 mineral solutes from dilute solutions. Although the independ- 

 ence of the processes of transpiration and of mineral solute ab- 

 sorption is established, it has not been proved that transpiration 

 does not indirectly influence absorption of solutes over an ex- 

 tended period. A certain upper limit of concentration exists for 



