The Plant as a Metabolic Unit 239 



The respiration of root cells, an activity which must proceed 

 in order that accumulation of solutes may take place, is also a 

 source of hydrogen ions, which can bring about the solution of 

 bases or of other elements from the solid phase of the soil. Great 

 importance thus is assigned to the displacement by hydrogen 

 ions of calcium, magnesium, and potassium from those colloidal 

 complexes of the soil in which the properties of base exchange 

 chiefly reside. In this displacement we find a very direct relation 

 between metabolic activities of root cells (also, therefore, an 

 indirect relation with activities of leaf cells) and the dissolving 

 of certain essential elements from the solid components of the 

 soil system. There are thus two important general consequences 

 of root metabolism as related to nutrition with respect to cations 

 — production of hydrogen ions, and disturbance of equilibrium 

 through absorption of bases by root cells. The latter process is 

 of course selective, potassium ions being withdrawn usually at 

 a far greater rate than calcium or magnesium ions. 



Among the anions absorbed by plants, nitrate is ordinarily of 

 greatest quantitative importance and this ion has a special sig- 

 nificance for soil-plant interrelations as determined by metabolic 

 activities of the plant. The nitrate is often removed from solution 

 in excess of bases, leaving bicarbonate or organic acid ions 

 derived from the tissue in concentration equivalent to that of 

 the excess nitrate removed. In effect, an exchange of ions occurs. 

 Nitrate ions may be removed from solution and concurrently 

 undergo reduction, or they may be accumulated in the cell sap 

 as nitrate ions and subsequendy reduced. Recent experiments 

 on barley plants indicate that the accumulation of nitrate ions as 

 such is controlled by the same metabolic activities that control 

 accumulation of other ions. As conditions become less favorable 

 for aerobic processes, the removal of nitrate ions from solution (as 



