Lecture 3 — 61 — Absorption 



infer that this vitamin was present in the roots be- 

 cause of previous translocation from the shoot. 



The salt accumulation by the roots is notably selec- 

 tive, that is to say, some ions are absorbed more 

 rapidly than others. Potassium is absorbed by the 

 barley roots much more rapidly than calcium or mag- 

 nesium. Sulphate is a very slowly absorbed ion, while 

 nitrate and halide ions are absorbed with relative 

 rapidity. These differential absorptions of ions imply 

 ionic exchanges between the root and the culture 

 medium and also changes in the hydrogen ion con- 

 centration of the culture medium. For example, from 

 a solution of calcium nitrate, nitrate ions may be ab- 

 sorbed readily and accumulated in the sap as nitrate 

 ione in higher concentration than that of the outside 

 solution (some reduction of nitrate also takes place), 

 while but little calcium enters the cells. An electro- 

 static balance must obviously be maintained in both 

 the internal and external solutions and this is accom- 

 plished externally by bicarbonate ions taking the place 

 of the absorbed nitrate ions in the culture solution, 

 and internally by readjustments of organic acid con- 

 tent. From a potassium sulphate solution potassium 

 ions are absorbed in excess of sulphate ions. As a 

 consequence the acidity of the culture solution in- 

 creases rapidly. In addition, calcium and magnesium 

 ions enter the solution from the roots, possibly by 

 secondary exchanges for hydrogen ions of the solution. 



The readjustments of the metabolism of the roots, 

 with a tendency to maintain hydrogen ion concentra- 

 tion in the root sap within relatively narrow limits 

 are of considerable interest, but this is a matter for 

 later consideration. Without citing the detailed evi- 

 dence, I should stress now the point that in these 

 differential absorptions, not only the physical-chemical 

 properties of the ions are concerned but also the state 

 of metabolism of the plant cells and their content of 

 salt at any given time (Hoagland and Broyer, 1940). 

 Further, it does not follow that two ions of a salt 



