EXPERIMENTAL MATERIALS AND METHODS 23 



2. Quantitative Determinations 



a. General. Absorption of salts by whole plants, or their parts, 

 can be determined quantitatively by analysis of samples of either 

 the medium or the tissue, before and after a period of uptake. In 

 practice, analysis of the medium is often preferred, since it is then 

 possible to measure absorption after several consecutive time 

 intervals with the same plant material. Moreover, analysis of the 

 tissue is often less reliable than that of the medium owing to the 

 presence of a greater number of substances, which may interfere 

 with the determination, and difficulty may be encountered in 

 extracting salt quantitatively, especially if some has been incor- 

 porated into organic cell constituents. 



If a tissue contains initially an appreciable amount of the salt 

 under investigation, the change in concentration as a result of 

 absorption may be smaller, and less easily determined with accuracy, 

 than are the concomitant changes in composition of the medium. 

 On the other hand, if the volume of medium is large relative to the 

 volume of tissue, and the concentration of salt is high, the reverse 

 may be true, and an analysis of the tissue may be preferred. In 

 general, the most satisfactory experiments are those in which it is 

 possible to analyse the medium at intervals, and then confirm the 

 total amount of salt absorbed by a final analysis of the tissue. 



b. Methods of analysis. 



i. Chemical analysis. Quantitative macro- and micro-analytical 

 methods are available for determining the amounts of the most 

 important elements taken up by plants (see Humphries, 1956a). 

 Although these methods are accurate, they are frequently laborious, 

 and are often replaced by more convenient physical methods, when 

 such are available. 



ii. Physical methods, a. Conductivity determinations. A change 

 in the total concentration of ions in a solution can be estimated from 

 the change in electrical conductivity. It must be noted, however, 

 that if uptake of one ion is accompanied by release into the medium 

 of a different ion with the same electrical charge, little or no change 

 in conductivity may be detected even though an appreciable amount 

 of a particular ion may have been removed from the solution. 



^. Polarography has been successfully used for the determination 



