INTRODUCTION 



11 



emphasized. It was gradually realized that since those saits which 

 are absorbed by plants are dissociated in aqueous solution, the 

 problem is primarily one of ionic, rather than molecular transport. 

 In the early years of the present century, the unequal uptake of the 

 two ions of a single salt was demonstrated both with isolated tissues 

 (Meurer, 1909; Ruhland, 1909), and whole plants (Pantanelli, 

 1915). Attempts were made to explain these observations in terms 

 of the establishment of Donnan equilibria, adsorption and ion 

 exchange. 



In the decade from about 1930, it was clearly established, mainly 

 through the investigations of Hoagland in the United States of 

 America, Lundegardh in Sweden, and Steward in England, that 

 salt uptake depends largely on aerobic metabolism. This led to 

 speculation concerning the relationship between absorption and 



No* K* Ca** Mg CC 



D U 



Protoplasm 



Cellulose wall 



. D D D 



Pond water 



No* k"" Co** Mg** CL" 



D 



Protoplasm 



Cellulose wall 



en 



nU 



Sea water 



(a) 



(b) 



Fig. 2. Diagram showing the relative concentrations of various ions in the 



sap of Nitella clavata (a), and Valonia macrophysa (b); and in the medium 



in which the plants were grown. The vertical scale of (a) tc is that of (b). 



Redrawn from Hoagland (1944) 



