CHAPTER 



/ The Mechanism of Ion 

 Absorption 



HANS BURSTROM 



D 



uring recent years there have appeared complete 

 theories of the mechanism of the active absorption of ions, which in a 

 rather definite way have brought together the vast experiences con- 

 nected with the subject. It is an easy task to explain an active absorp- 

 tion and accumulation of one kind of ion, and this can be brought 

 about in experiments with models of different kinds. However, the 

 crucial point in every such theory is to explain the simultaneous absorp- 

 tion of anions and cations and their accumulation together against the 

 diffusion gradient. 



The first of these theories was advanced by Lundegardh (j), and 

 was founded on experiments in progress since 1933; a second theory, 

 a modification and development of the former, has been advanced by 

 Robertson (6). Although the main features of both theories are proba- 

 bly well known to you, I feel it necessary to review them in some 

 detail before entering into a discussion of the present situation of this 

 field of research. 



Lundegardh's theory is based on four assumptions, which perhaps 

 today may be regarded as generally accepted facts. 1) The absorptions 

 of anions and cations are independent of each other to such an extent 

 that different mechanisms must be responsible for each. 2) The absorp- 

 tion of cations takes place in two steps, one involving the absorption 

 of ions from the external solution and the other the excretion of the 

 ions from the cytoplasm into the vacuole. The latter step constitutes 

 the real accumulation. A number of recent investigations have cor- 

 roborated this view for different plant materials such as roots, storage 

 tissues, and even leaves. Arisz (/) has shown that leaves of Vallisneria 



