260 Mineral Nutrition of Plants 



anions. This means that the cytochrome system saturated with organic 

 anions should be much less active than when it contains attached in- 

 organic anions. The function of the cytochrome system depends upon 

 its ability to carry electrons between the iron atoms. If the latter bind 

 anions, the activity must depend upon the rate of exchange of the elec- 

 trons for the anions, a reaction which certainly must vary with the kind 

 of ion. The fact that large bivalent organic anions are much less ex- 

 changeable than the inorganic ions, might explain the activation of the 

 respiration system by the added mineral salts. 



In this way the organic acids occupy a central position of interest. 

 Their role is, however, far from clarified and can hardly be tackled 

 from common physiological points of view. If this theory gives a true 

 picture of the mechanism of the ion absorption, biochemical studies are 

 necessary for further progress within this field. 



There are still other difficult questions to answer. One is the real 

 cause of the specificity of the ion absorption: why do not all plants 

 absorb all ions in the same proportions, for instance always potassium 

 more rapidly than sodium, as was to be expected from the physical 

 properties of the elements and their importance for the exchange phe- 

 nomena regulating the absorption. It is obvious that the recorded theo- 

 ries can only claim to explain the principles and the fundamental 

 mechanism underlying the absorption of ions, but there are details 

 which hitherto have not been seriously considered at all. I do not think, 

 however, that such gaps in the theories detract from their value as a 

 solid working basis for explaining the active absorption of ions. 



REFERENCES 



i. Arisz, W. H., Proc. Koninkl. Nederhwd. Akad. Wetenschap., 50: 3 

 (1948). 



2. Burstrom, H., Ar\iv. f. Bot., 32 A, Nr. 7 (1945). 



3. Lundegardh, H., Arkjv . f. Bot., 32 A, Nr. 12 (1945). 



4. •, Ann. Roy. Agr. Coll. Sweden, 16: 339 (1949). 



5. •, Ann. Roy. Agr. Coll. Sweden, 16: 372 (1949). 



6. Robertson, R. N., and Wilkins, M. J., Austr. J. Sclent. Res., B r: 17 

 (1948). 



7. Ulrich, A., Am. J. Bot., 29: 220 (1942). 



8. Wanner, H., Ber. schweiz. botan. Ges., 58: 383 (1948). 



