Hans Bur strom 257 



not the same thing to work with a histologically highly organized root 

 as with a uniform piece of storage parenchyma. 



So much for the theories. They are obviously speculations to some 

 degree, but, nevertheless, in conformity with all or most experimental 

 data and also with our knowledge of respiratory mechanisms. Several 

 points need to be further clarified, however. 



The first question is, as already discussed by Robertson, why this 

 special respiration system does not work in the absence of external 

 salts. All integral parts are present in the cell: substrate, enzyme, and 

 oxygen. Here we can only guess. We may refer to an activation of 

 the system by the anions, but that does not explain anything. A second 

 point is, how is it possible to get unequal absorption of anions and 

 cations? If the anions are absorbed in excess, the problem is simple. 

 The excess is combined with hydrogen ions and the vacuolar sap is 

 acidified. The cell possesses no means of preventing such a decrease 

 in pH. Usually there is, however, an excess accumulation of cations, 

 because the rapidly absorbed anions, notably nitrate, are assimilated to 

 a large extent in the cytoplasm and an excess of cations appears in the 

 vacuole. 



These two questions are probably intimately connected. The second 

 one has been partly elucidated by the work of Ulrich (7) and the 

 author (2). Ulrich showed that the excess of cations in his material 

 was balanced by a production of malic acid which was equivalent to 

 the excess cations, with some exceptions. Our observations showed that 

 this equilibration without exception takes place not at the point of in- 

 take of the ions but at the point of their accumulation or, with the 

 adopted terminology, at the /-level. When an excess of cations is re- 

 leased to the vacuole, it is accompanied by an equivalent amount of 

 malic acid or, at the prevailing pH, of malate ions. 



As to the source of this malic acid, it is almost certainly formed as 

 an intermediate in the acid cycle of the respiration. This acid cycle 

 forms the fundamental part of the dehydrogenase systems and is, ac- 

 cording to the picture outlined above, localized to the /-level. In this 

 cycle malic acid may be formed from succinic acid and itself trans- 

 formed into oxaloacetic acid. Of course other transformations are also 

 possible. Here malic acid, which is a weak acid, reacts as the undis- 



