258 Mineral Nutrition of Plants 



sociated acid, but it must always be dissociated to a fixed extent ac- 

 cording to the prevailing pH, malic acid ^ malate . If an 

 excess of cations is added to such a system, this dissociation shifts to 

 the right, and an amount of malate equivalent to the cation excess is 

 withdrawn from the respiration cycle and is apparently excreted to- 

 gether with the cations into the vacuole. If the excess diminishes, the 

 dissociation reaction is reversed and the acid disappears in the respira- 

 tion cycle again. This is a simple matter of equilibria, and Ulrich has 

 shown directly that the formation and disappearance of the malic acid 

 in connection with the ion accumulation is effectuated by the respira- 

 tion system. 



We may go one step further. If the ferric ions of the cytochromes 

 are able to bind anions, why should they restrict themselves to the 

 mineral anions, why not attract organic anions as well? Robertson has, 

 in fact, raised this question, and Lundegardh (4) has answered it by 

 assuming that in intact roots, a major part of the salt respiration de- 

 pends upon the action of a respiration system combined with organic 

 anions, called "native" anions. If we assume that the ferric atoms are 

 normally equilibrated by malate, or other organic acids formed in the 

 cell, it is easy to explain why an excess of cation accumulation causes 

 malate formation. 



As soon as a mineral anion enters the cytoplasm, it is attracted by a 

 ferri-cytochrome and there replaces a malate ion, which disappears in 

 the respiration. Then the anion is moved inward and, if it should 

 happen to be assimilated as regularly occurs with nitrate ions, its place 

 is immediately occupied by a malate ion again. At the /-level the 

 anions (mineral ions or malate ions) are set free and combined with 

 cations in the vacuole (Figure 1). It is, of course, not necessary to 

 assume that this role must be played by malate or malic acid, although 

 it is so in the cases investigated so far. In other plants other acids may 

 fulfill the same function, depending on, for instance, the existing pH 

 of the cytoplasm, the dissociation constants of the acids, etc. 



A very strong argument in favor of the assumption that the dehy- 

 drogenation system, producing both hydrogen and organic acids, is 

 located at the /-level where the accumulation of the ions is regulated, 

 is given by Lundegardh (4) in a very recent paper. He has shown that 

 the bleeding from a root is dependent upon the release of ions to the 



