256 Mineral Nutrition of Plants 



of Lundegardh, on which it is largely based. Robertson assumes the 

 same function of the cytochromes, the same polarity, and the same 

 mechanism of combination of anions and cations at the /-level. He has 

 made one change, however, which may be rather important. 



He does not postulate a successive exchange of anions along a 

 cytochrome bridge and against an electron wave, but assumes that the 

 cytochrome system circulates with the cytoplasm between o- and /-levels. 

 At the o-level, where ferric iron is formed by oxidation, it catches an 

 anion and the whole complex is moved to the /'-level. Here the iron is 

 again reduced by absorbing an electron and the anion is set free. The 

 iron, in the reduced state, returns to the o-level, where it is oxidized, 

 thus completing the cycle. It is worth mentioning that Arisz (/) has 

 pointed out a third possibility, namely, that the transfer of ions between 

 the plasma constituents is conditioned by the continuous breaking down 

 synthesis and of organic compounds, and this means that a high 

 degree of instability of the cytoplasm should be one of the causes of 

 the change in position of the mineral ions. 



At present we cannot decide which of these two or three assumptions 

 is correct, but this is certainly an important question, because it may 

 contribute to an explanation of the quantitative relationship between 

 the salt respiration and the salt accumulation, or, which is the same 

 thing, the efficiency of the salt respiration. It is quite certain that there 

 is no stoichiometric relationship between salt respiration and the 

 amounts of ions absorbed. Robertson has computed, however, the maxi- 

 mum amount of ions that can be absorbed per unit of respiration if 

 the theory is correct. This is very simple since one electron corresponds 

 to one equivalent of anions and one molecule of oxygen corresponds 

 to four electrons. Thus, one molecule of oxygen consumed in respira- 

 tion may cause the absorption of, at most, four equivalents of anions. 

 His experimental data show that the highest ratios of ion equivalents 

 accumulated to oxygen consumed for storage tissues and excised roots 

 are between three and four. This is no proof of the correctness of the 

 theory, but is quite consistent with it. In a very recent paper Lundegardh 

 (5) has shown, however, that in intact roots this quotient does not 

 exceed 1, which is explained by a respiration connected with an internal 

 transport of salt in the normally functioning roots. In practice, it is 



