Hans Burstrom 253 



salt is added to the external solution, as demonstrated by Lundegardh 

 long ago, but also, if salt is withdrawn from the external medium, the 

 rate again goes down to approximately the initial level. Thus, this 

 effect of salt on the respiration is reversible, which it would have to be, 

 if it is connected with the absorption of ions and not only with the 

 presence of ions in high concentrations within the tissues. 



This salt respiration has been further defined by its high sensitivity 

 to the heavy metal inhibitors — hydrocyanic acid, carbon monoxide, and 

 sodium azide — to which agents the ground respiration is almost in- 

 sensitive. This point, initially brought out by Lundegardh, has been 

 verified by several investigators who have shown that the intake of ions 

 can be wholly inhibited by hydrocyanic acid. Especially convincing are 

 the experiments of Robertson, according to which an addition of hy- 

 drocyanic acid stops both salt intake and the associated increase in 

 rate of respiration at once, but leaves the ground respiration almost 

 unaffected. Consequently, with roots or tissues in water, hydrocyanic 

 acid has little effect on the respiration. 



According to prevailing concepts, this would indicate that the salt 

 respiration is catalyzed by hemin compounds, while the ground respira- 

 tion is catalyzed by a different enzyme system, the nature of which is 

 unimportant in this connection. The hemin compounds contain iron 

 as the active constituent with which the mentioned inhibitors form 

 complexes and thus inactivate the enzyme. 



On the basis of the above evidence, Lundegardh assumed, as did 

 Robertson later, that the salt respiration is catalyzed by cytochrome 

 systems, because cytochromes are probably universally present and 

 contain hemin compounds which are easily inactivated by hydrocyanic 

 acid. Lundegardh and Robertson go even further and claim that the 

 cytochromes themselves play a fundamental role in the absorption of 

 the anions. This remains to be proved, but is at present an integral 

 part of the theories advanced by both men. 



The active part of the cytochromes is the iron atoms which change 

 their valence by taking up and giving off electrons according to the 

 formula, Fe+++ -f- e~ ' Fe++. Thus the cytochromes serve as 



carriers of electrons and function at one end of the respiratory system. 

 Now it is assumed, or is really an established fact, that when the Fe+++ 



