274 TEXTBOOK OF PLANT PHYSIOLOGY 



the intermediate products of alcohol fermentation, goes on 

 smoothly. But when the normal structure of the protoplasm is 

 disturbed, for instance by mechanical injury, through influence of 

 narcotics, freezing, etc., then oxygen has an easier access to the 

 cell, and the oxidation of the chromogen goes on until it results in 

 stable, dark-colored products and the whole process is checked. 

 This explains the cessation of respiration in dead cells, as well as 

 their turning brown in contact with air, a phenomenon observable 

 in cut or frozen apples, or in sap pressed from potatoes. Some- 

 times these stable ultimate products of oxidation of the respiratory 

 chromogens are not brown-colored, but of more brilliant colors. 

 Thus, for instance, the fruiting bodies of many fungi turn blue, 

 green, or red at the broken surfaces. 



Palladin's theory has not been verified conclusively and will 

 no doubt undergo revision in the course of further investigations. 

 In recent years, however, new evidence in favor of this theory has 

 been supplied through the work of chemists, as well as of plant 

 physiologists. This theory has been applied to the physiology of 

 animals, because essentially respiration is the same phenomenon 

 in the whole organic world. Then, too, it is in harmony with the 

 modern ideas as to the role of water in all kinds of oxidation proc- 

 esses. 



The nature of respiration has been considered by Warburg 

 from quite a different point of view. On the basis of a number of 

 observations, he attempts to prove that respiration is connected 

 with the presence of a permanent colloidal structure. He con- 

 siders it to be essentially an adsorption process, taking place on the 

 surface of the colloidal particles of protoplasm. Warburg suc- 

 ceeded in constructing an artificial model of respiration. He used 

 blood charcoal as an adsorbing body with a large surface, and as 

 respiration material a solution of cystine and other amino acids. 

 In the presence of oxygen, such a model showed a resemblance to 

 the respiration process with absorption of oxygen and formation of 

 C0 2 , H 2 0, NH 3 , and S0 3 . 



Iron was found to be an exceedingly important agent in the 

 working of this respiration model. A prominent role in physi- 

 ological oxidation has been ascribed to iron for some time, but it 

 could not be proved by precise determination. Warburg's experi- 

 ments have shown that respiration in his model is one-third to one- 

 half as fast, when, instead of using blood charcoal, which always 



