INTRODUCTORY DISCUSSION. 9 



100 hours. During this time the amount of protein and other nitrogen 

 compounds in the leaves does not change; thereafter, and when all carbo- 

 hydrate has disappeared, the nature of the respiratory process changes 

 radically: the proteins are broken down, a variety of non-protein nitro- 

 genous substances appears, and ammonium salts are formed. It is a matter 

 of common knowledge that when a fungus is fed solely on proteinaceous 

 material there is formed a large amount of ammonia, and that this is com- 

 pletely reduced or inhibited when sugar is made available to the organism. 

 These phenomena indicate the widely different paths which the course of 

 metabolism may follow under varying conditions and which, unfortunately 

 for a clearer understanding of the subject, have been ascribed to obscure 

 regulatory devices of the organism. They furthermore suggest the exist- 

 ence of a condition in the nature of a balance between or interdependence of 

 carbohydrate and protein catabolism in the mature vegetable organism. In 

 young and growing portions of a plant the state of affairs is of course quite 

 different and more complex. 



In spite of the enormous amount of work which has been done and its 

 great economic importance, no definite conclusion has been reached even 

 for the mammalian organism. Thus, to quote Lusk (Science of Nutrition, 

 p. 277, 1917) : 



" Thomas calculated that during the period of minimal ' wear-and-tear ' pro- 

 tein metabolism, 0.4 calories were derived from the metabolism of 1.5 milligrams 

 of protein per kilogram of body-weight every hour, while 0.96 calories were 

 derived from the oxidation of 259 milligrams of glucose. In other words, pro- 

 tein furnished only 4 per cent of the energy required by a man at rest. Since 

 mechanical work scarcely influences the * wear-and-tear ' quota of protein 

 metabolism, although it largely increases the oxidation of carbohydrate, it is 

 evident that protein may play a very small role as a producer of energy for the 

 maintenance of the function of life. When carbohydrates are given in the diet, 

 it is possible to establish nitrogen equilibrium at a much lower level than when 

 protein alone or protein and fat are ingested." 



Besides furnishing methods for the study of metabolic products, chem- 

 istry has been able to be of service to physiology largely because it has sup- 

 plied many valuable analogies between chemical phenomena outside the 

 cell and the processes in the living organism itself. These analogies are of 

 closer relation than mere resemblance or likeness, and can with profit be 

 reasoned from, so that it is safe to infer that other and often deeper rela- 

 tions exist. Such analogies have aided us greatly in our thinking and have 

 influenced strongly the formulation of the theories of many vital processes. 

 The method is, of course, open to some criticism, and its value depends upon 

 the real identity in important aspects of the cases compared. Physiological 

 experimentation is often associated with such great difficulties that its 

 course can be opened only after the simpler, purely chemical relationships 

 have been formulated. Then, as has often been the case, physiological 

 investigation contributes to a clearer understanding of the chemical phe- 

 nomena and the thread which binds the two is found to be the identity of 

 a general principle. 



