228 PHYSIOLOGY OF NUTRITION 



an excess of proteins. Etiolated bean leaves, which are rich in protein but con- 

 tain only a little carbohydrate, produce carbon dioxide at an exceedingly low rate ; 

 Palladin 1 found that ioo g. of such leaves, at room temperature, gave off carbon 

 dioxide for three successive hours at the rates of 102.8, 95.9, and 70.2 mg., 

 respectively, with an average rate of 89.6 mg. per hour. The same leaves were 

 floated upon cane-sugar solution in darkness for two days, by which treatment 

 their carbohydrate content was markedly increased without serious alteration 

 of their protein content, and they then gave off carbon dioxide for four succes- 

 sive hours at the rates of 152.6, 147.5, 146.8, and 144.5 m g-> respectively, with 

 an average rate of 147.8 mg. per hour. 



If etiolated bean leaves are kept upon cane-sugar solution longer than two 

 days their carbohydrate content continues to increase, but this further increase 

 in carbohydrates is without influence upon the rate of elimination of carbon 

 dioxide. After forty hours upon cane-sugar solution 100 g. of these leaves pro- 

 duced 144.5 m g- 0I carbon dioxide in one hour. After forty-two hours longer 

 upon the sugar solution they gave off 144. 1 mg. of carbon dioxide in an hour. 

 The longer period upon sugar solution, although resulting in higher carbohy- 

 drate content, did not produce any alteration in the respiration rate; the 

 protein content of the leaves remained unchanged and the supply of carbohy- 

 drates was adequate in both cases. This experiment shows that there exists 

 no constant relation between the rate of evolution of carbon dioxide and the 

 supply of carbohydrates. During the shorter period upon sugar solution 

 these leaves had absorbed enough sugar so that the sugar content of the tissues 

 was adequate for the maximum respiration rate with the given amount of 

 proteins, and still further addition of sugar was without influence upon the 

 rate of elimination of carbon dioxide. 



An excess of carbohydrates is to the living cell what a coal supply is to a 

 manufacturing establishment; as long as there is sufficient coal on hand to oper- 

 ate the machinery at maximum speed, the amount of the coal supply determines 

 only how long the factory can be kept in operation, and is without influence upon 

 the daily rate of production. The daily output from such an establishment, so 

 long as enough coal is available to operate the machines at their maximum speed, 

 is dependent only upon the capacity of the machines themselves. Similarly, 

 only the duration of the respiration process in a cell is dependent upon the supply 

 of carbohydrates present, providing only that the supply is adequate for the 

 maximum rate, and this maximum rate depends upon the capacity of the living 

 protoplasm to carry on the respiratory process. Other conditions remaining 

 the same, this capacity depends upon the amount of protoplasm present in the 

 cell. Regarding the cell as a factory, carbohydrates are the coal and the proto- 

 plasm is the machinery. Only upon the amount of protoplasm present does the 

 rate of the life-processes thus depend, assuming the supply of carbohydrates, 

 water, etc., to be adequate and the temperature, etc., to be optimum. 



Carbohydrates are not directly acted upon by the protoplasm, but their de- 



1 Palladin, W., Recherches sur la respiration des feuilles vertes et des feuilles 6tiolees. Rev. gen. bot. 

 5: 449-473. 1893- 



