20 THE CARBOHYDRATE ECONOMY OF CACTI. 



in such terms as to permit of any well-defined experimental investigations. 

 The form in which energy is utilized in the plant is still a matter of much 

 conjecture. This is, of course, aside from the utilization of solar energy in 

 photosynthesis. 



Many animal physiologists have assumed for a long time that dextrose 

 is not burned as such in the body, but that by the action of protoplasm it is 

 first dissociated (in a very similar manner as has been described through the 

 action of alkali) to form various dissociation products, which in turn are 

 either oxidized or are reduced or condensed to form other metabolic 

 products. 1 These opinions were greatly influenced by the work of G. von 

 Liebig, who showed that the legs of frogs continued to react and produce 

 CO 2 in an oxygen-free atmosphere, and by that of Pettenkofer and Voit, 

 who demonstrated that the total protein dissociation is independent of 

 oxygen. Yoit showed that for proteins the metabolism was not proportional 

 to the oxygen supply, that metabolism was a process of cleavage of sugar, 

 fat, or protein molecules into simpler products which then united with 

 oxygen ; in brief, that absorption of oxygen is not the cause of metabolism 

 but rather that metabolism, the process of dissociation, determines the 

 amount of oxygen which is used. This whole conception has been tersely 

 stated by Lusk : " Metabolism vivifies the energy potential in chemical 

 compounds." ' 



That the production of heat is roughly proportional to the oxygen-supply 

 has been a familiar fact since the well-known researches of Garreau and of 

 Erikksson. That many plants live and grow to some extent for a limited time 

 in an atmosphere freed of oxygen is also a matter of common knowledge. 

 Many animals exhibit the same phenomenon, although to a more limited 

 extent Under these conditions the organism obtains its energy through 

 so-called intra-molecular respiration. In the higher plants and animals 

 anoxybiosis usually produces substances which are toxic to the organism, 

 and in most cases suffocation is the result of the accumulation of these 

 substances to an extent causing death. The fact that in general plants are 

 more resistant to these effects can probably be explained by their high 

 synthetic power. Thus, for example, alcohol and many hydroxyl acids are 

 converted into substances which can be drawn into the stream of anabolism, 

 especially in the light. 



From observations on the temperature rise in the fermentation of sugars 

 by yeasts, Fitz comes to the conclusion that each gram molecule of dextrose 

 fermented yields 57,500 calories. The heat of combustion of dextrose is 

 684,000 calories.* These figures give some indication of the cause of the 

 differences in the thermal relations of the two processes of oxybiosis and 

 anoxybiosis. However, it is an open question whether the thermal prop- 

 erties alone express the true energy relations in so complex a process as 



* Von, C. VON. Handbuch der Physiologie des gesammtstoffwechsels under der Fort- 



pflanzung, 6, 279, 1881. Leipzig. 



* LTTSK, GRAHAM. The elements of the science of nutrition, page 33, 1917. 



H. Grundlagen und Ergebnisse der Pflanzenchemie, II, page 161, 1909. 

 Braunschweig. 



