328 PHYSIOLOGY OF NUTRITION. 



only a very limited interchange of gases with the exterior. Oxygen 

 is not at the disposal of their cells in excessive quantity, and the 

 combustion of the carbohydrates is therefore only incomplete. It 

 does not, under certain conditions at least, proceed as far as the 

 production of Carbon dioxide and water, but considerable quan- 

 tities of organic acids accumulate as products of incomplete 

 oxidation. 



The process of acid decomposition which is constantly going on 

 side by side with the process of acid formation in the tissue of the 

 Crassulaceee is, as regards the energy with which it proceeds, 

 highly dependent on conditions of temperature and illumination. 

 High temperature and exposure to light very considerably ac- 

 celerate the decomposition of the acids. Hence the acidity of the 

 tissue in Crassulaceous plants diminishes when they are exposed 

 in the dark to a high temperature, or when they are submitted to 

 the influence of light. In darkness, on the other hand especially 

 at a low temperature the acidity of the sap of the Crassulacen? 

 rises. And indeed we know that the acidity of the sap in these 

 plants undergoes a daily periodic variation. In. the daytime the 

 sap is slightly acid in reaction, at night strongly acid. 



The decomposition of the acids is essentially an oxidation 

 associated with production of Carbon dioxide. The acid already 

 formed by oxidation undergoes further and complete oxidation, 

 and the" question now arises how it happens that the rays of light 

 accelerate the process. It has been already mentioned that the 

 Crassulaceae can for various reasons carry on only a limited inter- 

 change of gases. This determines the collection in the tissue of 

 large quantities of acid. In darkness there is a deficiency of Oxygen 

 in the tissues ; access of light, on the other hand, markedly increases 

 the quantity of Oxygen in the tissues, since Oxygen is set free in 

 assimilation. The assimilating chlorophyll grains do not take 

 part directly in the process of acid decomposition, but they do 

 indirectly by causing the liberation of considerable quantities of 

 Oxygen in the tissues, which in turn effect complete oxidation of 

 the acid. The Carbon dioxide thereby originating can again be 

 worked up in the chlorophyll corpuscles, and the Oxygen set free is 

 once more in a position to accelerate the decomposition of acid. 



These relations are of the very first significance in explaining 

 the increased decomposition of acid, and they are of themselves 

 sufficient to account for the acceleration of this process under the 

 influence of light. But it appears probable to me that the rays of 



