58 



THE CARBOHYDRATE ECONOMY OF CACTI. 



After 44 days of desiccation these plants lost 71 grams or 19.5 per cent of 

 their original weight At the end of the experiment the water-content was 

 YT.O per cent, which, of course, does not equal the extreme desiccation the 



TABU: 22. Carbohydrate-content of Opuntia phceacantha before and after 44 days in 

 the dark at 20 C. in percentages of the dry material. 



plants attain under natural conditions. The processes which take place 

 during such a period of desiccation are, of course, by no means simple, and 

 it can not be considered as a simple loss of water. In the first place, the 

 factor of respiration or the consumption of carbohydrates is to be taken into 

 consideration. If it is assumed that the reaction is of the empirical nature 

 C 6 H 12 O 6 + 6O 2 -^6H*O -f 6CO 2 , there is formed for each gram of sugar 

 burned 0.6 gram of water, which can be considered as remaining within the 

 plant. For each gram of total loss in weight of the plant due to carbohy- 

 drate burning, 2.5 grams of sugar must be consumed, resulting in the 

 formation of 1.5 grams of water. Thus the respiration of the plant tends 

 not only to reduce the dry mass but as well to increase the water-content. 

 A second complicating factor is introduced when it is considered that the 



TABLE 23. Proportional values of carbohydrate-content of Opuntia phceacantha 

 before and after 44 days in the dark at 20 C. 



hydrolysis of polysaccharides takes place accompanied by the absorption of 

 water. This water is incorporated in the chemical composition of the 

 sugars. The withdrawal of water by this process tends to increase the dry 



