EFFECT OF WATER ON THE CARBOHYDRATE-CONTENT. 57 



VI. EFFECT OF WATER ON THE CARBOHYDRATE- 

 CONTENT. 



It has already been pointed out that the platyopuntias respond quickly 

 to available water, and that although the rate of water-loss is low, the plant 

 is capable of undergoing relatively great fluctuations in its water-content. 

 Under natural conditions these plants are exposed alternately to conditions 

 of extreme drought and to abundant water-supply. The changes in the 

 carbohydrate-content occur especially at the times of the well-defined periods 

 of rainfall. Low temperatures influence the carbohydrates in the same 

 manner as high water-content does; both result in a condition of general 

 inversion. It was necessary, therefore, to determine independently the 

 influence of changes in water-content on the carbohydrate equilibrium. For 



TABLE 20. Loss of weight of joints of Opuntia phceacantha 

 in a humid atmosphere. 



TABLE 21. Loss of weight of joints of Opuntia phceacantha 

 in a dry atmosphere. 



this purpose a number of healthy joints of the same age were cut from the 

 same plant. These were divided into three sets of seven joints each ; one 

 set was analyzed immediately, another set was placed under a large bell-jar 

 over a dish of water, and the third set was placed under a similar bell-jar 

 over a dish containing anhydrous calcium chloride. A slow stream of moist 

 and dry air respectively was passed through the bell-jars. These were kept 

 in the dark in a constant-temperature chamber at 20 C. The experiment 

 ran for 44 days ; at intervals the plants were weighed to follow the loss of 

 weight. In table 20 the results of these weighings are given. 



Thus the joints which were left in a humid atmosphere lost in 44 days 

 only 6 grams or 1.5 per cent of their total weight. At the end of this time 

 the water-content had fallen from 81.5 to 81.2 per cent. The joints kept 

 over calcium chloride lost much more water, as is shown in table 21. 



