376 H. A. SPOEHR 



is no accumulation of either pentose sugars or of pentosans in 

 the old material. In fact, in several other determinations the 

 proportion of pentose sugars in the young was sHghtly higher 

 than in the old joints. There is, therefore, no evidence here 

 which supports the opinion that pentoses and pentosans are end 

 or waste products which accumulate in the older portions of the 

 plant. 



In the cacti the daily variation in the carbohydrate content is 

 slight. Davis, Daish and Sawyer note that the pentoses and 

 pentosans rise steadily during the night in the mangold leaf 

 (mangel wurzel). This was not observed in the cacti, but rather 

 a slight diminution of these sugars during the night. 



During the course of the year there is considerable variation 

 in the total carbohydrate content of the cactus as well as in the 

 proportion of the various kinds of sugars. These variations are 

 the result of environmental influences; especially the amount of 

 available water and temperature. A rational conception of the 

 carbohydrate metabolism of a plant can be gained only through 

 the study of the equilibrium of the various sugar components 

 and the factors which influence these equilibria. This paper is 

 confined to a brief consideration of the pentose sugars. These 

 show a decided increase both in actual amount and in the pro- 

 portion to the total sugars as the water content of the plant de- 

 creases. In table 5 are given the results of monthly analyses 

 of joints of Opuntia sp. of the same age and from the same plant 

 growing out of doors. 



The first analyses were made during the dry fore-summer— 

 the pentose and hexose sugars are present in about equal pro- 

 portion. The summer rains began the middle of July and con- 

 tinued through August. During this period of rain the total 

 sugars are reduced, and especially the pentoses, bringing the pro- 

 portion of hexose sugars considerably higher. 



September, October and November were very dry months (less 

 than 1 inch rainfall) with a httle rain to December 20. The 

 increase in pentoses during these dry months is very noticeable. 

 There was regular rainfall during the last week in December and 

 January. The March samples were taken a few days after a 



