52 



THE CARBOHYDRATE ECONOMY OF CACTI. 



This series runs from the arid fore-summer through the humid mid- 

 summer, the arid autumn, and into the beginning of winter with its higher 

 rainfall and lower temperatures. What, then, are the changes which occur 

 in the carbohydrate-content of these cacti when exposed to these decided 

 changes in the environment ? The values in tables 13 and 14 for June repre- 

 sent the effect of extreme desiccation and high temperatures ; there fell only 

 0.68 inch of rain in the preceding April, none in May, and but a trace in 

 June. It is evident that here the polysaccharides, both in actual amount 

 and in the proportion to the other sugar groups, are high. The mono- 

 saccharides, on the other hand, by the same criteria, are low, while the 

 pentosans and total pentoses are relatively high. In the analysis of July 5 

 the desiccation has proceeded, though this was taken immediately after the 

 first rain and the effect on the carbohydrates is hardly perceptible. By 

 July 31, the next analysis, 3.37 inches of rain have fallen: the water-con- 



TABLE 15. Seasonal variation in carbohydrate-content. Analyses expressed in 

 percentage of the dry material. 



tent is decidedly higher, though the temperatures continue high. The 

 polysaccharides, both in actual amount and in proportion to the other sugar 

 groups, have fallen. The monosaccharides and the hexoses are higher than 

 during the dry periods. The pentosans show a marked decrease with the 

 increased water-content. The September analysis represents the beginning 

 of the arid autumn, there has been little rain, and the water-content has 

 dropped. The total polysaccharides have again risen, also in proportion to 

 the other sugars ; the monosaccharides show a corresponding drop, and the 

 pentosans have also risen. During the following months, October, Novem- 

 ber, and December, the drought continues, with corresponding low water- 

 content. The amount of the polysaccharides again rises during October. 

 In November and December, however, in spite of the continued dryness, 

 the relative values again decrease. So also the monosaccharides, while very 

 low in October, increase in November and December. The pentosans show 

 a constant increase until they drop slightly in December. It appears, there- 

 fore, that the effects of the drought are first moderated and then counter- 

 acted by the lower temperatures prevailing during November and December. 



