DEPARTMENT OF BOTANICAL RESEARCH. 



77 



polysaccharide to monosaccharide, and resulting in a relatively reduced 

 rate of consumption and consequent conservation of the total stored 

 food material. Thus, for example, two identical sets of Opuntia 

 joints kept in the dark, the one (A) at 28°, the other (B) at 12° for 5 

 weeks showed the following values in percentages of the dry material : 



Dry weight 



Total polysaccharides . . 

 Total hexose sugar. . . . 

 Hexose polysaccharides 



Monosaccharides 



Disaccharides 



Hexoses 



Total pentoses 



p. ct. 



33.00 



16.00 



6.62 



5.49 



0.70 



0.73 



0.4S 



10.13 



p. ct. 



33.60 



16.63 



7.11 



5.47 



1.01 



.96 



0.81 



10.74 



It is evident that although the total hexose sugars and the total 

 polysaccharides are a little lower in A than in B, the proportion of 

 monosaccharides to total polysaccharides is: A = 0.044 and B = 0.061; 

 and the proportion of hexoses to the hexose polysaccharides is : A = 0.087 

 and B = 0.148. The influence on the pentose equiUbrium is not nearly 

 as marked as on the hexose sugars. The importance of these results 

 on the effect of temperature on the carbohydrate equilibrium for further 

 experimental work on the sugars produced in photosynthesis and on the 

 problems of translocation of food material is self-evident. 



EFFECT OF WATER-CONTENT ON CARBOHYDRATE EQUILIBRIUM. 



The seasonal variations of the carbohydrate-content of the cacti 

 are markedly affected by the rainfall. These plants respond quickly 

 to available water, and the changes in the carbohydrate-content occur 

 especially at the times of the well-defined periods of rainfall. With 

 decreasing water-content the proportion of monosaccharides and 

 disaccharides decreases; conversely, with increasing water-content 

 the equilibrium is shifted in favor of the simple sugars. Variations 

 in the water-content are associated with great differences in pentose 

 sugars. These show a decided increase both in actual amount and in 

 the proportion to the total sugars as the water-content of the plant 

 diminishes, and a decrease as the plant takes up more water. Thus, 

 lower temperature and higher water-content cause the carbohydrate 

 equilibrium to shift in the same direction. In view of these facts most 

 of the features of the seasonal variations can be understood. 



Experimentally these phenomena can be clearly reproduced by allow- 

 ing one set of similar joints to absorb water, while another set is kept 

 dry under otherwise identical conditions. In view of these results and 

 those on temperature many of the apparently contradictory conclu- 

 sions regarding the first sugar formed in photosynthesis and the sub- 

 sequent metabolic changes thereof can be reconciled. 



