74 



THE CARBOHYDKATE ECONOMY OF CACTI. 



The gain in water-content of the plants which had been placed with 

 their roots in solutions is very apparent. The effect on the carbon-dioxid 

 production is very striking in the case of the cane-sugar and dextrose ; both 

 show a very decided increase. The plants which were in the sunlight show 

 but a very slight gain. In none of the cases is the gain in carbohydrate- 

 content very great. This was somewhat surprising, as the plants before 

 the starvation contained 20.60 per cent total sugars, 4.50 per cent hexoses, 

 and 8.28 per cent total pentoses. The plants in a solution of cane-sugar 



TABLE 38. Carbohydrate-content in percentages of the dry material of Opuntia 



ph&acantha. 



A starved 189 days, B subsequently for 60 days in 1 per cent cane-sugar, C in 1 per 

 cent dextrose, D in sunlight. 



show a greater gain in disaccharides, while the plants in dextrose show a 

 greater gain in hexoses. The joints which had been in the sunlight exhibited 

 but very slight gains, the hexoses being even less than in the original 

 starved condition. This is probably of considerable importance for an 

 understanding of the nature of the photosynthetic process. It has been 

 found that a similar condition exists in thin leaves, the rate of photosyn- 

 thesis varying with the rate of respiration, suggesting an intimate associa- 

 tion between photosynthesis and the respiratory activity. Investigation on 

 this subject is now in progress. In all three cases the pentosans show the 

 usual diminution with increased water-content. 



