SEASONAL VARIATIONS IN THE CARBOHYDRATE-CONTENT. 55 



TABLE 17. Carbohydrate-content of young and parent joints of Opuntia phwacantha. 

 Values given in percentages of the dry material. 



The analyses were made in October. From the same plant were taken a 

 number of joints formed in the spring of the year 1916, as well as a number 

 of joints which had been formed three years or more previous, as was 

 indicated by their position on the plant; i. e., the old joints were fourth in a 

 row counting from the youngest joints. The results are given in table 18. 



TABLE 18. Carbohydrate-content of young and old joints of Opuntia phccacantha, 



expressed in percentages of dry material. 



Although the cellulose is higher, it is noticeable that the pentosan content 

 is lower in the old joints. This was found in a number of other analyses 

 as well. In general, the carbohydrate content of the younger joints is 

 higher than in older ones, though the proportion of the various groups of 

 sugars to the total sugars is about the same in the two sets of joints. 



As yet it has not been possible to obtain entirely satisfactory results on 

 the photosynthetic activity of the mature cacti. The complexity of the gas 

 interchange and the influence thereon of the high acidity as described by 

 Richards, has already been alluded to. The diurnal variation in carbo- 

 hydrate content is slight in these plants. In table 19 are given the results 

 of analyses made before and after insolation. 



The loss of water during the night, resulting in an increase of the total 

 dry weight, is evident. This is a phenomenon commonly observed in these 

 plants. The increase of the disaccharides and monosaccharides, both in 

 actual amount and in the proportion to the polysaccharides, is considerable. 



