56 



THE CARBOHYDRATE ECONOMY OF CACTI. 



How much of this increase is due directly to the photosynthetic activity it is 

 difficult to establish, as the independent influence of the changed water- 

 content on the proportion of simple sugars to polysaccharides in all prob- 

 ability comes into play. This will be discussed in the following section. 



TABLE 19. Variation in carbohydrate-content of Opuntia phwacantha during 

 21f hours. Results in percentages of the dry material. 



In perusing the results of the seasonal variation in carbohydrate-content, 

 of significance is the fact that the greatest activity of the plant comes at 

 the time when the content of monosaccharides and disaccharides is highest. 

 The vegetative shoots and flowers are developed in March, and it is at this 

 time that the general process of inversion has reached its maximum. This 

 process of carbohydrate inversion, taking place in the plant as a consequence 

 of low temperature and ample water-supply, can be prevented by artificially 

 keeping the plant either at a higher temperature or with a low water-supply. 

 In neither case does the plant then develop vegetative or flower buds at the 

 regular season. Thus, a number of plants of Opuntia phwacaniha were put 

 in soil, and so placed out of doors that they obtained only a minimum 

 amount of rainfall during the winter rainy season. These plants produced 

 no flowers or vegetative shoots. Other plants similarly treated, but allowed 

 to receive the normal rainfall, developed normally in the spring. Again, 

 a number of plants were kept at higher temperatures than normal by cover- 

 ing them with glass cages. Through the " trapping " of the solar radiation 

 thus effected, the temperatures within the cages were decidedly higher than 

 the surrounding air. The cages were not large enough to prevent the rain 

 from reaching the extensive root system. ISTone of the plants thus treated 

 produced either flowers or new joints in the spring. Similarly, plants which 

 had been kept in glass cages artificially heated during the Avinter showed the 

 same behavior. It can not be maintained, of course, that the accumulation 

 of relatively large quantities of the simpler sugars is the only prerequisite 

 to growth. It is not possible to indicate any single factor or substance to 

 which can be ascribed such properties, nor is such a state of affairs likely to 

 exist. Physiological activity in all probability represents the " resultant 

 of forces " and a supply of simple sugars above that required for the normal 

 respiratory activity seems to be one of the factors necessary for growth. 



