338 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



been prevented, for instance, by means of keeping the joints at a 

 raised temperature in the Hght during the winter time. However, 

 it need hardly be emphasized that the supply of simple sugars 

 can not be regarded as a single determining factor for growth or 

 the awakening of buds. Such material is essential for the con- 

 struction of new cells, but as yet no definite conclusions can be 

 drawn as to the exact physiological role of the various hexoses and 

 pentoses. When the joints are subjected to starvation, i. e., are 

 placed in the dark for periods of from one to nine months, these 

 simple sugars are used up more rapidly than they are formed from 

 the relatively large store of polysaccharides. With the decrease of 

 the supply of monosaccharides the accumulated organic acids, in- 

 termediate products of the normal respiration, are drawn into the 

 process and the total acidity of the organism is thus reduced. Re- 

 duced acidity is accompanied by an increased imbibition of the 

 cactus in water. It is also highly probable that other intermediate 

 and end products of metabolism that accumulate in the colloidal 

 substratum of the cell, and affect imbibition as will be shown in 

 the next chapter of this paper, are also removed, resulting in the 

 same effect on the water-absorbing capacity as the removal of the 

 organic acids. Thus cactus joints with a swelling capacity of 20 

 per cent, in water after being starved four months were neutral to 

 litmus indicator and showed a swelling of 100 per cent. During 

 this period the dry weight of the cactus remained the same. 



It is as yet impossible to determine definitely the carbohydrates 

 which make up the colloidal substratum of the cactus cells. Theo- 

 retical considerations would require that these be substances of rela- 

 tively slight physiological reactivity, i. e., substances which are not 

 utilized in the course of metabolism as sources of energy, and are 

 little susceptible to enzymatic disintegration. Of special importance 

 in this connection are the unfermentable sugars which have been 

 found to be present in relatively large amounts, mostly in the con- 

 densed form as pentosans. 



