378 



H. A. SPOEHR 



TABLE 6 

 The effect of the water content upon the proportion of total pentose sugars 



A 

 B 



DRY WEIGHT 



16.34 

 23,25 



TOTAL SUGARS 



8.29 

 11.90 



TOTAL HEX- 

 OSE SUGARS 



6.70 

 6.36 



TOTAL PEN- 

 TOSE SUGARS 



1.50 

 5.22 



TOTAL PEN- 

 TOSE SUGARS 

 TO TOTAL 

 SUGARS 



0.181 



0.439 



TOTAL HEX- 



OSE SUGARS 



TO TOTAL 



SUGARS 



0.808 

 0.534 



It is highly probable that this fact explains the observation of 

 various workers that the total pentoses increase with advancing 

 age of the plant, as in general the water content decreases as the 

 plant grows older. Unfortunately, however, in these older in- 

 vestigations no data are given concerning the amount of water. 

 Furthermore, in using the old methods the cellulose of the walls 

 and vessels is usually also hydrolysed; this fibrous material in- 

 creases with age in the plant and is known to contain a high per- 

 centage of pentose. The fact that the pentose sugars tend to 

 disappear with increasing water content is of special interest in 

 considering the theory previously mentioned of the formation 

 of rubber from pentoses in the rubber plants. 



In the commercial culture of these plants the effect of water 

 is best expressed in a proverb used by the Mexicans of the Guay- 

 ule rubber country: "much water, little rubber."!^ This, of 

 course, must be regarded as rather circumstantial evidence in 

 favor of the pentose origin of rubber. 



The salient feature of these experiments is that the pentose 

 sugars accumulate only under conditions of low water content. 

 Little light is thrown on their origin. While Davis, Daish and 

 Sawyer in the mangold leaf observed a rise in the pentoses ap- 

 parently at the expense of the hexoses, the large number of 

 experiments with cacti are not univocal herein. The problem, 

 however, is very complex involving factors some of which are 

 impossible to control. 



The course of carbohydrate consumption during starvation of 

 joints of Opuntia sp. throws some light on the utilization of the 

 various sugars. A large number of joints of the same age and 

 cut from the same plant were kept in the dark at a constant 



'^ From a private communication of Dr. W. B. MacCallum. 



