120 



Guayule. 



of bark to wood has increased during the period between the dates given 

 above, as shown in brief in table 47. 



As will be seen, this change in volume in the irrigated cortex is to be 

 referred chiefly to an increase in the rubber-content. 



Table 46. Relative amount of bark and wood in guayule, by volume {dry). Field 



plants {Sept. 1908). 



No. 1, basal portion of trunk; No. 2, a lateral root; No. 3, a tap-root; Nos. 4 

 to 18, series from a single plant; No. 19, San Isidro, Chihuahua, large plant; No. 20, 

 Chihuahua, trunk, seedling. 



A discrepancy which always appears between the ratios by weight, 

 not here given, and by volume is greater for the irrigated plants on ac- 

 count of the larger rubber-content of the wood cylinder in the field plants. 



The smaller ratios for the smaller twigs (Nos. 12 to 18 inclusive) for 

 field plants in table 46 as compared with those in table 42 are due in 

 part to the fact that the material was collected in the height of the grow- 

 ing-season, and hence the new growths have something of the character 

 of the irrigated plant in its low rubber-content especially. The impor- 

 tance of this fact in its relation to practice is shown elsewhere. 



Table 47. Ratios of bark to wood in Cedros irrigated plants collected in 

 September 1908 and in April 1909. 



It is interesting to note, further, that the ratios for root-tissues are 

 similar in both types of plant, but are smaller in irrigated plants. They 

 are also much smaller than those for the stem in each type. This fact 

 should be considered in making up averages to indicate the relative eco- 

 nomic value of cultivated guayule. The roots, in proper practice, should 

 not enter into a calculation for returns in manufacture, and by excluding 

 them the average ratios (those, namely, for stems only) are higher, but 

 relatively higher for field plants, as shown in the tables above. 



