The Origin and Occurrence of Rubber. 



185 



ruption from August 1908. Rubber was found in dense rounded agglom- 

 erations throughout, but evidently not reaching a maximum content 

 (plate 43, fig. 2). 



(c) A branch from a plant grown at Caopas, from stocks transplanted 

 by Don Teofilo Delgadillo about January 1908 and taken in October 1909. 

 These had less irrigation than the above-mentioned Cedros plants. 1908 

 growth: the rubber was densely agglomerated in the whole of the con- 

 junctiva (plate 43, figs. 3, 4), in amounts exceeding that in Cedros material 

 (plate 43, fig. 2); the 1909 growth contained rubber in coarse granules 

 more or less agglomerated throughout. 



6. Irrigated seedlings of all ages up to five months were examined. 

 Very young individuals were seen which contained no rubber at all. A 

 five-months-old seedling (plate 20, fig. B) contained rubber in coarse 

 granules throughout the conjunctiva, being in sufficient quantity in the 

 secondary cortex to become agglomerated. 



The method which was used in obtaining the foregoing data, despite 

 its limitations, could doubtless be used by the grower of guayule, enabling 

 him to follow the behavior of the plants under his charge. The evaluation 

 of the evidence is somewhat difficult, but it could be mastered, as may 

 be seen, I think, on examining plates 40 to 43. The final control must, 

 however, be had by chemical analysis. Tables 52 to 54, which follow, 

 contain a few results which comport with the evidence preceding. 



RUBBER-CONTENT BY CHEMICAL METHODS. 



The analysis of the guayule plant in order to determine its rubber and 

 resin content presented unexpected difficulties, but the results attained, 

 after these had been met, are undoubtedly more reliable than earlier 

 analyses. I therefore adopt them as exposed in table 52 (Whittelsey. 

 1909, pp. 3, 5). 



Table 52. Percentage of rubber in various parts of guayule shrub. Field plants. 



'The percentage of pure rubber in the whole trunk is 9.9, the whole 

 root 7.8, the branches and leaves 9.7, and in the whole plant 9.5, * * * 

 based on perfectly dry material. If 'mill weight' is taken as a basis, the 

 percentage of pure rubber in the whole plant is 7.8." This result is found 

 to correspond very closely to factory experience and the more accurate 

 published results, and we may therefore adopt it as exact enough for the 

 present purpose. 



The only figures available for irrigated plants are given in table 53 

 on the following page. 



