188 Guayule. 



RELATION OF RUBBER AND RESIN. 



A notion has been widely entertained that the amount of rubber in 

 the guayule plant is in some way related to the amount of resin. This 

 naturally grew out of the fact that commercial rubbers always contain 

 more or less resin, and that resin is abundant in the guayule. In the 

 preparation of the commercial article from the guayule the resin becomes 

 intermingled with the rubber to the amount of 20 per cent (Whittelsey, 

 1909). There appears, however, to be no adequate evidence in support 

 of this notion, while on the other hand there is strong evidence to show that 

 the physiological processes involved in the secretion of these two materials 

 are quite distinct: 



1 . The canals which are laid down in the protogenic tissues become 

 actively secreting as regards resin long before rubber appears at all. This 

 is strikingly evident in irrigated plants, in which the amount of growth 

 is very much in excess of that in field plants. 



2. Resin is secreted in other Compositae in which rubber does not 

 occur. In the closely related mariola (Parthenium incanum) resin is 

 abundant, while rubber is very meager in amount; and this is true of 

 many others. 



3 . In irrigated plants the amount of resin is correlated with the ana- 

 tomical conditions within the organism, while the secretion proper appears 

 to be neither retarded nor advanced by the presence of water. Water, on 

 the other hand, affects markedly, though probably indirectly, the rate of 

 rubber secretion, which lags behind growth. But the lagging behind of 

 rubber secretion is not in inverse relation to any possible increase which 

 may be shown to occur in the secretion of resin. 



4. The distribution of starch appears to be connected with the secre- 

 tion of resin, as in other well-known instances (e.g. , Pinus] . The secretion 

 of resin appears, as above pointed out, to be extra-protoplasmic, and in 

 harmony with the view expressed by Tschirch, already alluded to. 



5. Rubber, however, appears in the tissues independently of the dis- 

 tribution of starch referred to in (4) above. However, the starch found 

 in the young tissues near the growing apex may serve as a source of ma- 

 terial for the elaboration of rubber. 



6. The appearance of rubber in the canal-cells might be cited-to sup- 

 port the view under discussion, but for the fact that the rubber is merely 

 accumulated in these cells and that this occurs later than the secretion 

 of resin. Further, rubber occurs in other tissues, e.g., parenchyma rays, 

 far removed from resin secretion. Resin in the canal-cells has not been 

 demonstrated, but in the meatus only. 



THE SIGNIFICANCE OF RUBBER. 



The inevitable question as to the use of rubber to such a plant as the 

 guayule, subject as it is to the severe conditions of the desert, has been 

 raised and must be met in some wise. I have already briefly discussed 

 the matter (Lloyd, 1909) with but meager satisfaction, as will appear to 

 those inclined to find a use for everything in animate nature. I can only 

 repeat here what I have already said. 



