THE PRINCIPAL RUBBER- YIELDING PLANIS 43 



Largo quantities of Jelutong are exported from Singa- 

 pore to Eui'ope and America for use in rubber manu- 

 facture. 



V. COMPOSITAE 



21. Parthenium argentatmn, A. Gray, the Guayule 

 rubber plant. 



This plant is of interest as it is the only member of the 

 natm-al order Compositae which furnishes rubber in 

 sufficient quantity to be of commercial importance. It 

 is a small shrub which attains a height of about 3 ft. 

 and has a much branched stem bearing small silver-grey 

 leaves and yellow flower heads. The plant formerly oc- 

 ciu-red wild over a large portion of the " bush prairies " 

 in northern Mexico, but owing to the extent to which it 

 has been exploited durmg recent years the supply is 

 rapidly diminishing. 



The latex is contained in isolated cells which are princi- 

 pally situated in the bark, but also occur in the pith and 

 the medullary rays. The rubber cannot be obtamed 

 by making incisions in the stem as the latex does not 

 exude, and it must, therefore, be extracted by a me- 

 chanical process or by treatment with solvents. The 

 crude rubber contains a rather large proportion of resin 

 (20 to 30 per cent.), but a superior product is obtained 

 by removmg the greater part of the resin by treatment 

 with a suitable solvent. The yield of crude rubber is 

 stated to be from 8 to 12 per cent, according to the 

 amount of moistme present in the plants treated, and the 

 yield of the purified rubber from 7 to 8 per cent. 



