194 TROPICAL AGRICULTURE 



or chemical nature or both. The resulting product retains 

 its elasticity much longer and through a much greater range 

 of temperature than is true for pure rubber. Rubber may 

 be vulcanized by heating a mixture of rubber and sulphur, 

 or by dipping in melted sulphur, or by treating the rubber 

 with sulphur monochlorid. In the simple process of heating 

 a mixture of rubber and sulphur, the rubber is ordinarily 

 mixed with 4 to 40 per cent, of sulphur and heated to a 

 temperature of 125 to 150 C. 



Rubber is an elastic substance belonging to the hydrocarbon 

 series and having the chemical formula (C 10 H 16 )x. It is 

 obtained by coagulating the latex of a number of plants which 

 are native chiefly to Central and South America and Central 

 Africa. The milky juice or latex of rubber-bearing plants 

 is contained in the series of latex tubes and communicating 

 structures which together constitute the latex system. The 

 arrangement of the latex tubes is somewhat different in dif- 

 ferent species of trees. In Kickxia they are distributed chiefly 

 just under the epidermis, immediately outside of the cambium 

 and in the outer part of the pith bordering on the woody 

 tissue. There are numerous strands of the latex system 

 connecting the latex tubes in the outer and inner portions of 

 the bark. The main latex system in Hevea or the Para rubber 

 tree is an inner belt located about halfway between the epi- 

 dermis and the cambium and no latex tubes occur in the pith. 

 In Ficus, the latex tubes are chiefly found in the bark near 

 the cambium. In young trees, however, they occur throughout 

 the pith. In the Castilloa, the main latex system is in the 

 bark and a few strands of latex tubes are located in the pith 

 immediately underneath the wood tissue. In Ceara rubber 

 trees, the latex tubes are found almost exclusively in the bark 

 outside of the cambium. In order to obtain a full yield of 

 latex, therefore, it is unnecessary to injure the cambium. The 

 large number of connecting tubes between the main longi- 

 tudinal trunks of the latex system is conspicuous in Ceara 



