WHALEY: RUBBER 23 



Rubber is then produced freely during the late part of the season. The proce- 

 dure calls either for some irrigation during the early part of the season in 

 semi-arid regions, or in California by fitting the time of planting to utilization 

 of winter rains to the greatest advantage. 



In the California plantings the small first-year plants grow fairly vigor- 

 ously all summer by drawing on the winter rainfall. As the plants become 

 larger in succeeding years they exhaust the available moisture, and thus growth 

 is slowed progressively earlier each year, leaving a longer period for rubber 

 formation. 



A very marked increase in rubber yield has been secured by selection of 

 high -yielding varieties of guayule. The plants first brought under cultivation 

 ranged, after five years growth, from less than one to over fifteen pounds in 

 weight. All of the large plants were found to be low in rubber content. It is 

 probable that most of the very large plants were progeny from crosses be- 

 tween guayule and Mariola, a related and associated plant with a 1 to 2% 

 rubber content. Selection, mostly of medium-sized plants, and the building up 

 of varieties in isolation has now established strains which consistently yield 

 20% rubber at the end of five years — an acreage yield of something just over 

 a ton of rubber. 



Guayule rubber, as extracted from the plant, is a highly resinous product. 

 When deresinated, however, it is a good quality rubber. 



Gryptostegia. Rubber from this genus was displayed at the Exposition 

 of Madras in 1856. By crude native methods rubber was produced from 

 Cryptostegia growing wild in both India and Madagascar during the nine- 

 teenth century. 



Cryptostegia grandiflora, the palay rubber vine, was introduced into 

 Mexico as an ornamental by a German sea captain. It spread rapidly in Mexico 

 and then to Florida and the West Indies. 



Cryptostegia madagascariensis ; the Madagascar rubber vine, was intro- 

 duced into Florida as an ornamental in the present century. In 1927 a natural 

 hybrid between these two species was discovered in Florida. Detailed descrip- 

 tions of these two species and the hybrid have been given by Polhamus, Hill 

 and Elder (1934). 



Cryptostegia is a much branched shrub or vine which under good condi- 

 tions, particularly as to soil and moisture relations, makes rapid growth. The 

 latex in Cryptostegia is in a latex tube system. It can be obtained by tipping 

 the stems, or by chemical or mechanical extraction from the leaves and stems, 

 though the latter has been done only in experimental studies and is not com- 

 mercially practical. The rubber secured from stem tipping is superior to that 

 extracted by other means but the yield per stem is extremely small and the 

 amount of labor involved is tremendous. 



