358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1945 



has a large pilot plant and laboratory seeking better methods of rub- 

 ber extraction, under the direction of F. Macdonald. 



Dr. W. G. McGinnies of the Forest Service heads a division which 

 has made an almost foot-by-foot survey of the probable range of the 

 plant, determining and classifying the areas adapted to its culture, and 

 studying the behavior of indicator plots. T. P. Cassidy of the Bureau 

 of Entomolog}' and Plant Quarantine has made an intensive study 

 of the plant's insect relationships. Other members of the organiza- 

 tion have invented or improved cultural machinery, including a new 

 and simple seed-collecting machine which enabled the project to har- 

 vest more than one million pounds, rough weight, last season. 



Past research in plant improvement has been confined almost en- 

 tirely to selection among varieties and strains, but the geneticists are 

 now tackling actual breeding up of the plant. It has certain peculiari- 

 ties that promise to challenge their ingenuity, but there is every reason 

 to suppose that the plant is susceptible of considerable improvement. 

 It is very choosy about its site requirements; it demands a friable, 

 well-drained soil and contracts various fungous diseases if conditions 

 are unfavorable. It also does not like temperatures much below freez- 

 ing. These characteristics greatly restrict its range, of course, and 

 the scientists are investigating the possibility of increasing its toler- 

 ance in those respects. 



RUBBER-EXTRACTION PROCESS 



The rubber-extraction process is virtually the same one the industry 

 started out with 40 years ago, and while it recovers a satisfactory pro- 

 portion of the rubber from the shrub, it is cumbersome and expensive 

 and also mixes with the rubber foreign substances that the processors 

 would very willingly dispense with. Briefly, the milling process is 

 about as follows. 



After being dug or pulled out of the ground, the shrub is cured 

 in the field 2 or 3 days, then baled and stored till needed. In the mill, 

 it is chopped up and the chips dried down to about 15 percent moisture 

 content. It is then crushed, mixed with water, and fed into a series 

 of pebble mills. These are revolving tubes lined with rough silica 

 bricks and about one-third filled with flint pebbles. The pebbles cas- 

 cading down the wall thoroughly macerate the material in the slurry, 

 freeing the rubber from the fiber. The slurry is then discharged into 

 a vat, where the water-logged wood sinks and the rubber floats and 

 is skimmed off. After certain other cleansing operations the rubber 

 is dried and pressed into 100-pound cakes. 



The mill at Salinas, which is capable of handling about 30 tons of 

 shrub in 24 hours, cost some $207,000 to build in 1931. There is not 



