22 T O R R E Y A 



The supply of wild guayule is strictly limited, however, and, except for the 

 imposition of conservation measures, would have been exhausted some time 

 ago. It is estimated that a constant production rate of 7,000 to 10.000 tons a 

 year would be possible in Mexico without depleting the natural supply. 



Any increase in the production of guayule depends upon cultivation of the 

 shrub. Two projects for the furtherance of guayule cultivation were begun 

 in the early days of the war. One of these has been directed toward intensive 

 cultivation of the plant in the United States, largely in California, the other 

 toward a considerably less intensive cultivation in areas of Mexico presenting 

 adapted lands but a vastly different economic picture (Brandes 1942a). The 

 procedures of maintenance, and to some extent those of planting and harvest- 

 ing, are unlike in the two areas, but fundamentally the problems of domestica- 

 tion are the same. 



To begin with germination of guayule seed when and where desired is a 

 difficult accomplishment. In the wild, germination hinges on the presence of 

 large amounts of moisture for the elimination of the inhibitors. Nursery prac- 

 tice consists of chemical treatment of the seed followed by preplanting 

 germination in chambers. The seeds are then mixed with sawdust or sand, 

 and kept very moist until the seedlings are established. Naturally this proce- 

 dure presents serious problems in disease control. The use of rigid sanitary 

 measures, strict water regulations, and use of resistant strains is dictated. 

 Seedlings are grown in nursery beds usually from March or April until 

 January of the following year. The length of time allowed between transplant- 

 ing and harvesting depends a great deal upon conditions. At least two years' 

 growth is necessary and maximum rubber content is not reached for many 

 years. The plan for cultivation in California originally called for harvesting 

 the major portion of the plants in the fourth year. Because of the urgency for 

 early production it is planned to harvest much of the area after only two years. 



Most interesting of the problems connected with the growth of guayule in 

 the field are those arising from the relation of growth to rubber formation. 

 It was early observed that if conditions for growth are very favorable plants 

 of guayule would grow to a very large size but would contain practically no 

 rubber. On the other hand small, but mature plants often contained large 

 percentages of rubber. Study has revealed that little rubber is formed during 

 periods of active growth, and that if growth has been very good little rubber 

 is formed in a subsequent less active growth period. In guayule the cortical 

 tissue is the main rubber-bearing region. This tissue is laid down, or at least 

 differentiated mostly during periods of slow growth, and only following a 

 general growth phase. A seasonal rhythm of growth and rubber formation 

 has been found and cultivation practices are now directed toward following 

 this rhythm. Attempts are made to produce fairly vigorous growth early in 

 the season and then permit a gradual drying to limit growth by mid-summer. 



