24 



Guayule. 



associates above mentioned, an "edaphic" species, found only where 

 the ground is stony. In the alluvial plains one meets only an occasional 

 isolated plant, but if the plain is traversed by a low ridge of gravelly 

 ground, even if the surface is raised only a few inches above the surround- 

 ing area, the guayule may be found. In the fine soil of the plain, on the 

 other hand, the mariola (Prathenium incanum H. B. K.) and the annual 

 species P. hysteropharus grow in abundance, though the mariola is com- 

 monly associated with guayule on the foot-slopes and hills. This asso- 

 ciation of guayule and mariola frequently misleads the inexperienced 

 observer in estimating the amount of guayule which may be found in a 

 given area. 



Why the guayule does not grow in the fine alluvium is not clear, 

 and is a question often asked by persons familiar with the facts. Any 

 reasons, aside from those mentioned above, which may be assigned are 

 at present of only speculative value, but some reference may properly 

 be made to them. 



FIG. 4. Dasylirion cedrosanum T release. Type material in the lower row. Above, for comparison 

 fruits of D. wheeleri at the left and of D. graminifolium at the right. X 3/1. 



Guayule is confined practically to the Cretaceous region of the Cen- 

 tral Plateau, and therefore to highly calcareous soil (see Chapter IX). 

 It may very well be that the plant is sensitive to even a slight acidity, 

 and therefore prefers a soil with a very small amount of humus. Certain 

 experimental results referred to beyond, while not conclusive, indicate that 

 this explanation may apply during the period of germination, but it has 

 been found that the absence of lime is not a hindrance to maturer plants. 



It is a popular notion that the plant "rots" in situations where 

 water is relatively abundant, and that for this reason it is not to be found 

 in "bajillos" or low places. It is true that for a considerable period in 

 the summer season practically mesophytic conditions prevail in many 

 areas within the flats, especially in the frequent slight depressions. Here 

 annual weeds grow in profusion, and a number of species of desert shrubs 

 flourish. Among these is the mariola, the seeds of which germinate 

 freely among the dense vegetation of shrub and weed, and in one season 



