The Environment. 25 



attain a height of a foot or two ; but this is not true of the guayule. That, 

 however, the mere quantity of water or the density of the vegetation 

 are not the determining factors is shown by experimental evidence, 

 while in the field are to be found numerous instances of plants which 

 have germinated in the dense shade and dampness found beneath the 

 dead leaves of the sotol and in crowded conditions produced by other 

 plants, such as the lechuguilla. Indeed, these are frequently the only 

 conditions under which the plant gains a foothold. It therefore does 

 not appear probable that the abundance of water or the density of the 

 vegetation is the determining factor in preventing the guayule from get- 

 ting a start; hence we may infer that the conditions below the surface 

 must be understood before an explanation may be had. The edaphic 

 habitat of the plant suggests that the mechanical conditions of the allu- 

 vial soil are unfavorable, owing to meager aeration, in connection with 

 which the humus conditions also may have to be taken into account. 



DENSITY OF GROWTH. 



Of great importance economically as well as to the student of vege- 

 tational problems is the number of plants per unit of area, both abso- 

 lute and relative. The operations of the forester rest upon this datum 

 in the first instance, as this, together with the size of the individuals, 

 forms the basis of calculations of the available tonnage per acre. It 

 will readily be understood that any estimate on a large scale will involve 

 a necessarily large error, since it would be impossible to do more than 

 proceed on the basis of sample counts combined with acreage and esti- 

 mates of size. This can frequently be done with great accuracy by persons 

 who have had practical experience in taking guayule from the field, 

 especially if the judgment be checked by survey and sample counting 

 and weighing. The following tables, the data for which were obtained 

 by accurate measurement, will, however, serve a useful purpose in indi- 

 cating a method of making estimates, as well as in furnishing indications 

 of actual conditions. For the purpose, quadrats of ioo square meters 

 were laid out by means of a steel tape, the data obtained attaching to 

 the guayule plants within each such quadrat. 



The weights in the following tables are field weights. For dry 

 weights a reduction of 20 to 25 per cent is necessary. As field weight is 

 usually assumed, however, I have followed the usage and have not applied 

 the above correction. 



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