62 The Pi.ant World. 



A few other species are of occasional occurrence in this spec- 

 ial habitat of Larrea. Thus on the slope at the foot of Tumamoc 

 Hill a very few individuals of Fouquieria splendens are established, 

 and in places Opuntia fulgida is frequent; none of these, however, 

 approach the creosote-bush as the universal and distinctively 

 characteristic species of this zone. 



As the slope advances upward with the long continuance of 

 disintegration and erosion, the creosote-bush advances with it, 

 and the presence of this plant, both as the successful pioneer 

 and the final possessor of the soil, may be seen almost every- 

 where throughout its range. Thus we have the interesting case 

 of a well-marked habitat, with a single species forming in it, 

 at least in places, a perpetually renewed close association of its 

 own. 



The description just given applies to the slope as seen near 

 the foot of Tumamoc Hill and in situations such as the "alluvial 

 fans," where the conditions of slope and soil characters are es- 

 sentially similar. There are wide areas commonly included 

 under the term "mesa" upon which a greater variety of con- 

 ditions, with corresponding differences of vegetation, prevails. 

 An example of this is the ground included in the Laboratory do- 

 main lying to the west of the wash presently to be described. 



(h) Franseria Association. 



Beyond the wash, on the Tucson slope west of the Laboratory 

 and elsewhere on the domain, areas of considerable 

 extent occur where there is a soil of coarse texture, 

 approaching gravel, on which Franseria deltoidea and Opuntia 

 fulgida constitute the greater part of the vegetation. The former 

 is often very numerously represented, and in many places is not 

 accompanied by the cholla, but where the two attain their best 

 development they are likely to occur together. Frequently 

 Krameria canescens and a few other species are associated with 

 them. As far as appears from present evidence, this association 

 is one that is determined first of all by soil relations. These are 

 described in a later paragraph. 



(7) The Hill. 



Above the Larrea zone the hill is the habitat of a large 

 number of conspicuous and characteristic perennials, which, 

 although they may be classed together as a single association, 

 differ among themselves to such an extent in ecological traits 



