92 The Plant Wori.d. 



and the association of Franseria and cholla, which (within our 

 limits) occurs just beyond the wash on the west half of the Lab- 

 oratory .iomain. Each of the three is characteristic and well- 

 defined. They are evidently deter'xiined by soil relations, in 

 which water-supply is a dominant factor. 



The different behavior of certain species of these associations 

 as regards strict maintenance of habitat choice is especially 

 worthy of note. While the catclaw, for example, hardly passes 

 beyond the limits of the wash, the creosote-bush not only invades 

 this, but grows there far more luxuriantly than within its own 

 special habitat. The relations are complicated, but it is plain 

 that this behavior of the creosote-bush is the result of a greater 

 capacity of adjustment on its part to differences in amount of 

 soil-water than is possessed by such plants as the catclaw and 

 palo verde. 



Associations of the hill. — Up to this point it is manifest, even 

 upon casual observation, that soil relations have had a prepond- 

 erating influence in determining the different associations of 

 plants and their limits. On coming to the hill, however, it is 

 apparent that another factor, ntimely, aspect, or directior of slope 

 has also exerted a marked influence in determining the composi- 

 tion and place of several of the associations. This is seen with 

 special clearness in the case of the Lippia asso;ij,tiDi ani that 

 ot the sahuaro and Encelia jarinosa, the former occurring here ex- 

 clusively on north exposures, the latter well represented only on 

 the east, west, and especially on the south sides of the hill. 

 Plants of the two remaining associations are entirely indifferent 

 to aspect, and in this respect stand at a decided advantage. 



The association of annuals, belonging to superficial soil- 

 layers, though well represented on the hill, is not confined to it. 

 Temperature and soil-water are obviously the factors which de- 

 termine the appearance in their seasons of the plants composing 

 the two widely different biological groups of this association. 



To avoid unnecessary diversion the parasitic, symbiotic, 

 and miscellaneous introduced species are here omitted from dis- 

 cussion. Excluding these, there are in the four natural groups 

 that have been described 12 well-defined associations of plants 

 exhibiting habitat preferences which, on the evidence obtained 

 from careful observation, must be correlated first of all with 



