Plant Associations at the Desert Laboratory. 



65 



even more conspicuously than perennials. It is evident, how- 

 ever, from the facts cited in the case of Lippia, that this choice 

 of aspect is not necessarily constant, and that direction of slope 

 determines distribution merely as it presents a combination of 

 conditions which may change so greatly, even within a few miles 

 as to completely reverse the aspect preference of a given species. 



{d) Plants of the cliffs; Hyptis-Nicotiana association. 



One species of each of the genera just named is found grow- 

 ing almost exclusively on the abrupt cliffs of Tumamoc Hill and 

 on similar clifTs elsewhere. Celtis pallida, though by no means 



Fig. 4. Deltais of vegetation near Desert Laboratorj-. Encelia jariiiosa in flower. 



confined to the cliffs, is of common occurrence on them, where it 

 evidently finds a congenial home. The factors concerned in de- 

 termining the choice of habitat exhibited by members of this 

 association are not known. It may well be that where there is 

 so little soil the result may in part be the outcome of competition, 

 but regarding this point we are without positive knowledge. 

 Hyptis and Nicotiana are rarely seen growing elsewhere, though 

 the latter is sometimes found growing in the sandy soil of washes, 

 and in this latter habitat Celtis is of rather frequent occurrence. 

 I am disposed to think of water supply as again the main factor. 



