Plant Associations at the Desert Laboratory. 63 



and choice of habitat as to call for consideration under separate 

 groups. 



(a) Plants of general occurrence on all exposures; association of 

 Fouquieria and Parkinsonia micro phylla. 

 The species of this group, more than any others, give its 

 character to the vegetation of the hill as a whole. Growing as 

 they do, equally well on all exposures, to differences of which 

 members of other groups show remarkable sensitiveness, they 

 form at once the most widely-spread and most typical representa- 

 tives of the vegetation of Tumamoc Hill. Their adjustment to a 

 wide range of physical conditions seems well nigh perfect, yet it 

 is exhibited in many different ways. Ths habits of the 

 Fouquieria and Parkinsonia, to go no further, are very different 

 as regards the production and fall of foliage leaves, arrangements 

 for protection, and other ecological characters, but they grow side 

 by side on all exposures and on various soils, so that, however 

 limited some of them are as to geographical range, it is plain that 

 within its limits they have attained a high degree of adaptation 

 to widely varying soil and atmospheric conditions, and are to be 

 reckoned as highly successful desert species. 



(6) Plants occurring on southern exposures and largely wanting 

 on northern ones; association of sahuaro and Encelia farinosa. 

 The species just named are closely and widely associated and 

 are both far more numerous on southern exposures, as well as 

 eastern and western, than on northern ones. This is strikingly 

 shown on the two sides of the gulch adjacent to the Desert 

 Laboratory on the southwest. On the right side of this gulch, 

 with a southern and partly western aspect, there are upwards of 

 70 sahuaros and the ground is almost covered with a flourishing 

 growth of Encelia, numbering thousands of individuals. The 

 left bank, on the other hand, with its generally northern aspect, 

 has less than a dozen sahuaros and only a few scattering groups of 

 Encelia. Observations of temperature and humidity and ex- 

 periments with seedlings have been in progress for some time, 

 with a view to determining as far as possible the factors to which 

 this striking limitation as regards local habitat is due. 

 (c) Plants occurring on northern exposures, but wanting on 

 southern ones; Lippia association. 

 On the left side of the gulch just referred to, and near its 

 head, Lippia Wrightii forms an extensive patch, and in the vicin- 



