6 THE RELATION OF DESERT PLANTS TO 



The problems here dealt with concern the relations between certain 

 desert plants on the one hand and their physical environment, consist- 

 ing of soil and atmosphere, on the other. The importance of animal 

 life as an environmental factor in the desert is undoubtedly very great, 

 but no careful studies were made along this line. The results of the 

 investigations can be best presented under the three headings, "Soil 

 studies," "Atmosphere studies," and " Plant studies, " these to be fol- 

 lowed by a discussion of the interrelations existing between the facts 

 brought out by the three lines of inquiry. 



It was more expedient and seemed altogether more desirable to make 

 a rather thorough study of the conditions obtaining on the shoulder of 

 Tumamoc Hill, in the immediate vicinity of the Desert Laboratory, 

 than to attempt broader and therefore less thorough studies embracing 

 other localities, such as the mesa below.the hill and the erosion channels 

 and washes of the Santa Cruz River, Rillito Creek, etc. , or of the more 

 distant and more varied Santa Catalina Mountains. A remarkable 

 uniformity in soils and vegetational characters is exhibited by all the 

 peaks and buttes of the Tucson Range, and Tumamoc Hill may be 

 taken as a type of these. Thus the results of the present investiga- 

 tions may be regarded as applicable to the whole range. All these 

 peaks are distinctly desert mountains, not attaining a sufficient alti- 

 tude to have moisture conditions which will allow any form of plant 

 growth less xerophytic than the Parkinsonia-Cereus society which covers 

 Tumamoc Hill. This society comprises, besides the giant cactus or 

 saguaro (Cereus giganteus) and palo verde (Parkinsonia microphylla) , 

 a number of Opuntia species, both of the arborescent and prickly-pear 

 types, the barrel cactus (Echinocactus Wislizeni) , ocotillo (Fouquieria 

 splendens), cat's claw (Acacia greggii) , and occasional creosote bushes 

 (Covillea tridentata), together with several other shrubs and numerous 

 smaller plants. This vegetation has been briefly described by Coville 

 and MacDougal (1903) and also by Lloyd (1905). 



The Santa Catalina Range, which rises on the opposite side of the 

 mesa, is more extensive than the Tucson Range and much higher. The 

 foot-hills and rugged slopes toward the mesa are very similar in soils 

 and vegetation to the Tucson Mountains, but as the ascent is made new 

 conditions are encountered, largely those of increased moisture and 

 lower temperature, and in the higher altitudes of the Catalinas are 

 streams of running water and forests of oak and needle-leaved trees. 

 The series of vegetational transitions from the willow and ash margined 

 Rillito Creek, across the great sandy washes, where the latter widens 

 in time of flood, on which dwarfed mesquite (Prosopis velutina) forms 

 practically the whole vegetational cover in the dry season; across the 



