86 CARNEGIE INSTITUTION OF WASHINGTON. 



the valleys at the same elevations. The coldest temperatures of winter 

 at 8,000 and 9,000 feet are much milder on an isolated mountain, sur- 

 rounded by desert, than they are at the same elevation on extensive 

 plateaus. These and other features of the work on the Santa Cata- 

 linas have emphasized the difference between the altitudinal gradient 

 of phj'sical conditions on a small mountain and on a larger gently tilted 

 plain which lies through the same elevations. The relation of environ- 

 mental conditions to the vegetation is likewise different in the two. 

 Both of these cases form an important part of the general problem of 

 the relation of vegetation to climate. 



The Vegetistic and Floristic Features of the Pinaleno Mountains of Southern 



Arizona, by Forrest Shreve. 



During September 1914 an expedition was made from Tucson to the 

 Pinaleno Mountains (Mount Graham), in Graham County, Arizona. 

 These mountains are 60 miles distant from the Santa Catahnas, reach 

 an elevation of 10,500 feet, and are built chiefly of gneiss. The object 

 of the visit was to compare the general vegetistic features of these two 

 desert mountains, which are of approximately the same age, in nearly 

 the same state of dissection, and constructed of the same mineralogical 

 material. The gently rolling summits of the Pinaleno range lie chiefly 

 above 9,500 feet and are clothed with a fir and spruce forest much 

 greater in extent than the analogous portion of the Santa Catahnas. 

 The numerous canons which have eaten the edges of the summits are 

 very precipitous. This circumstance has limited the extent of the pine 

 forests and has presented conditions favorable for the high occurrence 

 of the trees and shrubs which are characteristic of the encinal, or ever- 

 green-oak, region. The existence of well-watered canons and steep 

 slopes has caused a pronounced interdigitation of the highland and low- 

 land vegetations, so that the plants of streamways are carried nearly 

 1,000 feet lower than they are in the Santa Catalina Mountains, while 

 the encinal is carried about 1,200 feet higher than it extends in the 

 Santa Catahnas. 



The existence of lofty and sharply dissected alluvial aprons on the 

 northeast side of the mountains, falling to an elevation of 2,800 feet 

 at the Gila River, and the existence of a sub-level plain on the south- 

 west side, lying at 5,500 feet, causes marked dissimilarities in the 

 vegetation of the lowest slopes on the two sides and also influences the 

 vertical limits of the vegetations on the two faces of the mountain. 



Inasmuch as the Pinaleno Mountains had not been visited by bota- 

 nists for 40 years (since the Whipple Expedition), a full collection of all 

 plants in suitable condition was made, with the cooperation of Profes- 

 sor J. J. Thornber. Many plants of the Sonoran-Sinaloan region, 

 characteristic of the lower elevations of the Santa Catahnas, were not 

 found in the Pinaleno Mountains, while in the higher elevations of the 



