246 The Plant World. 



and the other zone on the side opposite. The bluB's facing- the 

 fiercely burning sun are usually clothed with such types as 

 Acacia constricta, A. greggii, mesquite, other thorny shrubs, 

 and cacti. The sides of the bluffs facing north, particularly if 

 they are steep and well set back between other protruding bluffs, 

 are usually clothed with perennial grasses, dotted v.ith Yucca, 

 Nolina, Agave or Dasylirion, and not infrequently, particularly 

 toward the base, are found thickets of such shrubs as Rhoeidium, 

 Schmaltzia, and Moms celtidijolia, true t^-pes of the Upper 

 Sonoran. Similarly, a canyon bottom or the top of a ridge in 

 the mountains may give us a clean-cut division between Upper 

 Sonoran and Transition societies. The altitude for the canyon, 

 forour region.will be near 6,000 feet and for the ridge often above, 

 7,000 feet. The Upper Sonoran has now reversed its position 

 with reference to the sun. Again, about 1,500 feet higher, 

 another ridge or canyon having the direction of a parallel will 

 give a similar abrupt change to the Canadian zone, and the 

 Transition has changed its position. Higher still, as for instance, 

 on the summits of the Chiricahuas and less distinctly so on ]\Iount 

 Lemmon, the Canadian in turn is found to have appropriated 

 the south sides, having yielded the cold boreal slopes to the 

 Hudsonian. 



Without multiplying instances, it may be stated that this 

 reversion of aspect with altitude is a general truth, at least in 

 the Southwest. From desert to mountain top, no species has 

 been observed to behave to the contrary. The same tendency 

 may be discerned between the heart of the Arizona desert and 

 sea-level. The fact holds true not only for the great character 

 species upon which our zones are based, but appears true also 

 for those of discontinuous distribution, like Lippia urightii. 

 It mav be expected to hold in similar mountainous desert re- 

 gions in the northern as ^\ell as southern hemisphere, in the 

 latter with the points of the compass resersed, It should not 

 hold for equatorial mountains, but, except as modified by other 

 influences, should be true for more humid regions, and increas- 

 ingly true with ascending latitude, for it appears to be simply a 

 question of the obhquity of the sun's rays, their effect probably 

 being intensified in acting through a highly rarified desert 

 atmosphere. The primary cause of the floral difference between 



