FLOWERING PLANTS AND FERNS OF ARIZONA 11 



plateau exhibits conditions that are similar to those at sea level in 

 some other place, although never exactly like them. By reason of its 

 size and rugged topography Arizona comprises a great many squares 

 on the checkerboard. In fact, there are only two or three States that 

 would comprise more than Arizona does. It is 395 miles from the 

 southern boundary of the State to the northern, which would make 

 considerable difference between the climate of the northern part and 

 that of the southern part, even if the State were flat. In fact, the 

 State covers one-third of the distance from Mexico to Canada, and 

 because of the mountainous character of some of it and its position 

 with reference to major climatic provinces, the diversity of conditions 

 is multiplied many times. As a result the vegetation is rich and di- 

 versified. 



Arizona is essentially a desert State. Its plains and valleys are 

 desert, hi both a physical and a biological sense, from the lowest 

 elevations to an altitude of about 4,000 feet in the south and to nearly 

 6,000 feet in the north. To the ameliorating conditions of the higher 

 altitudes are due the areas of grassland and forest that are so often 

 likened to islands surrounded by a sea of desert. A close acquaintance 

 with the vegetation of the higher altitudes brings to light many features 

 in which the nearness of the desert and the fringe of desert conditions 

 are of considerable moment in the life of a vegetation that is other- 

 wise so much like that of moist regions. 



Not only do the hills and mountains carry plants to higher and less 

 arid levels, but even at low elevations they afford habitats that are 

 more favorable than the outwash plains, although not so favorable as 

 the flood plains of the larger valleys. The soil of a slope that is covered 

 with stones has more favorable moisture conditions. The pockets of 

 soil filling the depressions and cracks in the buried rocky surface of a 

 hill are very favorably located for the infiltration of rain water and for 

 its retention. Innumerable localities might be cited in which the 

 vegetation of an outwash plain is replaced on the adjacent hillsides by 

 a different type of vegetation of much higher water requirement. 



Approximately 60 percent of the surface of Arizona is desert. The 

 vegetation in the various parts of the desert is far from being uniform; 

 in fact, it is much more varied than that of the forested areas. Every- 

 where the perennial plant covering of the desert is made up of a small 

 number of species. Passing beyond the distributional limit of any one 

 of these will bring a striking change in the appearance of the vegetation 

 and in the character of the landscape. Uncommon in Arizona are the 

 areas of shifting sand or barren rock that are commonly called to 

 mind by the word "desert." The regions of low and uncertain rainfall 

 are covered by bushes, dwarf trees, half woody perennials, and cacti 

 growing in an abundance, which is truly remarkable in view of the low 

 rainfall, the high temperature, and the almost continual sunshine in 

 daytime. The nonsucculent and the succulent desert plants grow 

 side by side in varying relative abundance. The latter arc less ubiqui- 

 tous than the former, as they are sparsely represented in the driest 

 and also in the coldest parts of the desert. Where they are at their 

 best, however, the cacti are the dominant plants in some of the most 

 striking landscapes in the State. 



The peculiar character of the vegetation of southern Arizona is 

 largely due to the close mingling of plants, which differ greatly in 

 size, form, habit of growth, and manner of adjustment to the adverse 



