FLOWERING PLANTS AXD FERNS OF ARIZONA 7 



Mountains (continuing: until the present time); and P. C. Standley in 

 the Carrizo Mountains. 



In the 1920's, in addition to some of those who began their work in 

 the preceding decade, the following made important collections: W. X. 

 Clute in the Painted Desert region; Mrs. Rose E. Collom in Gila and 

 Coconino Counties (continuing until the present time); M. F. Gilman 

 and F. A. Thackery, chiefly in the Baboquivari Mountains; W. W. 

 Jones chiefly in Yavapai County; Susan D. McKelvey in many parts 

 of the State (continuing in the 1930's); Pauline Mead on the Kaibab 

 Plateau; and the writers and their colleagues, with headquarters at 

 the United States Field Station, Sacaton, throughout the State (con- 

 tinuing until the present time). 



The last decade (1930 to 1939) was notable for the large number of 

 persons engaged in collecting plants in Arizona. Among these were: 

 H. C. Cutler in northern Arizona; Alice Eastwood and J. T. Howell 

 in Coconino, Navajo, and Apache Counties; J. W. Gillespie in Mari- 

 copa and Pinal Counties; Elbert L. Little, Jr., on the San Francisco 

 Peaks; Bassett Maguire chiefly north of the Colorado River and in 

 Graham County; Aven Nelson in many parts of the State; M. J. A. 

 Wetherill in the vicinity of Navajo National Monument ; A. F. Whiting 

 and his colleagues of the Museum of Northern Arizona in Coconino 

 and Navajo Counties; and Ira L. Wiggins in southwestern Arizona. 



GEOGRAPHICAL RELATIONSHIPS OF THE FLORA 



The geographical position of Arizona and its great diversity of topo- 

 graphic and climatic conditions make it a veritable "melting pot," 

 where floral elements from nearly all parts of the North American 

 continent are to be found, if not side by side, at least within a few 

 miles of one another. It is the only State in which occur both alpine 

 and subalpine plants, of which some 50 species are found on the 

 higher mountains of the northern part of the State, and representa- 

 tives of genera and families that are mainly tropical, in the lower 

 country near the Mexican boundary. It may surprise those who know 

 Arizona only in its semidesert aspect that no fewer than 20 species of 

 those humus-loving plants, the terrestrial orchids, are found within 

 its borders. 



The diversity of life forms is remarkable. First and foremost are 

 the Cactaceae, culminating in the huge sahuaro {Cereus giganteus) 

 which Arizona has chosen, appropriate!}', as its State flower. In 

 southern and western Arizona, especially, one is impressed by the 

 bizarre forms of Yuccas, Agaves, crucifixion-thorn (Holacantha), ele- 

 phant tree (Bursera), and ocotillo (Fouquiiria). In the southwestern 

 corner occur two remarkable parasitic flowering plants, the tiny Pilo- 

 styles thurberi, of the mainly tropical family Rafflesiaceae, and the 

 sandfood (Ammobroma sonorae), one of the very few members of a 

 strictly American family. Even a species of palm (Washingtonia) is 

 native in Arizona. 



The 3,200 species of flowering plants, ferns, and fern allies known 

 definitely to occur in Arizona may be classified roughly, on the basis 

 of their distribution outside the State, into 12 geographical categories. 

 Obviously these are not hard and fast, and many species have been 

 placed somewhat arbitrarily in one or the other of them. Table 1 



