10 MISC. PUBLICATION 4 2 3, U. S. DEPT. OF AGRICULTURE 



mopolitan). Remarkable examples of discontinuous distribution are 

 afforded by two ferns, Asplenium exiguum and Cetarach dalhousiae, 

 found only in a few widely separated localities in North America and in 

 the Himalaya Mountains. Both species are extremely local in 

 Arizona. 



Adventive. — Plants introduced by the agency of man from other 

 parts of the world, chiefly from Eurasia. The category includes 

 numerous species that have become naturalized in Arizona and others 

 of sporadic occurrence that have not yet established themselves as 

 components of the flora. A large majority of the introduced weeds 

 are common in California and probably reached Arizona from that 

 State. There are also a few species of central Asiatic origin that 

 occur here and there in the Great Basin region and in Arizona. It 

 seems probable that these were introduced with seeds of alfalfa from 

 Turkistan and Siberia, there having been formerly rather extensive 

 importations of such seed into the western United States. 



THE VEGETATION OF ARIZONA 3 



Contributed by Forrest Shreve 



In whatever direction one might traverse the United States he would 

 continually encounter new species of plants and new types of vegeta- 

 tion and would look in vain for some of the ones that had previously 

 been familiar in the landscape. Many plants would be seen again 

 and again for long distances, whereas others would be found only in 

 restricted localities. If time were taken to travel completely around 

 the area occupied by each species, it would be found that scarcely any 

 two of them coincide exactly. The traveler would also find that 

 changes in temperature conditions are encountered on going from 

 north to south, and that changes in moisture conditions are found on 

 passing from east to west. 



Any scheme of representing these gradations of conditions on a 

 map would result in a gigantic checkerboard with squares perhaps 

 150 miles in diameter, the symmetry of which would be greatly modi- 

 fied by the mountains and elevated plains. No one plant species 

 would be found to occur in all of the squares of a map of the western 

 United States. Although there would be no two of the squares on 

 which the conditions for plant life would be identical, nevertheless 

 there would be many plants found on a large number of the squares. 

 Indeed, there would be very few plants found on just one or two of 

 them. Some plants are able to adjust themselves to a wide range of 

 conditions, so that they are able to grow in many of the squares. Per- 

 haps they range widely in a northern and southern direction, en- 

 countering great differences in temperature, or perhaps their greatest 

 extension is from east to west, showing them to be capable of with- 

 standing a considerable range of moisture conditions. The character 

 of the physical conditions of each square serves to admit or exclude 

 and thereby rigidly to control the plant population of the area. 



The modifications in the symmetry of the checkerboard caused by 

 mountains and other elevations are due to the existence in every 

 elevated place of a set of conditions that would not prevail there if the 

 land were flat and at sea level. In the warmer latitudes an elevated 



s Citations to literature on the vegetation of Arizona are given on pp. 1036. 



