FLOWERING PLANTS AND FERNS OF ARIZONA \) 



Chihuahuax.— The center of distribution of species of this category 

 is the Rio Grande drainage basin in western Texas, southern New- 

 Mexico, and northern Mexico, but many of them reach Arizona, 

 especially the southeastern part of the State, and some of them extend 

 into the second tier of States of the Republic of Mexico (San Luis 

 Potosi, Zacatecas, Durango, Sinaloa). This is the largest single 

 geographical category of the Arizona flora, unless the Rocky Mountain 

 category be considered as including species found also in the Pacific 

 Coast States. 



Tropical or Subtropical. — Species ranging farther southward in 

 Mexico than those of the Sonoran and Chihuahuan categories, many 

 of them extending to Central or South America and several occurring 

 also in the West Indies and the subtropical portion of the eastern 

 United States. Because of the relatively low latitude and altitude of 

 southern Arizona, subtropical species are much more numerous than 

 in any other State west of Texas. A number of species, mostly 

 belonging to the Pacific category, that are found in temperate North 

 America and in southern South America but not in the intervening 

 Tropics, are, of course, not included here. 



Rocky Mountain. — Many species of this category range from the 

 Canadian Rockies to the Sierra Madre in northern Mexico, but not a 

 few of them are known only from Arizona and New Mexico. There 

 are also mam^ Rocky Mountain species that extend westward into the 

 Pacific Coast States. The higher mountains of Arizona and the 

 elevated plateaus in the northern part of the State offer congenial 

 habitats for characteristic plants of this category. Such genera as 

 Penstemon, Erigeron, and Artemisia are well represented. 



Great Plains. — Species found chiefly east of the Rocky Moun- 

 tains, including some that extend even farther eastward into the 

 prairie States. This is a relatively small component of the Arizona 

 flora, confined chiefly to grassy plains in the eastern part of the State. 



Atlantic — Species mainly of the southeastern United States, 

 extending often as far west as central Texas and in some cases along 

 the northern coast of the Gulf of Mexico into Mexico, but absent in 

 western Texas and New Mexico. The following 11 species, all of 

 which occur only in the southern part of Arizona, have this peculiar 

 distribution: Ophioglossum engelmannii, Corallorrhiza wisteriana, 

 Hexalectris spicata, Cerastium te.ranum, Crotalaria sagittal)'*, Clitoria 

 mariana, Acalypha ostryaefolia, Chimaphila maculata, Isanthus brack ia- 

 tus, Galium pilosum, Cyclanthera dissecta. 



North American. — Species of wider distribution on this continent 

 than those of any of the foregoing categories. A large number of these 

 range across the continent in Canada and the northern United States, 

 reaching lower latitudes only in the higher mountains. 



Transoceanic. — Species that are believed to be indigenous in both 

 the Eastern and Western Hemispheres. A large majority of them are 

 temperate or circumpolar, occurring in the northern part of Europe, 

 or of Asia, or of both continents, and have a distribution in North 

 America similar to that of many species in the North American 

 category. Some of them, however, are found in the tropical and sub- 

 tropical parts of both hemispheres. It is always uncertain, of course, 

 whether plants having the latter distribution are really indigenous in 

 both hemispheres. Some of the aquatic and marsh plants of the 

 Transoceanic category are of almost world-wide distribution (cos- 



