40 VEGETATION OF A DESERT MOUNTAIN RANGE. 



Some of the members of this group are of wide distribution in the 

 north, as Populus iremuloides, Achillea lanulosa, and Disporum trachy- 

 carpum. In the northern mountain contingent are also a few species 

 which range eastward to the Atlantic coast, a few which are found at 

 least as far south as Maryland {Heracleum lanatum, Rudbeckia lacini- 

 tata, Apocynum androscemifolmm, Vicia americana, and Asplenium tri- 

 chomanes), not to mention Achillea lanulosa, which scarcely deserves 

 separation from the cosmopolitan Achillea millefolium. 



The relationship with northern California and the northwestern 

 states is weakly expressed in the occurrence of Salix lasiolepis and 

 Prunus emarginata. Genera characteristic of the sub-arctic regions are 

 sparingly represented at higher elevations by species of Primula, Saxi- 

 fraga, and Androsace. 



Some of the most conspicuous components of the vegetation belong 

 to northern genera, but to species which are characteristic of the Mexi- 

 can Cordillera, as Pinus arizonica, Pinus strobiformis, Alnus acuminata, 

 Salix bonplandiana, Quercus hypoleuca, and Quercus reticulata. Such 

 genera of herbaceous plants as Solidago, Eupatorium, Erigeron, Pentste- 

 mon, Mimulus, Potentilla, Gilia, and Gentiana — all of which are richly 

 developed in the Rocky Mountains — are chiefly represented in the 

 Santa Catalinas by species not found in Colorado nor Wyoming. The 

 extent to which these species are characteristic of the Arizona-New 

 Mexico region or are components of the flora of the higher Mexican 

 mountains is only partially known. 



The relationship of the Forest flora to that of the extended mountain 

 regions to the south is still further strengthened by the occurrence of 

 members of genera which are not found in the Rocky Mountains of 

 Colorado and northern New Mexico, as Arbutus, Calliandra, Micro- 

 stylis, Drymaria, Cologania, Stevia, and Tagetes. 



To sunamarize for the mountain as a whole, it may be said that the 

 floristic relationships of the Desert and Encinal regions are almost 

 wholly with the Mexican deserts and foothills to the south, while those 

 of the Forest region are divided between the Mexican Cordillera and 

 the Rocky Mountains. The Mexican group is the more conspicuous 

 in the make-up of the vegetation, while the Rocky Mountain contin- 

 gent is apparently preponderant in number of species. 



It will be impossible to summarize the floristic relationships of the Santa 

 Catalinas in a thorough manner until very much more is known of their 

 own flora and also of the floras of the many adjacent mountain ranges and 

 desert valleys, both in the United States and in Mexico. For the explana- 

 tion of these relationships a closer acquaintance is needed with the 

 actual mechanisms of transport which are effective in the dispersal of the 

 seeds of desert and mountain plants. A fuller knowledge is also required 

 of the fluctuations of climate within recent geological time, and of the 

 consequent downward and upward movements of the Encinal and Forest 

 belts of all the southwestern mountains. Such movements would alter- 



