FLOWERING PLANTS AND FERNS OF ARIZONA 23 



season, and mainly covered with loose volcanic cinders in which it is 

 difficult for plants to become established. It seems very certain, in 

 fact, that timber line on the San Francisco Peaks is not a truly 

 climatic line, but one in which the character of the substratum 

 cooperates to bring the tree limit down to a somewhat lower level 

 than would exist on a mountain that presented favorable soil condi- 

 tions. An approach to timber-line conditions is found on Baldy 

 Peak (Mount Thomas) in the White Mountains and on Mount 

 Graham, in the Pinaleno Mountains. 



The vegetation of the summits of the San Francisco Peaks is of 

 interest on account of the character of the few plants that are able to 

 persist there and also on account of their distributional relation- 

 ships. No species is very abundant and there are few spots in which 

 the vegetation covers the substratum. The most abundant species 

 are those forming mats in the crevices of rock, as SUene acaulis. 

 Saxifraga caespitosa var. lemmonii, and PotentUla sibbatdi. Small 

 size and low habit are characteristic, but large flowers make many of 

 these plants conspicuous. 



In common with other alpine areas in the United States the flora 

 above timber line on the San Francisco Peaks embraces several plants 

 that range north to the Arctic Circle, and a few that have very close 

 relatives in the high latitudes of Europe and Asia. The following 

 list includes the most characteristic and abundant species; an asterisk 

 indicates plants found also in the Arctic regions of North America. 

 Poa rupicola, *Phleum alpinum, Carex bella, *Luzula spicata, *Oxyria 

 digyna, *Cerastium beeringianum, *Arenaria sajanensis, *Arenaria 

 vema, *SUene acaulis, ^Ranunculus eschscholtzii, Thlaspi fendleri, 

 Draba crassifolia, Saxifraga caespitosa var. lemmonii, *S. flagellar',*, 

 PotentUla el ire rsi folia, *Potentilla sibbaldi, EpUobium saximontanum, 

 Pseuelocymopterus montanus, *Moneses uniflora, Primula parryi, 

 *Androsace septentrionalis var. subumbellata, Gentiana memantha, 

 Polemonium confertum, Penstemon whippleanus, Pedicularis parryi. 



Vegetation of the Grand Canyon 



The scenic grandeur and geological interest of the Grand Canyon 

 have long made it the most outstanding natural feature of Arizona. 

 In the present-day distribution of plant and animal life from the rim 

 to the river the Canyon is equally remarkable. Crowded into the 

 vertical space of 1 mile and the horizontal space of 10 to 20 miles 

 are nearly all phases of environment and vegetation to be found in 

 the entire State. It is possible to descend in a few hours from pines 

 and firs to mesquites and cacti. At the same time that the Canyon 

 is an epitome of the vegetation of the State, its conditions and th«» 

 distribution of its plants are very complicated. The climatic differ- 

 ences due to altitude are modified by slope exposure, by the shade of 

 the great cliffs, by seepage of water, and by the ever-changing cur- 

 rents of warm and cool air. Plants may be found growing near each 

 other that have very different life requirements and are elsewhere 

 foimd only many miles apart. Only a very few of the plants of 

 Arizona appear to be peculiar to the Canyon, but on its walls may be 

 found a large percentage of the species of plants known to occur in the 

 State. 



