I20 PLANT WORLD. 



rise still higher. By frequent checking of the aneroid with 

 several bench marks just established by the U. S. Geological 

 Survey, it was possible to attain a high degree of accuracy in 

 the determinations of altitude about Paradise. 



While the distribution of the species is profoundly in- 

 fluenced locally by geologic origin of the rock, and probably 

 less so by still other factors, for the sake of clearness these 

 are at present left out of account. The two things that 

 appear from the foregoing facts are: 



1. The constancy of the species in question in choos- 

 ing a steep and more or less rocky surface as a habitat. 

 whether this be outcropping bedrock, boulder-strewn areas, 

 or transported material distributed usually in banks and 

 bluffs along the water courses. 



2. Its variability of aspect as governed by altitude. 

 While at an elevation of 3000 feet and under, which 

 is probably near its lowest limit, it grows only in more or 

 less protected places of north aspect, it appears to lose its 

 aspect preference as the altitude rises to about 5000 feet, 

 but near 6000 feet and over, it is just as definitely limited to 

 southerly exposures as near 3000 feet and under it is con- 

 fined to northerly ones. 



A parallel case of higher range is that oiOuercus reticu- 

 lata HBK. In the Chiricahua Mountains this oak, at 6000 

 feet, was found as a leading species among the other oaks on a 

 steep north slope in Riggs' Canyon, while from a west slope 

 immediately adjoining it was absent. Like Lippia ivrightii, 

 it is not at all of general distribution in the region observeci, 

 and it was at first a surprise to find at the head of Pine Can- 

 yon 2000 feet higher in almost pure growth a dense chaparral 

 of this oak on a very steep slope facing directly south. The 

 two slopes are less than 10 miles apart. At the first-men- 

 tioned altitude it is also t^^und in the bottom of deep, narrow, 

 well-watered canyons, notably Rock Creek Canyon, and thus 

 furnishes a second high altitude parallel, this time to the case 

 of the following species: 



