64 



The Plant World. 



ity of Tucson it frequently occurs on similar northern exposures, 

 never, as far as observed by the writer, on a southern exposure, 

 but in the Chiricahua Mountains and elsewhere, as shown by 

 Blumer, it exhibits a marked variability in choice of ascent^as 

 governed by altitude; thus at elevations of 3,000 feet and under, 

 it grows only in protected places of north aspect, but loses its 

 aspect preference at altitudes approaching 5,000 feet, while at 

 those near 6,000 feet it is definitely limited to southern exposures. 



Fig. -5. West slope of Tumamoc Hill. Thick choUa in the foreground. 



It appears, then, that this plant completely changes its aspect 

 preference within a range of not more than 3,000 feet. At the 

 lower levels it requires protection from too severe desert condi- 

 tions, and finds this on sheltered rocks of northern aspect. At 

 the higher altitudes apparently requiring protection from cold, 

 it finds a congenial home on warm southern exposures, while at 

 intermediate elevations, in the neighborhood of 5,000 feet, the 

 extreme conditions of both higher and lower altitudes are so far 

 modified that it grows on all exposures. 



Various other species show at this place a distinct prefer- 

 ence for northern exposures of the hill. Among these are 

 Abuiilon incanum, Brodiaea capitata, and many others, annuals 



