74 



The Plant World. 



and only very occasionally, if at all, in the flats. Some species 

 which occupy the footslope are to be found in great abundance 

 also, but in definitely restricted habitats, in the hills, and it is 

 with reference to two plants which show this peculiarity that I 

 wish to refer. These are the alvarda or ocotillo (Fouquieria 

 splendens) and palma samandoca (Samuel/a camerosa). 



Both of these plants are practically certain to be found 

 associated on the footslope and on the low ridges which merge 

 into the foothills Those familiar with the southwestern deserts 

 will recall that the ocotillo is, like the sotol, an "edaphic'' 

 species, growing in stony ground, and this is true also of the 

 palma samandoca, although to this general rule there are ex- 

 ceptions. In the higher reaches of the footslope and in the ad- 

 jacent hills. the soil is very stony; and, in the hills, even displaced 

 to various degrees by the underlying rocks, outcrops of which 

 are readily visible. 



As one enters the hills by passing up an arroyo, he sees at 

 once that the steep slopes on either hand bear different congeries 

 of plants, and the most obvious contrast is due to the fact that, 

 on the slopes facing toward the northeast, north, and northwest, 

 the palma samandoca is present in numbers large enough to 

 make it the characteristic element, while on the opposite slopes 

 the chief element is the ocotillo. This feature of local distri- 

 bution is so evident and constant that it is discoverable in photo- 

 graphs with the greatest ease (Figure 1). Similar differences 

 in the vegetation of opposed slopes have been repeatedly ob- 

 served in various desert regions by Coville, MacDougal, Spalding, 

 Bray, and others, but I do not recall that the species remarked 

 have in any instance been shown to have overlapping habitats, 

 and it is about this point that the interest of this paper centers. 



Whatever mav be the cause of this condition, it appears 

 quite probable, if not certain, that the factors which bring it 

 about do not operate on the mature plants. This follows from 

 the fact that on both slopes both the species in question do oc- 

 cur, so that the differences are to be seen in the ratios between 

 the numbers of individuals present on the north and south 

 slopes, and not in their entire absence from either. Indeed, 

 we may be observing the gradual colonization of both slopes 



