PLANT ASSOCIATIONS AND HABITATS. 19 
(c) PLANTS OCCURRING ON NORTHERN EXPOSURES, BUT WANTING ON 
SOUTHERN ONES; LIPPIA ASSOCIATION. 
On the left side of the gulch just referred to, and near its head, Lippia 
wrighttt forms an extensive patch (plate 10), and in the vicinity of Tuc- 
son 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 (1908), it exhibits a 
marked variability in choice of aspect 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. 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 conditions, and finds 
this on sheltered rocks of northern aspect. At the higher altitudes, appar- 
ently 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 preference for the 
northern exposures of the hill. Among. these are Abutilon imcanum, 
Brodiea capitata, and many others, annuals even more conspicuously 
than perennials. It is evident, however, from the facts cited in the case 
of Lippra, that this choice of aspect is not necessarily constant, and that 
direction of slope determines distribution merely as it presents a combi- 
nation of conditions which may change so greatly, even within a few 
miles, as to completely reverse the aspect preference of a given species. 
(d) PLANTS OF THE CLIFFS; HYPTIS-NICOTIANA ASSOCIATION. 
One species of each of the genera just named is found growing almost 
exclusively on the abrupt cliffs of Tumamoc Hill and on similar cliffs 
elsewhere. Celtis pallida, though by no means confined to the cliffs, is 
of common occurrence on them, where it evidently finds a congenial 
home. ‘The factors concerned in determining the choice of habitat exhib- 
ited by members of this association are not known. It may well be that 
where there is so little soil the result may in part be the outcome of com- 
petition, but regarding this point we are without positive knowledge. 
Hyptis and Nicotrvana are rarely seen growing elsewhere, though the 
latter is sometimes found growing in the sandy soil of washes, and in this 
latter habitat Celtis is of rather frequent occurrence. I am disposed to 
think of water-supply as again the main factor. There are probably 
pockets or fissures in the rocks where Hyptis and other plants obtain 
what they need, and the occurrence of Celtzs and Nicotiana in washes 
looks to this also. 
