REVIEW AND DISCUSSION. 129 
holds good as it approaches its southern limits we are not in possession 
of data to determine. 
From the studies thus far made we are also in possession of the key 
to a probable explanation of the association of this species with plants, 
such as the palo verde and others, of very different and considerably 
wider geographical range and of very different physiological requirements 
from those of the sahuaro, though at this place characteristic members 
of the same association with it. The superficial root-system of the 
sahuaro, as we have seen, gives it a decided advantage, as far as its 
capacity for utilizing light rains is concerned, but with their deeper root- 
systems the palo verde, the creosote-bush, and others are in a_ position 
to make use of water that has reached a lower level. ‘These plants, 
accordingly, which are so constantly associated here, experience a mu- 
tual advantage in the differences of their root-systems. 
To put it sharply, general similarity of climatic and dissimilarity of 
certain edaphic relations appears, in this case at least, to favor the asso- 
ciation of the plants in question. Not all of the roots of the palo verde 
and other species referred to extend deeply; some are superficial, and it 
may well be that in the layers of soil near the surface there may often 
be sharp competition between them and the sahuaro; but if so, a modus 
vivendi is found in the differences of their root-systems that have been 
pointed out, and it seems at least highly probable that this is an impor- 
tant means by which they are enabled to develop normally side by side. 
In marked contrast with the sahuaro in many respects, though fre- 
quently associated with it, is the creosote-bush (Larrea tridentata). ‘This 
plant presents no extraordinary structure or mechanism in any way 
comparable to that of the sahuaro, yet it is far more successful than 
the latter as a desert species. Both are well protected against excessive 
transpiration, a characteristic which they share with desert perennials 
in general, but in other respects it would be hard to find two plants more 
unlike. The distribution of the root-system of Larrea places it at once 
at a great advantage, making it capable of drawing its water-supply 
both from higher and lower levels. It can maintain itself with an exceed- 
ingly meager percentage of soil-water, as its occupation of the Larrea 
slope shows, but it luxuriates in an abundant supply, as is clearly seen 
by its vigorous growth in the wash and wherever plenty of water is given 
it. It bears successfully a far wider range of temperature than does 
the sahuaro, and with these physiological characteristics we find it, as 
would be expected, ranging far beyond the latter in both latitude and 
altitude, and in its local distribution affecting indifferently all exposures 
and all sorts of soils from the wash to the summit of Tumamoc Hill. To 
put all in a word, the tolerance of wide extremes, as regards both tem- 
perature and water-supply, on the part of this plant corresponds with 
its relatively wide geographical range and its local occupation of a great 
