REVIEW AND DISCUSSION. 17 
ever, in their response to these factors differences of degree, if not of 
kind, and in the study of desert plants, which are exposed to wide ex- 
tremes of conditions in closely approximate areas, it is perhaps easier 
to estimate with an approach to certainty the relative influence of dif- 
ferent factors than it is in the case of plants of regions of abundant rain- 
fall. This has been attempted with a few plants of the Laboratory 
domain which exhibit conspicuous and definite habitat preferences. 
CONCURRENT ACTION OF DIFFERENT FACTORS. 
The case of Cereus giganteus is one of the best for the present purpose 
and will be taken first. This plant, a most striking feature of the land- 
scape wherever it occurs, has naturally attracted the attention of all 
observers, and its occurrence has been noted by so many that its western, 
northern, and eastern limits are perfectly well known. On the west it 
ranges to the close vicinity of the Gulf of California, and in two or three 
places passes into the State of California, just beyond the Colorado 
River. To the northward it is found as far as Bill Williams River and 
the Tonto Basin, and on the east to the San Pedro Valley. Only to 
the south, the distance it extends into Mexico is not yet recorded. 
The habits of the sahuaro, as they have been observed from what is 
probably the central part of its range to its northern limits, are such 
as to at once attract notice and, taken together, appear to admit of but 
one explanation. It is preeminently an inhabitant of rocky slopes and 
southern exposures. Its seeds germinate only at high temperatures. 
Its behavior at the limits of its northern growth, and at the limit of its 
growth in altitude, goes to show that it is a plant of high temperature 
requirements. On the north side of Tumamoc Hill it is dying out; at 
all events, a considerable number have died there within the past two 
or three years, and reproduction is nearly at a standstill. 
It has been shown that this species has a superficially placed root- 
system and that it utilizes with remarkable promptness the light rain- 
fall, which, over certain parts of its area of distribution, probably amounts 
often to no more than 2 or 3 inches a year. Its storage system is per- 
fectly adapted to the requirements of such a situation, being adjustable 
to either a heavy downpour, which may occur in the form of torrential 
summer showers, or to the more usual light rains which are separated by 
long intervals of drought. ‘Thus far, observations of temperature where 
the difference of density on south and north exposures in close proximity 
‘is well marked, go to show that the conditions for its growth are very 
nearly optimum where the minimum winter temperature of the soil at 
12 inches below the surface is 5° F. higher than on the nearby northern 
exposure, where the sahuaro is almost wanting and shows signs of dying 
out. 
