66 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
It is possible that an analogous condition obtains in the fleshy cacti par- 
ticularly, in which the formation of leaves, those most active agents for 
the promotion of gaseous exchange, is entirely wanting. This circum- 
stance, together with the large size of the plants and their intensely 
xerophytic habit, would cooperate to reduce gaseous exchange with the 
surrounding air to a minimum and might make necessary the most 
favorable conditions of aeration for the roots, which would be found not 
far beneath the surface of the ground. 
The role played by the roots in the distribution of Cereus giganteus 
is probably a most important one, but its precise value remains to be 
determined. It naturally must be associated with the habit of the plant 
as being adapted to mild climates as well as dry ones. This must be taken 
into account, whether the general distribution or the local distribution 
merely is being considered. ‘The fact that the upper stratum of soil, where 
the roots lie, is subjected to great changes of temperatures and would 
afford little protection to the embedded roots and none to the subaerial 
portion of the plant would be an important factor and possibly a defini- 
tive one at the extremes of the range of the cactus, and may be of much 
weight in restricting the local distribution of the plant as well. For 
example, as elsewhere pointed out, in the vicinity of the Desert Labora- 
tory Cereus giganteus is most abundant on southern exposures, and in 
the neighboring Santa Catalina Mountains, where it is to be found at an 
altitude approximating 4,500 feet, it is wholly confined to this exposure. 
Temperature records made of the northern and the southern exposures 
indicate the importance of this thesis. 
Another factor limiting the range of the cactus is to be found in the 
entire dependence of the plant for its water-supply on the surface-water. 
The rains, therefore, must be of sufficient amount and kind to penetrate 
to the absorbing roots; and where this fails the plant will not be found. 
Although not demonstrated, the opposite extreme is probably as great a 
barrier, possibly from its effects of depriving the plant of proper aeration, 
as was mentioned above. And in this cause may possibly be found one 
of the factors operating to prevent the presence of Cereus on the heavy 
adobe soil of river-bottoms, where one of the imperative needs of the plant, 
that of safe anchorage, were the root-system a deep one, could be met. 
From the relations which the superficial character of the roots of Cereus 
hold to the temperature of the air, to the rainfall, to the plant’s need for. 
proper aeration and support, we find, therefore, important factors which 
must be considered in studies on the adaptation of the plant to certain 
types of habitats and in studies on its general distribution as well. 
