2l8 
W. A. CANNON 
not for the circumstance that this is the arid foresummer when root 
growth, and, broadly speaking shoot growth, also, of shallowly rooted 
plants does not occur. 
Enough has probably been given concerning soil temperatures to 
show that roots which occupy different soil horizons in the same habitat 
are exposed to widely different temperatures, not only in winter, when 
there is little, or no root growth, but also in summer when the growth 
of roots is most active. We will now consider how differences in soil 
temperatures influence the root growth rate of species having unlike 
root-systems, namely, Prosopis velutina, and Fouquieria splendens and 
Opuntia versicolor. 
Relation of Growth Rate in Roots to the Temperature of 
THE Soil. 
In order to observe the response of roots of typical desert perennials 
to variations in soil temperature three species were selected for study. 
These, as before mentioned, were Prosopis velutina, representing the 
deeply penetrating root type, and Fouquieria splendens and Opuntia 
versicolor, representing the type of root-system that lies near the 
surface of the soil. The growth rate of the roots was observed in 
several forms of experiments, and in very many experiments, which 
were carried out at the Desert Laboratory and at the Coastal Labor- 
atory at Carmel, and, it can be premised that, in spite of the widely 
different environmental conditions in the midst of which the two 
laboratories are situated, the general results of the cultures and ex- 
periments were in all instances consistent. 
A brief sketch of the leading types of experiments and cultures will 
be sufficient for the present purpose. In one form of experiments 
seedlings and relatively small plants were grown in tubes of different 
diameters and different lengths. These were either exposed to the 
temperatures of the air, or w^ere kept in thermostats of which the 
temperature was known. In another type of experiments the species 
were grown in a wooden or metal box with a sloping glass side, and 
which, again, was either permitted to follow the prevailing air tem- 
peratures, or was heated artificially. In addition to these methods of 
handling the plants, seedlings and young plants were grown in the 
garden both at Tucson and at Carmel. In the present paper only the 
leading results of these numerous as well as diverse experiments will 
be given. 
