ROOT GROWTH AND DEVELOPMENT 
217 
splendens Engelm. and in Opuntia versicolor Engelm, and active root 
growth also, as will appear below, does not occur until midsummer. 
t« I .ji— I I I I I I I I I 
Fig. 3. Integration of monthly soil temperatures above 10° C, Desert Labora- 
tory, 1910, at the following depths: 15 cm, ( ); 30 cm. ( ); ca. 2.6 m. 
( ). The horizontal intersecting lines are integrations for an average tem- 
perature of 30° C. {X), and 20° C. (7). 
It should be interpolated here that root growth in the latter species, 
with shallowly placed roots, would take place in May-June were it 
