220 
W. A. CANNON 
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 I 2 3 4 5 6 
NOVEMBER DECEMBER 
Fig. 4. Comparative daily growth rates of the roots of Prosopis velutina (A), 
Opuntia versicolor (B), and Fouquieria splendens (C), (much magnified), from 
November 12 to December 6, 191 3. The lower curve gives the temperature summa- 
tions above 10° C. in the root cage, and the detached curve is the summation when 
artificial heat was used in the cage. (From Year Book, Carnegie Institution, 1914.) 
the growth rate of Prosopis roots at the lowest temperatures was 
relatively fast, and, also, that, at high temperatures, the roots of this 
