222 
W. A. CANNON 
The last series of experiments to be mentioned was that of tube 
cultures, also, in which special temperature control was used and in 
which thermostats of special design kept the tubes at temperatures 
between 20.0° C, or less, and 42.0° C, for as long periods as was 
desired. The readings of the root growth were usually made each 
hour at which time the temperature of the thermostat was also ob- 
served. In figure 5 appears the average rate of growth of the roots 
of Fouquieria splendens and Opuntia versicolor, taken from many 
readings, for the temperatures given. The growth rate is necessarily 
greatly magnified, but the comparative growth rates are consistent. 
To give an idea of the actual growth rates it can be said that the 
maximum average rate for Prosopis, reached at a temperature of 35.0° 
C. is 2.86 mm. per hour, and the maximum for Fouquieria was about 
0.9 mm. per hour. The greatest growth rate observed in Prosopis 
roots was 3.0 mm. per hour. It will be seen that the rate of root growth 
falls away rapidly below 30.0° C. At 20.0° C. the rate for the roots 
of Fouquieria is very slow, practically ceasing at 15.0° C. In the case 
of Prosopis, on the other hand, a fairly active rate of root growth is 
maintained at 15.0° C, and, from other observations, it seems probable 
that growth continues at a temperature of 12.0° C. Thus the im- 
portant fact is established that root growth occurs in Prosopis at a 
temperature so low as to be unfavorable for growth in Fouquieria, 
and Opuntia versicolor, also, although the root growth of the last 
species is not included in figure 5. 
Root Development, Species Distribution, and the Reaction of 
Roots to the Soil Environment. 
During the season of most active root growth, midsummer, there 
are three features in the root environment which are of especial 
importance, namely, the soil moisture, its temperature and aeration. 
It can be stated, in brief, that at this time, owing to the rains, the soil 
contains sufficient moisture for the developing plants, and, also, that 
the temperature of the soil, at the depths attained by roots, is suit- 
able for their growth. Near the surface, in fact, as has already been 
shown, an optimum temperature for root growth obtains for a -rather 
long period. Within limits, also, the moisture content of the soil 
increases, and the soil temperatures decrease with the depth beneath 
the surface. In addition to these features is the condition of the 
aeration of the soil. Air movements, of whatever kind, in adobe soil 
