SOIL TEMPERATURE FACTOR IN ROOT GROWTH 65 



of the most characteristic shrubs of the Tucson region. Up to 

 this time, on the other hand, all attempts to grow it at the 

 Coastal Laboratory have failed. 



Hourly growth rate of ?'oo/.s of Covillea tridentata at different soil temperatures 



mm. 



15°C 0.1 



20°C 0.2 



25°C 0.4 



30°C 0.5 



32°C 1 . G 



In the above table is presented what may very likely be con- 

 sidered average rates of root growth at the soil temperatures 

 given and as established by unpublished- experiments. If w^e 

 consider any root we are aware that it represents responses to a 

 range of temperature which may or may not be considerable, 

 and, from the foregoing table of rates of growth at different 

 soil temperatures, it is clear that the total growth is composed 

 of units of different time values. It may better illustrate the 

 difference in effectiveness of the different temperatures of the 

 soil as factors in root growth if we imagine such a root to have 

 been exposed to the five different soil temperatures given above, 

 for example, but for an equal period of time to each. From the 

 rates of growth the percentage increase for each temperature can 

 be easily calculated. This would be as follows: 



per cent 



15°C 0.035 



20°C , 0.071 



25°C 0.14 



30°C . 178 



32°C 0.57 



Since 32°C. is about the optimum temperature for the growth 

 of the roots of Covillea it is also the temperature of maximum 

 effectiveness so far as root growth is concerned. Or, viewed 

 from the standpoint of the plant, at a temperature of 32°C. 

 it attains its maximum efficiency as to growth. The tempera- 

 ture of minimum efficiency, on the other hand, is about 15°C. 



2 Preliminary notice in Yearl)ook, Carnegie Institute, Washington, No. 15, 

 1916. 



