66 



W. A. CANNON 



The illustrations show, in brief, that, as related to Covillea, a 

 soil temperature of 32°C. is over 16 times as effective as one of 

 15°C. It is interesting to note also that the effectiveness of 

 temperature of 32°C. is over three times that of 30°C. 



As further illustrating the advisability of considering sep- 

 arately representative degrees of temperature in studies where 

 temperature forms a part, the TR for Carmel and Tucson for 

 one month can be given. Below is presented the total number 

 of hours during which the soil in August, 1916, at a depth of 

 30 cm,, at Carmel and Tucson, was at the following tempera- 

 tures, namely: 15°C., 20°C., 25°C., 30°C., and 32°C. 



By calculating the TR for each temperature we have the 

 following results, — the expected root growths: 



mm. 



Carmel, 744 hours at 15°C., 74.4 



, Tucson, 547 hours at 30°C., 273.5 



Tucson, 197 hours at 32°C., 315.2 



The TR for Carmel for August, therefore, is 74.4 while that at 

 Tucson for the same time is 588.7. 



It is very evident that the TR is so low for Carmel during the 

 month of August that anything approaching a vigorous condi- 

 tion of the species is not to be expected for that month, and as 

 a matter of fact the TR would be extremely small for all of the 

 other months as well. Survival of the species at this station 

 would hardly be expected. At Tucson, on the other hand, the 

 TR is relatively large. It appears therefore that the soil temper- 

 atures at a depth of 30 cm. at Carmel, and with reference to 

 Covillea, are not effective temperatures, whereas the soil tempera- 

 tures at Tucson are effective. What constitute effective soil 

 temperatures for any species must be a mooted question, but 

 for practical purposes it may be considered to be such as induce 



