DEPARTMENT OF BOTANICAL RESEARCH. 



91 



Evaluation of the Teviperature of the Soil as an Environmental Factor, 



by W. A. Cannon. 



Hitherto emphasis has been placed on the temperature of the air, 

 to the exckision of that of the soil, whenever temperature as a factor 

 in the distribution of species of plants is referred to. This practice 

 results from the lack of data both as to the temperature of the soil and 

 as to the reaction of a plant to soil-temperature; but with such series 

 of data in hand, not only can the importance of the temperature of the 

 soil as one element in the environmental complex be measured, but 

 such evaluation might aid in detei'mining what species 

 might be expected to survive under given conditions of 

 soil-temperature and thus be of economic importance. 

 The relative importance of the temperature of the soil 

 in plant distribution can be sufiicientlj^ illustrated by a 

 single example. 



One of the most striking species of plants in south- 

 ern Arizona is Covillea tridentata. Seedlings of the 

 species were gj-own at the experimental plot of the 

 Coastal Laboratorj'- to see what the plants would do in such a com- 

 paratively humid and equable climate, where relatively low tempera- 

 tures prevail most of the year, but where killing frosts rarely occur. 

 The seedlings made little gi'owth and did not survive. The leading 

 apparent reason for this is to be sought in the root-temperature rela- 

 tions of the species, as an effective growth-rate of the roots of Covillea 

 occurs only in fairly warm soil. The accompanying table gives the 

 approximate hourly rate of root-growth of Covillea as determined by 

 preliminary laboratorj^ experiments. 



The number of hours during which the soil was at the temperatures 

 15° C, 30° C, and 32° C. at the Desert Laboratory and at the Coastal 

 Laboratory at a depth of 30 cm. and for the month of August is pre 

 sented in the accompanying table. 



With the known growth-rate of the roots at the temperatures given 

 immediately above, the expected total root-growth of Covillea at 

 Carmel and at Tucson would be as follows : 



