324 



W. A. CANNON 



0.9 mm., 2.7 mm., and 2.8 mm., while that of the fresh plant 

 increased 4.3 mm. in length. It is apparent, therefore, that 41° 

 to 42°C. is not, for a period of 6 hours, the maximum tempera- 

 ture for root growth in mesquite seofilings. The curve of 

 growth, after the first 2 hours, was fairly level, although it is 

 questionable whether this rate would have been maintained 

 many hours. 



Root growth at soil temperatures between 28.5°C. and 3/i..Ji°C. 

 Most of the experiments reported at this time were conducted 



TABLE 1 



(*) In experiments XII, XIII and XIV readings were made twice daily. 



at temperatures clearly favorable for the growth of the roots of 

 mesquite. Observations were made every 3 hours and the ex- 

 periments extended over a period from 11 to 144 hours. The 

 experiments are numbered for convenience from I to XIV. 

 Table 1 gives the soil and air temperature, when known, to- 

 gether with the duration of each experiment. 



Root growth at a soil temperature of '28. 5° C. Two series of 

 experiments were conducted at a soil temperature of 28.5°C. 

 in one of which the initial root length was 20 to 22 mm., and 

 in the other the length of the roots at the beginning of the 



