328 



W. A. CANNON 



pears probable that the roots which are about 50 mm. in length 

 under parallel conditions, have a more rapid growth rate than 

 roots that are either much longer or much shorter than 50 mm. 



Fig. 3. Root growth of mesquite seedling, experiment X, plant no. 56. Soil 

 temperature, 31.5°C. Readings every three hours. Initial length of root, 90 

 mm. Growth X 50. 



Root growth at a soil temperature of 34-2° C. One series was 

 run at a temperature of 34.2°C., which is to be considered very 

 near the optimum for root growth in the mesquite. The ex- 

 periment lasted 45 hours. The initial root length was between 

 1 and 6 mm. The rate of growth was seen to be extremely 

 variable from observation to observation, but there was an 

 increase in the rate up to about the eighteenth hour, or later, 

 when the roots were between 35 and about 50 mm. in length. 

 In the case of plant 40, however, and plant 44 as w^ll, the maxi- 

 mum rate observed occurred within 6 hours after the experi- 



TABLE 4 



Root groioth of mesqiiite seedlings at a soil temperature of 15.5° to 16.5°C., in 

 millimeters. Twelve-hour periods 



