330 W. A. CANNON 



rate seen during the experiments. This occurred at a soil tem- 

 perature of 32,5° to 34°C., or shghtly lower than the temperature 

 of the experiment referred to in this paragraph. 



Root growth at an air temperature of 15.5° to 16. 5° C. Three 

 experiments were planned in which the air temperature was the 

 same, 15.5° to 16.5°C., but the temperature of the soil was un- 

 like. In the first one (no. XII) the soil was of the same tem- 

 perature as the air, in the second (no. XIII), it had a temperature 

 of 24° to 25°C., and in the third (no. XIV), the temperature of 

 the soil was 35°C. Readings were made every 12 hours. The 

 cultures were kept in a dark chamber which was illuminated 

 with one 100-watt nitrogen electric bulb placed about 2 meters 

 from the plants. The leading results are summarized in tables 

 4, 5 and 6. 



The length of the roots at the beginning of the experiment 

 was as follows: no. 65, ?; no. 66, 20 mm.; no. 67, ?; no. 68, 

 24 mm.; no. 69, 22 mm.; no. 70, 20 mm. 



The initial length of the roots in experiment XIII, table 

 5 was as follows: no. 75, 30 mm.; no. 76, 65 mm.; no. 77, 70 

 mm.; no. 78, 73 mm.; and, no. 79, 65 mm. In experiment no. 

 XIV, table 6, the root length at the beginning of the observa- 

 tions was as follows: no. 80, 12 mm.; no. 81, 5 mm.; no. 82, 

 10 mm.; no. 83, 8 mm.; no. 84, 8 mm.; and no. 85, 8 mm. 



An inspection of tables 5 and 6 will show that, with the ex- 

 ception of no. 82, an acceleration of growth takes place until 

 a well marked optimum, occurring a considerable time after 

 the commencement of the experiment, is attained. This is 

 followed by a gradual slowing in the rate. In the case of ex- 

 periment XIII, table 5, the conditions both of temperature and 

 of root length have operated to bring about a different result. 

 Not only is the rate of root growth greater, but, also, the most 

 rapid rate occurred at or near the beginning, and then gradually 

 became less. It is suggested that the rapid growth rate as 

 well as the decline in rate soon after the beginning of the ex- 

 periment are not solely associated with the relatively high soil 

 temperature to which the roots were subjected, but that the 

 roots were not far from the period of most active growth, the 



