DEPARTMENT OF BOTANICAL RESEARCH. 63 



fore, it is necessary to observe the rate of growth either at a constant 

 temperature or with sHght temperature range. Looking to this end, 

 experiments were devised, of which the following is a summary of 

 results now obtained, by which fairly constant soil temperatures were 

 kept for periods of from 6 to 9 hours, during which time readings of 

 root growth were made at intervals of from 30 minutes to periods of 

 3 hours each. 



The series of experiments were conducted at the Coastal Laboratory, 

 where in July the daily range of air temperatures is less than 10° C, and 

 the daily mean is about 20° C. The plants were grown in glass tubes 

 sunk in thermostats of the desired temperature, while the shoots were 

 exposed to air temperatures. In the first series a temperature varying 

 not more than 1° from 32.5° C. was maintained. Typical results at 

 this temperature, with readings made every 2 hours, were as follows: 

 1.2, 1.2, 1.6, and 1.3 mm. In another experiment the following 

 root growths, also at 2-hour intervals, were noted: 1.1, 0.9, 1.7, and 

 1.0 mm. These results were substantiated in other experiments. 



A second and a third series, differing from each other only in the 

 temperatures employed, gave results of which the following are rep- 

 resentative. In the second series a temperature of the soil was used 

 which did not vary more than 1° from 25.5° C. The roots were exposed 

 continuously to this temperature for 9 hours, and readings were made 

 at intervals of 9 hours. The increase in length of the roots for each 

 3-hour period was as follows: 1.1, 1.1, and 1.5 mm. 



In the third series of experiments the soil temperature varied less 

 than 1.5° from 20° C. Observations were made every half hour. 

 During the entire period, 7^ hours, a total root growth of 1.8 mm. was 

 made, which was slightly over 0.12 mm. for each half hour. An approx- 

 imately constant growth-rate was maintained throughout the experi- 

 ment. 



From these experiments it appears that roots exposed from 7^ 

 to 9 hours to a fairly constant soil temperature do not exhibit measur- 

 able decrease in growth-rate; but although the total root-growth is at 

 a rate somewhat different from that observed in other experiments, the 

 results, on the whole, are confirmatory of those previously obtained. 

 The probable rate of root-growth in the habitat of the species can now 

 be calculated with some accuracy if the daily course of the temperature 

 of the soil at the depth attained by the roots of opuntia is known. 

 Other factors, such as aeration of the soil and behavior of the shoot, 

 exert a minor influence. 



Soil Aeration and Root Growth, by W. A. Cannon. 



Observations on the root-systems of perennials growing in the 

 vicinity of the Desert Laboratory show that they may occup}^ different 

 positions in the soil. Some root-systems are deeply placed and others 



