RELATION OF THE RATE OF ROOT GROWTH IN SEED- 

 LINGS OF PROSOPIS VELUTINA TO THE 

 TEMPERATURE OF THE SOIL 



W. A. CANNON 



The Desert Laboratory, Tucson, Arizona 



SUMMARY 



This paper is a summary of numerous observations, a portion 

 only of which are presented, on the growth of the roots of mes- 

 quite in soil, and is one of a series on the physiological-ecolog- 

 ical relations of roots. The leading results can be given in 

 brief as follows: 



The range of temperature of the soil at which root growth in 

 seedlings of the mesquite will take place ranges from about 

 12°C., as the minimum, to about 42°C., as the maximum. 



The most rapid root growth observed in any experiment was 

 of a root with an initial length of 16 mm. which in a period of 

 12 hours grew 51 mm. The temperature of the soil was 32.5° 

 to 34°C., and the temperature of the air was 22.5° to 23.5°C. 



In roots about 50 mm. long, or less, at constant soil temper- 

 atures, the rate of growth may gradually increase during a 

 period of 48 hours, more or less. 



In roots about 50 mm. or more in length, the rate of growth 

 either remains approximately constant with the passage of 

 time up to 76 hours, more or less, or the rate gradually 

 decreases. 



In roots about 50 mm. in length there may or may not be an 

 acceleration of the growth rate during the first 3 to 6 hours of 

 the experiment. In the longer roots this is usually wanting, 

 but in the shorter roots it frequently occurs. 



Tha'ee types of growth rate variation are to be distinguished, 

 namely, the gi-adual increase or decrease, as above noted, and 

 which is associated with the length of the root, and major and 

 minor variations. The latter are independent of the age of the 



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