ROOT GROWTH AND TEMPERATURE 323 



RATE OF GROWTH 



Preliminary experiments on the relation between the rate ol 

 root growth in the mesquite and the temperature of the soil 

 indicated that the roots of the species respond to a relatively 

 large temperature range.- Root growth, although slow, does 

 however occur at a soil temperature of about 12°C., and the 

 rate of growth at a soil temperature of 15° to 16°C., as the fol- 

 lowing summary will indicate, is fairly rapid. Experiments 

 with seedlings, at a soil and air temperature of 15° to 16° C, 

 were conducted for a period of 114 hours with root growth as 

 follows: 8.5 mm., 15.5 mm., 8.3 mm., and 4.5 mm. What the 

 upper limit of temperature for root growth may be was not 

 learned although the following experiments at superoptimal 

 soil temperatures indicate that growth goes on, although slowly, 

 at a temperature of 41° to 42°C. Such experiments, that is, 

 those above the ''optimal" soil temperatures, were conducted 

 at the following temperatures: 36°C., 38° to 39°C., and 41° to 

 42°C. with results that can be briefly pressnted. At a temper- 

 ature of 36°C., the roots of four seedlings, which had an initial 

 length of 10 to 14.5 mm., increased 10.6, 13, 13.2, and 15.8 mm. 

 in length in a period of 12 hours. In two of the plants the maxi- 

 mum rate observed occurred 3 and 4 hours after the beginning 

 of the experiment. Following this rapid growth the curve was 

 a fairly level one. It was notable that the gradual increase in 

 rate, covering a period of several hours, which, as will appear 

 below, was observed at lower temperatures for young roots, did 

 not take place. Four of the plants which had been used three 

 days previously in the experiment above sketched were sub- 

 jected to a soil temperature of 38° to 39°C. for 12 .hours. The 

 roots of these plants grew 6.7 mm., 8.0 mm., 8.0 mm., and 7.2 

 mm. The root of another plant, not pre\'iously used, during 

 the same time increased 13.5 mm. in length. The experiment 

 was repeated with three, plants, which had been used in both the 

 preceding experiments, and with a fresh one not previously 

 used, at a temperature of 41° to 42°C., with the following re- 

 sults. In 6 hours the roots of the used plants grew 0.1 mm., 



