v.] 



VARIATIONS OF TEMPERATURE 



123 



curves show firstly (Fig. 9), the daily results during 

 a fortnight of April 1902, also (Fig. 10) certain hourly 

 readings obtained in the same month, in this case 

 beneath smooth, well-worked arable land. The diurnal 

 variations die away before the depth of 3 feet is reached, 

 nor are hourly variations perceptible at the depth of 

 one foot, except in the case of heavy precipitation and 

 a pervious soil. It will also be noticed from the 

 last curves that during part of the day the temperature 

 at the depth of 6 inches ran up to a point well above 

 the minimum required for germination, although the 

 mean soil temperature at 9 A.M. was near that limit 



Temperatures required for Growth. 



Reference has already been made to the fact that a 

 certain temperature is necessary before the vital 

 processes involved in growth become active; this 

 temperature is not always the same, but may be con- 

 sidered to lie between 40 and 45 F. for most of the 

 plants grown as crops in this country. 



The following table shows minimum, optimum, and 

 maximum temperatures of growth for a few plants. 



The next table shows the effect of soil temperature 

 uoon the growth of the root of maize. 



