CHAPTER XXIII 



TEMPERATURE 



THE vital functions of plants require a certain temperature for 

 their best performance. Plants may grow at other temperatures, but 

 they grow most vigorously at the optimum temperature, which for 

 different plants varies from KO to 100 degrees F. Below this growth 

 diminishes till at about 40 degrees F. it ceases for most plants. At 

 temperatures higher than the optimum growth is less vigorous till a 

 point is reached at from 5)!) to 115 degrees F., where it practically 

 ceases. A knowledge of the functions of heat in relation to germi- 

 nation, growth, physical phenomena, and bacterial activity, and the 

 means of its control, is of considerable practical importance to the 

 agriculturist. 



The Sources of Soil Heat. 1. Direct Radiation from the 

 Sun. The sun gives off both light and heat rays, and some of the 

 latter, striking the earth, are absorbed. This is the chief source 

 of heat. The amount received from the sun is enormous. Lang- 

 ley gives it as equal to 1,000,000 calories per hour per square 

 meter of surface from a vertical sun in a clear sky. If all of this 

 energy were absorbed by the plowed six inches of soil on a square 

 foot its temperature would be raised by 24.5 degrees in an hour. 

 The soil is always radiating heat, consisting of waves of lower pitch. 

 These are easily held by glass or water vapor, which is transparent 

 to waves of higher pitch or refrangibility. Hence the excessive heat 

 of the glass house and the oppressive heat when the air is laden 

 with moisture in the summer. 



The heat received by any part of the earth's surface from the 

 sun depends upon the -transparency of the atmosphere to heat. Dust 

 particles and water vapor in the atmosphere intercept some, while 

 dry air, free from dust, absorbs very little. 



Comparing the highest temperature reached by a blackened 

 thermometer in vacuo at Greenwich, England, near sea level, and at 

 Davos, Switzerland, 5100 feet above sea level, the temperature at 

 the latter place was 20.1 degrees F. higher in Xovember, 3f>.2 degrees 

 in December, 37.2 degrees in January, and 24.2 degrees in Feb- 

 ruary. The ground was continuously covered with snow at Davos. 

 While more heat is received from the sun at high altitude per unit 



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