84 



PLANT SOCIOLOGY 



sun is absorbed. The ecologically effective solar heat consists of the 

 total income of heat minus that lost by radiation. The curves in Fig. 

 45 show the total solar energy, with average clearness of sky, in high 

 mountains and in the Swiss lowlands. 



Direct solar radiation is measured by the use of the black-bulb 

 thermometer in a vacuum tube. Unfortunately this instrument has 

 various inaccuracies. The most serious is due to radiation from 

 neighboring objects, which cannot be measured. At best the black 

 thermometer gives only a very inadequate idea of the amount of heat 

 actually imparted to the vegetation, for the latter is influenced both by 

 the intensity of solar radiation and by the capacity for absorption of the 

 illuminated parts of the plant. 



^ fOOO 



r. 3000 



c 2500 

 «o 2000 



^ rsoo 

 ^ /ooo 



500 



/ 2 3 



Fig. 45. 



4 5 6 7 6 9 10 n 12 

 Months 

 -Solar energy with average clearness of sky in the high mountains (A) and 

 in the northern valleys (,B) of Switzerland. (After Maurer.) 



The outgo as well as the income of radiant energy differs according 

 to both time and place. Radiation (outgo) in middle latitudes, under 

 clear skies, at night, reaches 0.12 to 0.15 gram calorie per square 

 centimeter and is influenced by the depth of the atmosphere and its 

 humidity. Radiation is most marked just above the surface of the 

 earth. Hellmann {Sitzb. Berliner Akad. Wiss. 38: 1918) at Potsdam 

 found a mean annual difference of 1.80°C. between the temperatures 

 5 cm. and 2 m. above the ground. At Vienna the official thermometer 

 1.5 m. above the ground stood higher than the one just above the turf 

 by 3.5°C. in April, 4.0°C. in May, and 2.5°C. in summer (Hann, 1911). 



The mean minima at the surface of the ground and 1.5 m. above on 

 sod ground at Montpellier (France) were -8° and -5.4°C. in January, 

 1914. This explains how, in arid climates, water may freeze when the 

 temperature at the height of a man's head is several degrees above 

 zero centigrade. 



