CLIMATIC FACTORS 



83 



several or many representatives of the association (individual stands) in 

 different localities. Isolated measurements have little value, and that 

 only for comparison. For the measurement of climatic factors which 

 change rapidly from time to time (hght, heat, humidity, etc.) con- 

 tinuous observations in one habitat are necessary. Self-registering 

 apparatus as used by American investigators is very desirable. 



1. TEMPERATUREi 



Sunlight, the most important source of energy for organic life, 

 either is absorbed by the plant directly as heat, or functions as light 

 by being transformed into potential chemical energy. The heat and 

 Hght of the sun are due to different groups of rays. Light rays belong 

 mostly to the blue-violet part of the spectrum, while the most potent 

 heat rays lie in the yellow and red parts (Fig. 44). 



Infensiiy of 

 heafrays 



30 -to 50 60 10 80 30 100 110 120 130 m 150 160 770 ISO 130 ZOO 210 220 



Wave length In microns 



Fig. 44. — Distribution of energy (heat) in the normal spectrum of sunlight. {After 



Langley.) 



According to Langley (c/. Hann 1908), however, the maxima of heat 

 and light in the spectrum fall rather close together. In passing through 

 the atmosphere the short-waved blue rays are more dispersed than the 

 long-waved red rays. Whole groups of waves, moreover, at the red 

 end of the spectrum are completely absorbed by the atmosphere and 

 serve to heat the air. This is important for the temperature economy 

 of plants, since the radiation of heat by the plant body is thus reduced. 



Absorption and Radiation of Heat. — The heat of the sun on a clear 

 day at noon amounts to about one gram calorie ^ per square centimeter 

 at sea level or about two gram calories at high altitudes. In its passage 

 through the atmosphere more than 50 per cent of the energy from the 



' The centigrade scale is used throughout this volume in all temperature 

 measurements. 



- The amount of heat rec[uired to raise one gram of water from 0° to l^'C. 



