CLIMATIC FACTORS : RADIANT ENERGY 



121 



Temperature Records.— Because temperature is so extremely 

 variable, isolated or even numerous single determinations may be 



Fig. 63. Maximum-minimum thermometers of a standard type for air tem- 

 peratures. Installed in an instrument shelter, the holder permits whirling of 

 the maximum thermometer for resetting.— Courtesy Friez Instrument Divi- 

 sion, Bendix Aviation Corporation. 



completely useless. A continuous record is most desirable because 

 it gives the duration of extremes and variations. Although extremes 

 may be important in the reaction of a plant, their duration is apt 

 to be what determines the plant's response. Therefore, a thermo- 

 graph is desirable for thoroughly satisfactory work. The "mean 

 temperature" as computed by the United States Weather Bureau 141 

 is usually the average of the maximum and minimum for the day. 

 This is not accurate or truly indicative of plant-temperature rela- 

 tions because it ignores duration and is likely to run too high. The 



