LIGHT AND TEMPERATURE 203 



noon. Light disappears at night from 6 p.m. to 6 a.m., 

 that is to say, the period of minimum hght is prolonged for 

 12 hours. But the fall of temperature is gradual, and the 

 minimum is attained at or about 5 a.m., which is the thermal 

 dawn. The characteristic variation of these two important 

 factors should be borne in mind, since the diurnal move- 

 ments of plants are modified by the algebraical summation 

 of the effects of light and of temperature. 



Fig. 117. Record of diurnal variation of light and of temperature 



in summer. 



It is sometimes desirable to carry out researches during 

 a period when the intensity of light remains approximately 

 constant. Such a period occurs between 11 a.m. and 

 I p.m., for a variation is only ± 5 per cent, of the mean. 



The record given of the diurnal variation of light is 

 true of days when the sky is clear. But the passage of a 

 cloud causes change in the intensity which is accurately 

 recorded by the Radiograph. A record of such irregular 

 variation on a stormy day is given in the lower record of 



fig. 116. 



For facihty of treatment of the diurnal movement of 



