202 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS 



which will be designated as the light-noon. The intensity 

 of light then declined at a rate slower than the rise. But 

 after 5 p.m. the fall of intensity was extremel}^ rapid. 



An important point arises in connection with the diurnal 

 variation of light and of temperature, and determination of 



Fig. 116. Radiogram of variation of intensity of light from the 

 sky during 12 hours in winter. The upper record shows the 

 variation on a bright day, the maximum intensity being 

 attained at 12 midday. The lower record exhibits irregular 

 variation on a cloudy day. The horizontal record above the 

 base line shows that the electric resistance of the selenium 

 cell is practically unaffected by variation of temperature. 

 Successive thin dots at 15 minutes' interval, thick dots at 

 intervals of an hour. 



the times of their maxima and minima. For this purpose 

 records of diurnal variation of temperature and of light were 

 taken on the same day in summer with the Thermograph 

 (described in the next chapter) and the Radiograph. The 

 two curves are given in fig. 117. 



It will be seen that while the maximum intensity of 

 light is at 12 noon, the thermal maximum is at about 2 p.m. 

 The thermal noon is thus two hours later than the light- 



