164 CHAP. XIX. THE DAILY VARIATION IN ACTIVITY 



activity after 4 p.m. was therefore extremely rapid, and an 

 arrest took place by 5.15 p.m., the sun setting at 5.38 p.m. 



Summary 



An account is given of an investigation for determining 

 the changes of photosynthetic activity under the environ- 

 mental variation of light and temperature at different hours 

 of the day. 



A description is given of the complete apparatus, con- 

 sisting of (1) the Electric Photometer for determining the 

 varying intensity of light, (2) the Thermograph for recording 

 the variation of temperature, and (3) the Automatic Recorder 

 for photosynthesis. The intensity of light reflected from 

 the northern sky increased rapidly after sunrise at 6.46 a.m. 

 (January 24, 1923) ; it remained approximately constant 

 between 10 a.m. and 1 p.m. ; the intensity fell in the after- 

 noon, the decline being rapid between 4 and 5 p.m. There 

 was practically no light after sunset at 5.38 p.m. 



The temperature rose continuously from 17 C. at 7 a.m. 

 to 3o°*9 at 2 p.m., this being the thermal noon. It reached 

 30°-5 at 1 p.m., which was slightly above the optimum 

 temperature for photosynthetic activity. 



The resulting photosynthesis at various periods of the 

 day can be readily explained as the expression of the com- 

 bined effect of the factors of light and temperature. Under 

 increasing intensity of light and temperature (up to an 

 optimum), the activity was increased from 135-8 c.mm. at 

 7.30 a.m. to 531 -4 c.mm. at noon, the increase being nearly 

 four-fold. 



After 1 p.m. the temperature rose slightly above the 

 optimum ; consequently there was produced a sharp fall 

 of activity between 1 and 2 p.m. After this both light and 

 temperature underwent a decline, with resulting rapid fall 

 of activity. 



Photosynthesis was arrested at 5.15 p.m., when the 

 intensity of light had declined to 40 from 68 at noon. The 

 sun set at 5.38 p.m. 



