156 CHAP. XIX. THE DAILY VARIATION IN ACTIVITY 



The light from the sky varies with the course of the sun ; 

 it is extremely feeble at dawn, increasing rapidly as the sun 

 rises above the horizon. The light attains its maximum 

 at noon ; it then wanes gradually up to about 4.30 p.m., 

 after which the fall of the intensity is extremely rapid. 

 This relates to light from a point of the sky vertically over- 

 head ; the variation of the intensity of the reflected light 

 from the northern sky, as previously explained, is relatively 

 more uniform, the rise and fall being less abrupt. The 

 temperature also undergoes variation which is not exactly 

 parallel to that of light ; the thermal noon, for example, 

 is about two hours later than the light-noon. The fall in 

 temperature is gradual in the afternoon, whereas light falls 

 abruptly to zero in the course of less than two hours after 

 4 P.M. 



These are the two environmental factors which con- 

 spicuously affect the photosynthetic activity of the plant 

 growing in the open. The questions naturally arise, (1) How 

 quickly does the plant respond to the external changes ? and 

 (2) Does each factor produce the whole or only part of its 

 characteristic effect ? A complete answer to these queries 

 can be obtained only by simultaneous determination of the 

 photosynthetic activity and of the intensities of the two 

 factors which affect it. Three determinations have there- 

 fore to be made at the same time, for which special appliances 

 had to be devised. 



The investigation was undertaken with specimens grown 

 in the pond which was exposed to northern light from the 

 sky, direct sunlight being intercepted by a tall building 

 to the south. The method employed is so sensitive that the 

 plant responded to the slightest variation in the intensity 

 of light caused by reflection from two walls situated to the 

 north of the Institute, one of which was dark and the other 

 white. In the morning there was a feeble and diffuse 

 reflection from the dark wall ; later in the day, sunlight 

 was diffusely reflected from the white wall. The changes 

 in the light reflected from the neighbouring buildings were 



