ECOLOGY AND THE FLOWERING PROCESS 



29 



many objects by allowing their light to fall upon a photographic plate 

 for a number of hours, although his eye, with its short time limitation, 

 would be unable to detect these objects. 



It is important to note that response in photoperiodism will occur 

 under very low light intensities — only slightly more intense than 

 those detected instantaneously by the human eye. The time factor is 

 extremely important here again, and we will consider it in some 

 detail in Chapter 7. 



Figure 3-3 illustrates the fact that response in many light sensitive 

 plant processes is essentially linear to the logarithm of light intensity 



100 



80-- 



60 



40 



20 



Avena 1st. internode 

 12 days 



Bean hypocotyl 

 7 days. 



-2-10 I 2 3 4 .5 6 



LOGARITHM OF INCIDENT ENERGY IN log MICROJOULES/cm^ 



Figure 3-3 

 Response to the logarithm of light energy in three stem-growth mani- 

 festations. Indicated days and hours are exposure times. Note saturation 

 in Axena curve. Redrawn from R. B. Withrow (36). 



rather than to actual intensity itself. Thus the difference in response 

 between quantities of 0.01 and 0.1 microjoules/cm^ may be about the 

 same as the difference in response between 0.1 and 1.0 microjoules/ 

 cm2. In short, the plant is proportionately less sensitive in a given 

 response to very high intensities than it is to very low intensities. 

 In Chapter 7 we will be concerned with the idea of saturation 

 intensity. Allowing an exposure time of, say, 5 min, one finds that 



