28 



THE FLOWERING PROCESS 



RANGE OF LIGHT INTENSITIES 

 FOR BIOLOGICAL RESPONSES 



^4 -. 



Sunlight, noon, clear day, June (10,000 ft-c).— 

 Photosynthesis saturates in wheat (2,000 ft-c)- 



Sunlight, overcost sky. 



Photosynthesis compensation point 



-2 



10""-- 



10" 



10" 



-6 



10 



10"'-- 



I Threshold for incandescent light inhibition of 

 ["flowering in cocklebur (see figure 7-8). 



, End of twilight (0.4 ft-c). ' 



Maximum moonlight (0.02 ft-c). • 



^ Threshold for red light inhibition of 

 flowering in cocklebur (see figure 7-8). 



Limit of cone or color vision.— 



Diapause induction in Metriocnemus. ' 



-Threshold for phototropism in Avena (blue). 



Starlight (0.00008 ft-c). — 



-Threshold for beon hook response (red). 



Limit of rod vision, dork adapted eye. 



_8 I Threshold for photomorphogenesis (red), 

 '^ T Avena first internode, bean hypocotyl. 



10 

 + 10' 

 10' 

 10 



I. 

 J 



10 



-2o 



z 

 < 

 o 



10-3 H 



o 

 o 



10 



-5< 



Figure 3-2 

 The range of light intensities for biological responses. The agreement 

 between the foot candle scale and the absolute energy scale (microwatts/ 

 cm2) is at best an approximation. The foot candle (ft-c) is not a good 

 unit for measurement of light intensities which relate to any biological 

 response except human vision, since the unit is based upon the abiUty of 

 the human eye to respond to light. It is an excellent unit for measure- 

 ment of illumination for reading, working, etc., but its use in other 

 biological research should be avoided. Adequate light measurement 

 must express absolute energy in terms of the wavelengths being measured. 

 Such measurement is still very difficult, but the nature of the light source 

 can be expressed and the intensity given in energy terms. Unfortunately 

 data for some of the responses listed gave light intensity only in terms of 

 foot-candles, as indicated by the lines. Modified from R. B. Withrow (36). 



intensities during any given second, even though it must "add up" 

 the response over a long period of time before actual changes in the 

 organism can be observed. The plant response is quite similar to the 

 use of photography in astronomy. The astronomer is able to "see" 



