658 



PLANT MOVEMENTS 



duration, low intensities are effective only upon long exposures but with high 

 intensities extremely short exposures are sufficient. With a light intensity 

 of only 0.00017 meter candles an exposure of 43 hours was required to induce 



400 



420 



440 460 480 500 



WAVE LENGTH IN MILLIMICRONS 



520 



540 



POSITIVE 



Fig. 145. Relation between wave lengths of light and phototropic curvature of oat 



coleoptiles. Data of Johnston (1934)- 



phototropic curvature but with a light intensity of 26,520 meter candles an 



exposure of only o.OOi second resulted in phototropic curvature. 



The minimum quantity of light required for phototropic movement, the 



so-called "threshold value," varies 

 greatly with the portion of the 

 coleoptile tip that is illuminated. 

 The terminal 0.5 mm. of the coleop- 

 tile, for example, is nearly 1600 

 times as sensitive to illumination as 

 a zone only i. 5-2.0 mm. below the 

 tip (Lange, 1927)- 



Experimental results of a num- 

 ber of investigators have shown that 

 the degree of phototropic curvature 

 is controlled by the quantity of uni- 

 lateral light. The curvature is not 

 directly proportional to the amount 



of light but varies periodically as the quantity of light is increased (Fig. 146). 

 As the data in Fig. 146 indicate, once the threshold value is passed the 



coleoptiles curve toward the source of light and over a certain relatively 



*-75° 

 <r 

 ^*50° 



5 



a. 



D 



U c 



-25- 



2 3 4 5 6 



LOG. METER-CANDLE SECONDS 



Fig. 146. Relation between phototropic 

 curvature and quantity of light received 

 unilaterally. Data of du Buy and Nuern- 

 bergk (i934)- 



