GROWTH SUBSTANCES FOR PHOTOTROPISM 135 



to light stimuli is restored (Dolk, 1926; Reinders, 1934). It 

 should be mentioned that general illumination of colcoptiles 

 during the growth period lowers their sensitivity to subsequent 

 unilateral lighting (Filzer, 1930). 



Interesting data on the differential response of the tip and base 

 of the Avena coleoptile to varied intensity and controlled wave 

 lengths of radiation have been obtained recently by Haig (1935). 

 The extreme tip portions (1.5 mm.) of some plants and the sub- 

 apical regions of other plants were exposed to white light for 

 short periods, and the reaction time was measured for positive 

 phototropic curvature. The speed of the initial reaction-time 

 process was found to be proportional to the logarithm of the 

 light intensity up to about 1,000 meter-candle seconds, above 

 which there was a marked decrease in the rate of response. The 

 reaction-time curves for the tip and for the rest of the coleoptile 

 yielded separate loci suggestive of two distinct photoreceptive 

 systems. With white light, blue-green, and minus-blue regions 

 of the spectrum, the tip was phototropically more sensitive than 

 was the base. These results support the earlier work of Went 

 (1926), who reported a distinct difference in the light sensitivity 

 of the tip and base of the Avena coleoptile. 



The phototropic sensitivity of the Avena coleoptile to different 

 regions of the spectrum has been determined in recent years by 

 several investigators (Bachmann and Bergann, 1930; Johnston, 

 1934). It seems to be well-established that the shorter wave 

 lengths of visible light are most active in causing phototropic 

 curvature, while the longer wave lengths in the red region are 

 practically inactive (Fig. 43). The relative effectiveness in the 

 region of greatest sensitivity {i.e., in blue light) is represented by 

 a bimodal curve with maxima at about 4,400 and 4,800A. 

 (Johnston, 1934). As a matter of interest it may be mentioned 

 here that a similar bimodal curve with maxima in the same regions 

 (Fig. 44) has been found for the light inhibition of germinating 

 lettuce seed (Flint and McAlister, 1935). The phenomenon of 

 differential spectral sensitivity would appear to be of considerable 

 significance in connection with an analysis of phototropism and 

 growth. 



Conduction of the Stimulus. — The early studies (Rothert, 

 Fitting, Boysen Jensen) on the conduction of the phototropic 

 stimulus in the Avena coleoptile were described in the first chap- 



