118 PHOTOCONTROL OF GROWTH 



infrared cabinets was speeded up as compared to the control without 

 GA. The difference was especially obvious in the first 2 weeks. Later, 

 the differences were greatest in the "slower" colors: red, green, and 



blue + red. 



Next we compared the effect of red top illumination in both of 

 our blue cabinets, namely, the one with a small amount of near infra- 

 red and the one without any detectable near infrared. The same 

 amount of red light ( 2 tubes plus red glass on top of the cabinet with 

 26 blue tubes) depressed elongation much more strongly in the blue 

 cabinet without infrared than in that with infrared. The plants in the 

 former were even shorter than those in green light (Fig. 4, bottom). 



The explanation of these experiments involves the following con- 

 siderations. First of all, it seems beyond doubt that in our experiments, 

 near infrared strongly speeds up shooting and flowering in this annual 

 strain of Hyoscyamiis. Gibberellic acid enhances this influence and 

 induces it in case infrared is absent. The experiments also strongly 

 indicate that blue light has an effect similar to that of near infrared. 

 Furthermore, a red-infrared antagonism and a red-blue antagonism 

 are apparent, in which red suppresses elongation under the condi- 

 tions of our experiments. It may be asked whether all experiments 

 can be explained by the effectiveness of infrared and a red-infrared 

 antagonism alone. Notwithstanding the fact that this paper is confined 

 to high light intensity effects, it should be mentioned that in recent 

 years some effects of internode elongation formerly obtained with blue 

 supplementary light containing some near infrared (Wassink, Stolwijk, 

 and Beemster, 1951) could be reproduced by using this infrared con- 

 tent alone (Wassink, Bensink, and de Lint, 1957). These effects, 

 however, were not obtained with blue that contained no detectable 

 infrared. The last results with Hyoscyamus seem open to two different 

 explanations: (1) Near infrared and blue have similar effects upon 

 elongation in Hyoscyamus, which effects are suppressed by red. In the 

 latter respect, red inhibits blue effects much more strongly than it does 

 near-inf fared effects (or else blue effects require much higher intensi- 

 ties than infrared effects). (2) All responses, including those in blue, 

 are due to slight contaminations with near infrared, even with the blue 

 plastic filter. 



In view of the complete opacity of this filter for near infrared, we 



