120 PHOTOCONTROL OF GROWTH 



ually become relatively broader, successive leaves become relatively 

 narrower. In this case also internode elongation occurs, which is not 

 related to flowering. 



This internode elongation, as well as relative elongation of leaves, 

 can also be induced by supplementary near-infrared radiation, as was 

 found earlier (Wassink, Stolwijk, and Beemster, 1951). The effect 

 of supplementary radiation depends upon the intensity of the basic 

 white light period. If the basic light intensity is, for example, 25,000 

 ergs/ cm- sec, the effect of supplementary irradiation is much more 

 apparent than if it is in the order of 66,000 ergs/cnr sec (Fig. 5). 



Fig. 5. Lettuce, var. Meikoningin. Influence of 8-hr supplementary 

 irradiation with infrared (1000 egrs/cm- sec), after 10 hr light of two 

 different intensities. (1) 66,000 ergs/cm^ sec; (2) 25,000 ergs/cm2 sec, 

 photographed Feb. 10, 1953. 



As Stated above, successive leaves in a young lettuce plant grad- 

 ually become broader, so that length/breadth decreases. If now, for 

 example, at 12 leaves, the 6 latest ones are removed, the next leaf 



