106 PHOTOCONTROL OF GROWTH 



such infrared contamination. This explanation, however, does not hold 

 true, because in our experiments the same effect was obtained in pure 



blue hght. 



It is impossible, however, to grow plants in blue hght without the 

 influence of hght of other spectral regions. Plants growing in blue 

 light look reddish owing to the fluorescence of the chlorophyll. The 

 effect of blue light could therefore be due to such fluorescence. This 

 explanation does not seem to be correct, however, because in some 

 experiments it was shown that low intensities of blue light of about 

 70 /xw/cm^ were still effective, whereas green light of much higher 

 intensity (QOO^w/cm-) was inactive. 



Thus, it may be concluded that in photoperiodism, besides the 

 red-infrared photoreaction, still another photoreaction exists which is 

 very sensitive to near infrared and blue light. 



STEM ELONGATION 



There seem to be many different opinions concerning the influence 

 of colored hght on elongation of plants. Several investigators have 

 concluded that the absence of light of short wavelengths causes an 

 increase in stem length (Wassink et al, 1956, 1957). Roodenburg 

 (1940) was probably one of the first to state that the near infrared 

 had a strong elongation effect on many plants. Most of the older 

 experiments were carried out in light which had a predominance of a 

 special spectral region, but which contained light of other wavelengths 

 as well. 



Wassink and co-workers (1956) purified the light of different 

 colored fluorescent tubes by means of suitable filters. In their experi- 

 ments it was shown that elongation occurred in red, yellow, and green 

 light, but not in blue or in violet. However, when these colors were 

 given in low intensities as additional hght to a short day of white, the 

 effect was just the opposite. They then obtained strong elongation with 

 weak blue light and no elongation with weak red or yellow light. In 

 addition to this effect, there was a very strong elongating effect by 

 infrared. 



Vince and Stoughton (1957) confirmed for tomato plants the in- 

 hibitive effect of blue, whereas this part of the spectrum had a promo- 



