84 LIGHT, VEGETATION AND CHLOROPHYLL 



differences observed are therefore imputable not to the 

 differences of illumination, which are suppressed, but to the 

 differences in the composition of the Ught. 



Popp's experiments, in 1926, were conducted in this way. 

 Under glasses allowing only radiations of wave-length greater 

 than 5,290 A to pass, the stems were often longer, and always 

 thinner and weaker; the leaves were rolled up and the tissues 

 badly differentiated; the cells had thin walls; flowering was 

 retarded and the production of fruits and seeds much reduced 

 in comparison with the controls which had received light of 

 the same intensity but containing all the solar radiations. 

 The tissues were abnormally swollen with water, although 

 the total weight, even with this water, was less than the weight 

 of normal plants. Chlorophyll formed well. 



Note again that glasses opaque to the ultra-violet, but 

 allowing the blue and the visible violet to pass, do not 

 produce any of these effects on the plants that they screen. 

 This result, therefore, is not an effect of the ultra-violet but 

 of visible radiations of short wave-length, whose beneficient 

 action is indispensable for the formation of robust and well- 

 grown plants. 



Popp's experiments confirmed, under better operating 

 conditions, the observations which had been made previously. 

 They lead us to think that cultivation in purely artificial Ught 

 would be difficult with incandescent lamps, since these lamps, 

 unless they are very much overrun, emit a radiation which is 

 rich in red light and poor in blue. They would probably have 

 to be combined with supplementary sources of radiation 

 capable of providing the blue light necessary for the balanced 

 development of the plant. 



This regulating action of visible fight of short wave-length 

 is no doubt of photochemical origin, but its mechanism is 

 certainly very complex and completely unknown to us. One 

 of its effects, which is of the greatest practical importance, is 

 to increase photosynthesis. 



Other effects are probably connected with this. For 

 example, fight appears indispensable for hardening the plant 



