96 LIGHT, VEGETATION AND CHLOROPHYLL 



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radiations are difficult to produce with great intensity. The 

 mercury Une, 3,654 A, is still active and the extreme limit 

 appears to be near 3,300 A. These radiations are quite 

 invisible but their activity is small. The most effective wave- 

 lengths, situated between 4,000 A and 7,300 A, almost 

 coincide with those of the visible region of the spectrum. 



Outside this range, the infra-red and ultra-violet radiations, 

 inactive for photosynthesis, would have rather the effect of 

 disturbing and slowing down the action of the visible 

 radiations, but this fact does not seem to be estabUshed with 

 certainty. 



With equal energy, the monochromatic radiations of the 

 visible spectrum are not all equally effective for photo- 

 synthesis. It has been known for some time that the orange 

 yellow and the red have the maximum efficacy. This region 

 of the spectrum, near 6,500 A, coincides with an absorption 

 band of chlorophyll. 



More recent research has revealed a second, but lower, 

 maximum of efficacy in the blue, in the neighbourhood of 

 4,300 A or 4,400 A. In this region there is also an absorption 

 band of chlorophyll. Between these two maxima, the efficacy 

 of the monochromatic radiations is a Uttle less, but it is still 

 considerable. 



Among the experiments which have given these results 

 are those made by Hoover and described in the Smithsonian 

 Annual Report for 1936, page 364 (see also the Report for 

 1931, page 130). The plant used for the investigation was 

 wheat. The stem emerged from a double-walled glass tube, in 

 which a current of air, of controlled composition, circulated 

 slowly, while the carbon dioxide content of the air which 

 came out of it was continuously measured. 



Between the two walls was a solution opaque to the 

 excessive infra-red rays of the lamps and maintained at a 

 constant temperature. The lighting conditions in the tube 

 itself were measured by means of a special thermopile. 



To take advantage of the principal maximum in culti- 

 vations in artificial hght and thus obtain a higher efficiency, 



