LIGHT AND VEGETATION 97 



light sources are chosen to give the greater part of their 

 luminous emission in this spectral region. For this reason neon 

 tubes are recommended; the spectrum that they emit includes 

 several intense monochromatic radiations situated between 

 5,764 A and 6,402 A. 



The tubes with fluorescent walls which have been put on 

 the market during the last few years are made with various 

 casings enabhng the composition of the light emitted to be 

 modified within wide limits. Up to the present, there seems 

 to have been no attempt to do more than compose a light 

 agreeable to the eye, but when these tubes are used for plant 

 illumination the manufacturers will no doubt develop light 

 sources perfectly adapted to make the best possible use of the 

 maxima of efficacy for photosynthesis. 



The solar spectrum possesses a maximum of power in 

 the yellow, in the neighbourhood of 5,500 A, at equal distance 

 from the two maxima of activity for photosynthesis. In view 

 of the shape of the spectral distribution curve of the solar 

 radiations, which is analogous to that of the emission of non- 

 selective incandescent bodies, the adaptation of the curve of 

 efficacy, although it is not as perfect as it could be if its 

 maximum coincided with the solar maximum, is nevertheless 

 excellent; it could even be worse without any disadvantage, 

 for the light is generally superabundant. 



It is interesting to combine the solar curve with the curve 

 of efficacy and to obtain from them a third curve showing 

 the participation of each of the solar radiations in photo- 

 synthesis. It is obtained from the product, for each wave- 

 length, of the ordinates of the first two curves. This third 

 curve is very similar to the curve of efficacy ; the two maxima 

 are situated at the same wave-lengths, but as the smaller 

 is in a region where the solar power is greater, it is increased 

 and becomes about four-fifths of the maximum situated in 

 the red. In the interval between these two maxima, the 

 participation of the green and yellow solar radiations 

 remains more than half of that of tKe principal maximum 

 (Fig. I, 14). 



