LIGHT AND VEGETATION 101 



when the current changes direction, than that of incandescent 

 lamps; in the latter, the thermal inertia of the filament 

 reduces the fluctuations. The increase of efficiency, in the 

 conditions in which Warburg experimented, ought to be 

 about doubled. 



The advantages of these fluorescent tubes for plant 

 illumination have already been mentioned, first because their 

 radiation is poor in infra-red (infra-red, which is too abundant 

 when incandescent lamps are used, is a serious disadvantage), 

 and then because fluorescent cases can be chosen to emit 

 radiation the composition of which can be regulated, within 

 wide limits, to enable the power to be localized in the most 

 efficacious regions — the blue for the balanced formation 

 of the plant, the blue and the orange for photosynthesis. 



To these advantages is added the possibility of choosing 

 fluorescent substances of rapid extinction, which will con- 

 centrate the emission of light during the maxima of the 

 alternating current without prolonging it between the 

 alternations. 



Not only, therefore, is the efficiency excellent for the 

 conversion of electrical energy into luminous energy, but 

 there is the prospect of a better efficiency than that of dayhght 

 for the conversion of luminous energy into plant products. 

 Unfortunately, fluorescent tubes are expensive to instal. 



It must be added, however, that, although a rapid alter- 

 nation of fight and darkness is favourable, this does not hold 

 good for alternations of a longer period, one hour to thirty 

 seconds, for example. Alternations of one to five minutes 

 appear to constitute a real poison for the plant, which is 

 incapable of developing in those conditions, whatever care it 

 may receive. This strikingly illustrates to what point light is 

 essential to vegetation and that the way in which it is dis- 

 tributed matters quite as much as the total quantity offered. 

 Other examples of this occur in connection with photo- 

 periodism. 



