100 LIGHT, VEGETATION AND CHLOROPHYLL 



It is remarkable that, according to the experiments made 

 by Li, a sudden change of the colour of the hght, without 

 modification of the power of radiation, does not cause any 

 appreciable variation of the rate of photosynthesis. This would 

 seem to indicate that, apart from the variations of efficacy, 

 the plant regards the different active radiations as equivalent. 



The processes which come into operation in these various 

 reactions of the plant have not been elucidated; the move- 

 ments of the chloroplasts are obviously insufficient to explain 

 such large variations in photosynthesis. No doubt other 

 processes are affected in experiments in intermittent light 

 with periodic variations repeated several times per minute or 

 per second. 



Efforts have been made to obtain from these investi- 

 gations information on the nature of the complex chemical 

 reactions which result in photosynthesis. 



It is promoted by intermittent Hghting. The same quantity 

 of light (which therefore presupposes stronger illumination 

 during the flashes) is better utilized when it is given in small 

 quantities than when it is given continuously. There is a 

 surprising difference in the efficiency; with an interruption 

 every fifteen seconds, it is increased by 10 per cent. Flashes 

 at a frequency of 130 per second stimulate twice as 

 much assimilation as the same quantity of light suppUed 

 continuously. 



These results were obtained by Warburg with equal 

 periods of light and darkness. Emerson and Arnold experi- 

 mented with extremely brief flashes, the duration of which 

 did not exceed ten microseconds at a frequency of 50 per 

 second; the efficiency can then be increased fivefold. 



The interpretation of these results will be given later. 

 Note simply here that they can very easily be turned to 

 practical advantage in cultivations under artificial Hght by 

 using alternating current supplied from light sources of low 

 inertia which are extinguished at each alternation of the 

 current. The new fluorescent lamps seem well adapted to this 

 purpose because their light becomes fainter, at the instants 



