l66 CHAP. XX. EFFICIENCY OF DIFFERENT COLOURS 



greater in the more refrangible portion of the spectrum, 

 so that the intensity of light is relatively weakened- 

 in the blue-violet region. It is therefore necessary not 

 merely to obtain a pure spectrum, but also to determine 

 the energy of the different rays present in it. For the 

 detection and qualitative determination of photosynthesis 

 in the different regions of the spectrum, Engelmann devised 

 the Bacterium-method, in which aerotropic swarmers are 

 observed to congregate in larger numbers at the place of 

 maximum evolution of oxygen during photosynthesis ; 

 this method, though extremely sensitive, is not suitable for 

 quantitative measurements. Timiriazeff devised a highly 

 ingenious method by which he was able to map out the 

 photosynthetic action in different parts of the spectrum : 

 he threw the solar spectrum on to a leaf, and after several 

 hours developed it by first decolorising it with alcohol and 

 subsequently treating it with iodine ; the region of starch- 

 formation under the effective rays then came out black. 

 The most effective rays were found to lie between the 

 Fraunhofer lines B and C. This method, again, is rather 

 qualitative than quantitative. 



Various attempts have been made to establish a 

 relation between the wave-lengths of light and their photo- 

 synthetic efficiency. Timiriazeff (1877, 1885) and Reinke 

 (1884) agree that the maximum C0 2 -assimilation takes 

 place in the red, though they differ in regard to the exact 

 maximum point. Engelmann came to the conclusion that 

 instead of a single point of maximum activity there are two 

 maxima, one in the red and the other in the blue. Pfeffer, 

 however, could not detect this second maximum, though 

 he employed the same Bacterium-method. Timiriazeff 

 and Richter (1902) attributed the smaller assimilation in 

 the blue to the diminished intensity of radiation. Richter, 

 indeed, was of opinion that it is not the quality but the 

 energy of the light absorbed that determines effective 

 assimilation. Jost thus summarises the general results 

 of all the researches that have been made on the subject : 



