ACTION OF LIGHT ON VEGETATION, 



745 



efficacious in this process as all the others put together. The most refrangible 

 rays of the visible spectrum which act most energetically on silver chloride, &c., 

 play a very subordinate part in the process of assimilation.' 



Draper placed glass tubes filled with water saturated with carbon dioxide in 

 which he had placed green parts of plants, in the different coloured portions of a 

 solar spectrum. Seven of these tubes were exposed simultaneously in the same 

 spectrum. The following table gives the result of two experiments of this kind : — 



Part of the Spectrum. 



Dark-red 



Red-orange 



Yellow-green 



Green-blue 



Blue 



Indigo . 



Violet , 



Pfeffer experimented chiefly on leaves of the Cherry-Laurel and Oleander, which 

 were placed in air containing carbon dioxide (shut off by mercury) in suitable glass 

 vessels, and received the sunlight through coloured solutions (tested by the spectro- 

 scope). The following was the result of sixty-four experiments: — If the amount 

 of gas evolved in light which has passed through a stratum of water of standard 

 thickness is represented by 100, the numbers here given are the corresponding 

 quantities of carbon dioxide decomposed in light which has passed through equal 



thicknesses of the solutions named. 



» 



^ , ^ , , , Amount of carbon dioxide 



Colour of light. , J 



° decomposed. 



Red, orange, yellow, green 88-6 



Green, blue, violet 7*6 



Red, orange-green, blue, violet 53*9 



Red, orange-blue, violet 38*9 



Red, orange 32*1 



Red-orange, yellow, green 15-9 



Quite dark (14*1 carbon di- 

 oxide produced). 



Solution. 



Potassium bichromate 



Ammoniacal copper oxide 



Orcin 



Aniline-violet 



Aniline-red 



Chlorophyll 



Iodine solution 



From a comparison of these numbers Pfeffer deduced the following values for 

 the decomposing power of the different regions of the spectrum, the action of white 

 light being again placed at 100: — 



For Red-orange 

 Yellow 

 Green . 

 Blue- violet 



32'I 



46*1 



15-0 



7-6 



100 8 



and from these is deduced the first statement of Pfeffer given above. 



