ACTION OF LIGHT ON VEGETATION. 



667 



efficacious in this process as all the others put together. The most refrano-ihle 

 rays of the visible spectrum, and those 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. 



Gas evolved, 



Experiment II. 



GO 



24-75 



4375 



4-10 



I '00 



00 



O'O 



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. 



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 . . . • 32"i 



Yellow . . . .46-1 



Green i5'o 



Blue-violet . . • •7'^' 



:oo-8 



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



