THE PLANT AND ITS FOOD 25 



A solution of chlorophyll examined by 

 means of a spectroscope exhibits a number 

 of very definite absorption bands, due to the 

 absorption of certain of the coloured rays 

 of the spectrum (= the dark bands) while 

 the rest of the light filters through and is 

 unaffected. There are two very dark bands 

 in the red region of the spectrum, and others, 

 mostly fainter and more diffuse, in the yellow, 

 green and blue-violet regions. Furthermore 

 the extreme red and the violet end of the 

 spectrum are also obliterated. 



It is found, as might perhaps be anticipated, 

 that the rays of light which correspond to the 

 dark absorption bands in the red region of 

 the spectrum are those principally concerned 

 in promoting photosynthesis. The other rays 

 which are absorbed are not indeed without 

 influence, but they are of comparatively little 

 consequence from the point of view with which 

 we are just now concerned. We see from the 

 foregoing why it is so essential that the chloro- 

 phyll in the living plant should be directly 

 exposed to the light from the sky, inasmuch 

 as any light which has already traversed a 

 layer of chlorophyll will have been deprived 

 of those rays that are essential for photo- 

 synthesis. Such " filtered" light will of 

 course be unable to develop photosynthetic 

 activity in a chlorophyll-containing organ or 

 organism that may be exposed to it. The 

 apparent exceptions afforded by plants which 

 flourish in deep shade are due to the circum- 



