APPENDIX 343 



never to be observed in Nature! In their unsuccessful en- 

 deavours to represent Nature, these artists probably tend to 

 portray her in the clearest green possible, whereas the colour 

 of our vegetation is in fact a mixture of green and red.^ 



We must, however, return to the main object in view. We 

 wanted to know which rays are absorbed by the plant, and we 

 found that chlorophyll absorbs certain red, orange, and yellow rays, 

 as a result of which its spectrum presents a black band in place 

 of them. This fact can be tested even for a single chlorophyll 

 granule under a microscope. This time instead of throwing the 

 spectrum upon a wall, we can obtain it under the microscope by 

 means of a lens, and in this spectrum of the size of a pin's head 

 we investigate our chloroplast. We notice then that it appears 

 transparent green in the green part of the spectrum, transparent 

 red in the extreme red, and entirely opaque, as black as soot, 

 in the red rays (marked by BC in fig. 82) absorbed by the solu- 

 tion. This means that the living grains of chlorophyll also 

 absorb these rays. 



Thus when they fall upon a plant or rather upon the 

 chloroplasts enclosed in its cells, certain of the sun's rays 

 .become absorbed, cease to be light any longer. But there is 

 no loss of energy : it has only changed, passing into a state 

 of tension. What kind of work is done by these rays in 

 a plant ? Let us recall the conclusion we have just arrived 

 at, that sunlight decomposes carbonic acid in plants. May 

 not this work take place at the expense of just those rays 

 absorbed by the chlorophyll granules ? This suggestion gains 

 in probability when we learn that the chloroplast is the 

 very organ, the very apparatus in which the decomposition 

 of carbonic acid takes place. Priestley noticed that the de- 

 composition of carbonic acid and the giving off of oxygen 

 take place exclusively in the green parts of the plant, i.e. 

 in leaves or green stems. He was even able to prove that 

 this activity is due to the green substance. If a vessel of 



^ It is difficult to give any definite advice on tlie subject in the 

 absence of necessary technical information. The spectrum of chrome 

 green is nearest of all mineral green colours to the spectrum of chlorophyll, 

 its green colour being a mixture of red and green rays. At all events 

 we cannot obtain the green colour of foliage by mixing together yellow 

 and blue (the blue of the spectrum). 



