34 The Living Plant 



if, while one is observing this spectrum, a solution of chlorophyll 

 is inserted into the path of the light, a remarkable phenomenon 

 follows, for the green liquid blots out from the spectrum most of 

 the red and nearly all of the blue-indigo-violet, making those 

 parts of the spectrum quite black, while all of the rest of the colors 

 are left practically unaffected, as represented in our diagram 

 (figure 5, b). Chlorophyll, therefore, has power to absorb red 

 and blue rays out of the sunlight, ignoring the others, — in ob- 

 serving which fact the active scientific mind would jump straight 

 to the conclusion that these red and blue rays are probably the 

 ones which are useful in photosynthesis. This hypothesis also 

 is easily tested by experiment, for, obviously, if the red and blue 

 rays really are those used in photosynthesis, while the others are 

 not, then starch ought to be made under red light and blue light, 

 but not under any others of the colors of the spectrum. It is 

 possible to supply the different colored lights singly to the green 

 leaf, either by use of colored glasses or liquids or by throwing a 

 solar spectrum directly upon a leaf. The result of the experiment 

 is conclusive; a leaf can form starch very readily under red light 

 or blue light; but it can form none at all under the yellow, orange, 

 or green. It seems a safe inference, therefore, that chlorophyll is 

 a substance which picks out of white sunlight and applies to 

 photosynthetic work, just those rays which can be utilized in the 

 photosynthetic process, while rejecting the others; and all evidence 

 attests the correctness of this conclusion. 



This conclusion, however, raises a correlated question, which is 

 this, — for what particular purpose is light needed in photosyn- 

 thesis? Light, of course, is a form of energy, like heat and elec- 

 tricity; and energy is the source of power which underlies every 

 kind of work. Light, so physicists teach, consists of wave- 

 motions in a space-pervading medium called the luminiferous 

 ether; and the motion of these ether waves forms a source of 

 power that can accomplish work, just as surely as can the billows 

 of the ocean. Our problem, then, resolves itself into this, — is 



