50 THE NATURE OF LIFE 



it is unable to make use of all the dif- 

 ferent kinds of rays found in sunlight. It 

 absorbs principally the red and orange 

 light (as well as ultra-violet). Since it 

 can use only what it can absorb it is to 

 be expected that it will be more active 

 in photosynthesis in red and orange light 

 than in other colors. This seems in gen- 

 eral to be the case, as is shown by a very 

 beautiful experiment in which a spectrum 

 is thrown upon a green plant placed in 

 water. Certain motile bacteria which 

 seek oxygen are placed in the water and 

 they gather most thickly around that 

 portion of the plant which lies in the red 

 and orange light, indicating that oxygen 

 is given off most freely there. It should 

 be said, however, that these and other 

 experiments need repeating under con- 

 ditions which insure that the intensity 

 of all the different rays shall be equal: 

 there is some evidence that in this case 

 the result will be quite different. 



From sugar and starch fats may be 

 formed, but to make proteins nitrogen 



