METABOLISM 49 



accumulated it might poison the leaf 

 but it changes to sugar and this in turn 

 may produce starch. It is possible to 

 observe the chlorophyll bodies under the 

 microscope and to see the formation of 

 the starch grains within them. 



When starch is burned energy is given 

 oflF in the form of heat. This energy was 

 furnished by the sunlight and became 

 stored up in the starch. The process of 

 photosynthesis, by which starch is pro- 

 duced from carbon dioxide and water, 

 utilizes the energy of sunlight. It is this 

 which supplies most of the energy needed 

 by animals and plants and which likewise 

 runs our steam engines when wood or coal 

 is burned in them. 



The green plant is practically the only 

 place in nature where such storage of en- 

 ergy occurs and it therefore occupies a 

 place of peculiar importance. If we in- 

 quire how efficient it is we find that it 

 actually stores less than one per cent of 

 the energy which it receives from the 

 sun. This is due in part to the fact that 



