OUTLINES OF THE PHYSIOLOGY OF NUTRITION 209 



carbon dioxide is a very difficult proeeso to carry out 

 in the laboratory. Thus, in order to reduce carbon 

 dioxide (C0 2 ) to carbon monoxide (CO) by heat, a 

 temperature of no less than 1300 C. is required. Yet 

 this same work is carried on by the plant at the 

 ordinary temperature of the air say 10 to 30 C. 

 The energy which enables a green leaf to do its work 

 is derived from the rays of the sun; plants will onlv 

 assimilate carbon in the light. This is easily proved ; 

 if we repeat the experiment just described in a dark 

 place, no carbon dioxide is absorbed, and no oxygen is 

 set free. Other sources of light may be substituted for 

 the sun. Thus, plants will assimilate actively and con- 

 tinuously when exposed to the electric light. As in 

 nature sunlight alone is in question, we may say that 

 all the organic material of the world, all the animal 

 and vegetable substance which it contains, has been 

 directly or indirectly produced through the assimila- 

 tion of carbon from carbon dioxide by green plants 

 in sunlight. There are not many rules in science 

 without an exception : it has lately been shown that 

 certain very simple organisms are able to obtain their 

 carbon from inorganic sources without the help of 

 light ; the amount of organic substance thus formed, 

 however, is too small to affect our statement appreci- 

 ably. It is only green plants, and parts of plants, 

 which can decompose carbon dioxide in sunlight. If 

 we had used for our experiments coloured or white 

 flowers, or colourless roots, or any part of a plant 

 containing no chlorophyll, w.e should have found that 

 no absorption of carbon dioxide or evolution of oxygen 

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