8 THE ECONOMICS OF FEEDING HORSES 



of energy, which is supplied by the rays of the sun, and 

 becomes latent or dormant in the components of the 

 plant tissue, and ready to supply the energy needed by 

 the animal. An animal is quite incapable of directly 

 using this solar energy, or of deriving nutrition from 

 simple inorganic compounds, and so is absolutely de- 

 pendent on the chemical activities of plants. To take 

 one example, the element which forms the largest 

 amount of the dry matter of both plants and animals is 

 carbon, familiar in the impure forms of wood and bone 

 charcoal. In the air there is a practically unlimited 

 supply of carbon, existing as carbon dioxide, a gaseous 

 compound of carbon and oxygen. Normally, the 

 atmosphere contains 4 parts of carbon dioxide in 

 10,000, and since the atmosphere weighs approximately 

 14 pounds to the square inch, it follows that there is 

 in the air above us something like 16 tons of carbon 

 dioxide, or 44 tons of carbon, to the acre. All this is 

 quite unavailable for animals — in fact, carbon dioxide, 

 if concentrated much above the percentage present in 

 air, is a narcotic poison to animals. Plants, however, 

 can take up this gas, and from it assimilate the carbon 

 into the plant cells and tissues — not indeed in the form 

 of the element carbon, but as an organic compound, 

 such as starch or sugar, of a more complex chemical 

 nature than the simple carbon dioxide which the plant 

 took in. Thus the energy of the sun, directed by the 

 vital activity of the plant, has formed from the inorganic 

 simple compound carbon dioxide, by combination with 

 water, a complex organic substance, such as starch, or 

 sugar, or some other member of the important group 

 of food substances known as carbo-hydrates. Carbo- 

 hydrates are one of the chief sources whence an animal 



