OUTLINES OF THE PHYSIOLOGY OF NUTRITION 207 



II. Assimilation of Carbon. We will now consider 

 the question how a plant containing chlorophyll obtains 

 its supply of carbon, the most important of all the 

 elements for nutrition. We have already learnt that 

 the plant certainly does not obtain this element by 

 its roots, and that the air is the only source of carbon 

 available. 



Now, the atmosphere contains carbon in the form 

 of carbon dioxide, more commonly, though inaccurately, 

 called carbonic acid. 1 The proportion of this gas 

 contained in the air is very small, amounting only 

 to three volumes in 10,000 volumes of air. Yet 

 from this relatively small amount the entire supply 

 of carbon for most green plants is obtained. 



Carbon dioxide is formed whenever carbon is com- 

 pletely burnt, or, in other words, completely oxidised. 

 It contains the maximum amount of oxygen with 

 which carbon can combine. Now we know that the 

 substance of plants is combustible ; in fact, all our 

 fuels are directly or indirectly of vegetable origin. It 

 follows, then, that the tissues of plants must be com- 

 paratively poor in oxygen, or they would not burn. 

 Hence we see that the carbon dioxide of the air must 

 lose the whole or part of its oxygen when it is 

 assimilated by a plant. 



That green plants in sunlight give off oxygen 

 is easily proved. Some pieces of any green water- 



1 Carbonic acid is, properly speaking, the compound of carbon 

 dioxide with water, and has the formula H.,C0 3 . Hence carbon 

 dioxide is sometimes called carbonic anhydride, the latter word 

 implying that the acid is deprived of its water. 



