196 Plant Physiology 



gen. A crude picture of the distribution and importance 

 of the carbon in plants is afforded by the well-known 

 process of charcoal making, a burning without free 

 access of oxygen. As a result, the hydrogen, nitrogen, 

 and oxygen of the plant are set free, while only carbon 

 and the small proportion of ash remain. When burned 

 with abundant oxygen, the carbon combines with the 

 oxygen by an oxidation or hydroxylation process, and 

 gaseous carbon dioxid is ultimately formed. In this case 

 a perfectly definite amount of energy, as heat, is released. 



105. Carbon dioxid the source of carbon in green 

 plants. We are now concerned with the source from 

 which plants obtain their carbon supply, the conditions of 

 intake, and the method by which the carbon is incorpo- 

 rated into food for the plant cell. Carbon in inorganic 

 form, especially as carbonates of lime and magnesia, con- 

 stitutes no inconsiderable portion (about twice as much as 

 phosphorus) of the minerals of the earth's crust. Yet, as 

 will be subsequently indicated at length, carbonates are 

 valueless as a source of the carbon from which to make 

 organic compounds for either plant or animal. Moreover, 

 both water and sand cultures abundantly demonstrate 

 that green plants are able to grow and to attain their full 

 development (necessitating the making of organic matter) 

 in nutrient solutions containing no carbonates and also 

 no organic matter whatsoever save that derived from the 

 seed. 



It is easy to convince ourselves of the requirements of 

 common animals with respect to organic matter. They 

 feed upon plants or other animals, or upon the products 

 of these. Moreover, there are the countless fungi and 



