58 THE NATURE OF LIFE 



as is easily shown when we follow the 

 process of combustion or oxidation by 

 means of the changes in color which it 

 sometimes produces. Many plants show 

 changes during the process of death. We 

 are familiar with the blackening which 

 occurs in plants as the result of frost 

 or of injury due to crushing. These 

 changes, which are due to oxidation, are 

 particularly well seen in the common 

 Indian Pipe, whose cells are well adapted 

 for observing the whole process under 

 the microscope. In this case the most 

 rapid oxidation (and consequently black- 

 ening) appears to occur in the cell-nu- 

 cleus. It has been suggested that this 

 may be connected with the fact that the 

 nucleus contains more iron than the rest 

 of the cell. 



It is possible to extract from plants 

 and animals colorless substances which 

 if allowed to take up oxygen turn yellow, 

 then red, and finally black. These pig- 

 ments are responsible for much of the 

 coloration of animals, in skin, feathers. 



