INTKODUCTOKY 



is not destroyed but escapes into the air. By means of 

 suitable apparatus it may be collected and examined. It 

 will be found to contain the elements hydrogen, oxygen, 

 nitrogen, carbon and sulphur. The proportion in which 

 these elements are present necessarily varies considerably 

 in different circumstances, but roughly, carbon may form 

 about half the total weight, oxygen a third, hydrogen a 



AVERAGE PROPORTIONS OF ASH AND ORGANIC MATTER IN THE 

 DRY MATTER OF PLANTS. 



tenth, nitrogen a twentieth, and sulphur less than a hun- 

 dredth part of the whole. 



It will be seen, on reference to the table, that the ash 

 rarely exceeds about five per cent, of the dry matter in 

 stems and leaves. In seeds it is often less than half that 

 amount. Nevertheless, the ash is just as important for 

 the growth of the plant as the organic matter. It always 

 contains the elements potassium, magnesium, calcium, 

 iron, phosphorus and usually also sodium, silicon, fluorine 

 and chlorine. Traces of lithium, manganese, and some 



