38 THE LIFE OF THE PLANT 



away and volatilised. If we carry out this combustion 

 with certain precautions and collect the volatile gases, 

 we discover that the part of the vegetable matter which 

 burns away consists of four elements : solid carbon 

 and three gases — oxygen, hydrogen, and nitrogen. 

 This combustible part, which always contains carbon, 

 as is shown by the fact that it chars before it burns, is 

 called the organic substance of the plant. It is called 

 organic because it enters into the composition of all 

 organisms. At first people thought that organic matter 

 could be formed only in living bodies, in organisms, and 

 that only less complicated substances, which make up 

 dead or inorganic nature, could be produced artificially 

 in laboratories. But this opinion has been shaken by 

 recent progress in organic chemistry. Chemists can 

 now produce a great number of bodies, the forma- 

 tion of which used to be considered a mystery of 

 the living organism. All organic substances do not 

 necessarily consist of all four elements ; some of them 

 are composed of three only, carbon, hydrogen, and 

 oxygen ; or only of two, carbon and hydrogen. More- 

 over, these same elements are combined in different 

 proportions in different substances, so that obviously 

 in different plants, or in different parts of the same 

 plant, the elements will be present in different pro- 

 portions. Nevertheless, by taking the mean of a 

 number of analyses of various plants and of their 

 component parts, we can form an estimate of the 

 average elementary composition of a plant. One 

 hundred parts of dry vegetable matter contain on an 

 average : — 



45 'o per cent, of carbon. 



6 '5 ,, of hydrogen. 



I "5 ,, of nitrogen. 

 42"o ,, of oxygen. 



5"o „ of ash. 



This table gives a clear idea of the ratios in which the 



