ABSORPTION OF CARBON 7 



tive microchemical reactions it is not difficult to demonstrate 

 the K, Mg, Na, SO4, PO4, and other ions in the plant. 



Besides an organic analysis, it is possible also to make an ele- 

 mentary analysis, i.e., to try to ascertain which chemical elements 

 are present and in what quantitative proportions they make up the 

 body of a plant. For this purpose it is necessary first to dry the 

 plant, since no less than 80 per cent of it is water, and then to burn 

 with oxygen the resulting dry matter, according to the accepted 

 methods of organic chemistry. In a great majority of cases, it 

 has been found that almost half of the dry matter consists of 

 carbon, and almost an equal amount of oxygen. Hydrogen, and 

 nitrogen are present in considerably smaller quantities. In addi- 

 tion to the elements volatilized on combustion, there remains a 

 certain amount of non-volatile compounds in the form of ash. 

 The average composition of the dried plant tissues may be expressed 

 as follows: 



C, 45 per cent N, 1.5 per cent 



0, 42 per cent Ash, 5 per cent 



H, 6.5 per cent 



Different plants and different parts of plants vary considerably 

 in their composition, hence the above-mentioned values must not 

 be regarded as absolute. The composition of ash is also very 

 complex and varied. Avoiding details, to which we shall return 

 in Chapter III, it must be remarked here that the following 

 metals are always found in ash: potassium, sodium, calcium, 

 magnesium, iron, and manganese; and of non-metals, sulphur, 

 phosphorus, silica, and chlorine. 



When plants are grown under natural conditions, in a soil 

 comparatively rich in humus, it is not possible to determine 

 definitely the source of the principal constituents. All these 

 elements may be found in the soil in the form of different organic 

 and mineral compounds. Some of them, namely nitrogen, oxygen, 

 and carbon (as carbon dioxide) are present also in the atmosphere. 

 The long continued observations of agriculturists, that plants 

 develop well and produce high yields only when grown on dark 

 soil rich in humus, led to the conviction that all of the necessary 

 substances are obtained by the plant from the soil nutrients. 

 This conviction, however, has proven to be erroneous, as may be 

 demonstrated best by the method of artificial cultures. 



