1 8 AGRICULTURE. [CH. 



in different kinds of plants in very different proportions. 

 One variety of plant is found to contain more of one 

 material than another kind of plant, and the total 

 quantity of ash also varies. Taking the entire range 

 of cultivated crops, we find that, with one exception 

 (24) all the inorganic substances named as present 

 in the soil (26) are taken up by plants and built into 

 their structures. It is also known that plants take up 

 this inorganic or mineral matter with some regularity, 

 selecting only that which they require, and refusing to 

 use that which is not desirable for their growth. 



31. That portion of the plant which is burnt off is 

 known as the organic matter. In the plant it exists 

 in a great variety of forms, but these have been grouped 

 into two classes those which contain nitrogen are 

 called nitrogenous bodies, whilst those which do 

 not contain nitrogen are called non-nitrogenous 

 bodies. This is a distinction which must be carefully 

 remembered, for these organic substances are not only 

 distinguished by this difference in their composition, 

 but the presence or absence of nitrogen also deter- 

 mines the work they can perform. 



32. The following is a list of the principal substances 

 which constitute 



THE ORGANIC MATTER OF PLANTS. 



Non-nitrogenous Bodies. 



Starch. 



Gum. 



Sugar. 



Cellulose and woody fibre. 



Oil. 



Nitrogenous Bodies. 



Albumen. 

 Fibrin (gluten). 

 Casein (legumin). 



The non-nitrogenous bodies are all composed of 

 the three elements carbon, hydrogen, and oxygen. 

 Because they contain carbon, they are often called 

 carbonaceous, but it is more convenient to describe 

 them as non-nitrogenous. The bodies named in the 

 nitrogenous group contain carbon, hydrogen, and 

 oxygen, but they also contain nitrogen, and hence 



