CHAPTER V. 



LEGUMINOUS PLANTS. 



From the foregoing- chapters the student 

 should have an understandine of the fact that 

 the food of plants must contain certain ele- 

 ments, and that these food elements must be 

 obtained from the air or as soluble material 

 from the soil, so that they can be absorbed by 

 the roots. 



One of the most important elements is nitro- 

 gen (see Chapter IV.). It is found in the pro- 

 toplasm of every plant cell. The nitrogenous 

 compounds in the plant, taken as a whole, are 

 called crude protein. No plant can live without 

 a supply of nitrogenous food. 



Now if this nitrogen is to be obtained from 

 the soil, and since the plant requires so great a 

 proportion of it, it will be easily seen that the 

 supph' in ordinary soils would in time be ex- 

 hausted unless some means were taken to 

 replenish it. This is usually done by the appli- 

 cation of a fertilizer — some salt of nitrogen, 

 which is the most expensive of fertilizers. 



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