172 CHEMICAL COMPOSITION OF PLANTS 



protein or albuminoid substances, amides and a few other less 

 important organic substances ; it is also found in the inorganic 

 nitrates which are frequently present in small quantity in the 

 cell-sap of plants. 



The amount of nitrogen present is especially high in the seeds 

 of leguminous plants, being in peas 4'8 per cent., in beans 5 

 per cent, and in yellow lupins as much as 7 per cent, of the 

 dry matter : in starchy cereal grains such as wheat, barley, and 

 maize the amount is usually less than 2 per cent. 



The vegetative parts of leguminous plants are generally richer 

 in nitrogen than those of most other plants : for example, in red 

 clover and lucerne cut in bloom the amount present is from 2 

 to 2| per cent, while in grasses the average amount is about if 

 per cent of the dry matter. 



With the exception of leguminous plants which derive most of 

 their nitrogen from the free nitrogen of the atmosphere (see 

 p. 792), green plants take up this element from the soil chiefly in 

 the form of nitrates. It has been proved by means of water 

 cultures that they are also able to absorb and utilise the nitrogen 

 of ammonium compounds, but as the latter when applied to the 

 soil become changed into nitrates in the process of nitrification 

 (see p. 785) it may be said that nitrates are the chief natural 

 source of nitrogen for green plants. 



Although it has been shown that most plants can grow equally 

 well with nitrogen in the form of ammonium salts as with nitrates. 

 Maze found that solutions of the former when more concentrated 

 than about "5 gram in 1000 damage the plants, whereas bad effects 

 are not visible with nitrates until the solution applied to the roots 

 contained 2 parts in 1000 of water. 



Nitrogen when supplied to plants in considerable quantity 

 specially increases the luxuriance of their leaves, stems and 

 vegetative organs ; such plants are dark green in colour, and 

 show little tendency to produce reproductive organs and seeds. 



(iv) Phosphorus. — Phosphorus is a constituent of several kinds 



