6 CHEMISTRY OF PLANT LIFE 



is suitable here. However, the general principles of the utilization 

 of soil elements by plants for their nutrition and growth may be 

 fairly well understood from the following statements. 



Nitrogen is a constituent of all proteins (see Chapter XIII). 

 Proteins are apparently the active chemical components of proto- 

 plasm. Since it is in the protoplasm of the green portions, usually 

 foliage, of plants that the photo-synthesis of carbohydrates and 

 the synthesis of most, or all, of the other tissue-building materials 

 and reserve food substances of the plant takes place, the impor- 

 tance of nitrogen as a plant food can hardly be over-emphasized. 

 Nitrogen starvation produces marked changes in the growth of a 

 plant. Leaves are stunted in growth and a marked yellowing 

 of the entire foliage takes place; in fact, the whole plant takes on a 

 stunted or starved appearance. Abundance of nitrogen, on the 

 other hand, produces a rank growth of foliage of a deep rich color 

 and a luxuriant development of tissue, and retards the ripening 

 process. In the early stages of growth, the nitrogen is present 

 most largely in the leaves; but when the seeds develop, rapid 

 translocation of protein material into the seeds takes place, until 

 finally a large proportion of the total supply is deposited in them. 



Nitrates are the normal form of nitrogen in the soil which is 

 available to plants. During germination and early growth, the 

 young seedling uses amino-acids, etc., derived from the proteins 

 stored in the seed, as its source of nitrogen; and experiments have 

 shown that similar forms of soluble organic nitrogen compounds 

 can be successfully fed to the seedling as an external food supply. 

 Soluble ammonium salts can be utilized as sources of nitrogen by 

 most plants during later periods of growth, particularly by the 

 legumes. But for most, if not all, of the common farm crops 

 whose possibilities in these respects have been studied, it has been 

 found that a unit of nitrogen taken up as a nitrate is very much 

 more effective in promoting growth, etc., than is the same unit of 

 nitrogen in the form of ammonium salts. 



While the proteins are finally stored up largely in the seeds, 

 or other storage organs, they are actively at work during the grow- 

 ing period in the cells of the foliage parts of the plant. Hence, 

 the popular statement that " nitrogen makes foliage " is a fairly 

 accurate expression of its role. Inordinate production of straw 

 in cereal crops and of leaves in root crops often results from liberal 

 supplies of available nitrogen in the soil early in the growing sea- 



