XI. NUTRITIVE METABOLISM 



287. Essential Constituents of the Food of Plants. A careful 

 chemical control of the medium in which a plant lives and the 

 substratum to which it is attached, demonstrates that the elements 

 necessary for growth and existence, comprise carbon, oxygen, 

 nitrogen, hydrogen, sulphur, phosphorus, calcium, potassium, 

 iron, and magnesium. Calcium, however, is not necessary for the 

 fungi. The supply of some of these elements may be partially 

 replaced by others which may be themselves essential or non- 

 essential. Thus, for instance, it is found that sodium and calcium 

 may partly fill the place of potassium and magnesium under some 

 conditions of growth. The analysis of the bodies of plants re- 

 veals the fact that many other substances are often present, and 

 it is to be said that almost any element in the soil may be taken 

 up in such quantity as to form a noticeable proportion of the ash. 



The presence of sodium and chlorine in the substratum is often 

 an important condition : although these substances may not be 

 actually used in the plant, yet they exert a tonic influence upon it 

 by stimulating the absorbing organs, and play an important part 

 by their chemical action on the other constituents of the sub- 

 stratum (97). 



Carbon is obtained from the carbon dioxide of the air by green 

 plants and from organic compounds by chlorophylless forms. 

 This element is perhaps the most important as it enters largely 

 into all compounds from which the organism is constructed. 



Oxygen is a constituent of the combustible substances of the 

 plant, and is obtained from the air, water, salts and oxides taken in 

 from the soil. Hydrogen is absorbed in the form of water by 

 green plants, in the form of ammonia and its compounds, and 

 sparingly in the form of complex compounds by the higher plants, 

 which form a large proportion of the food of the bacteria and 



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