PLANT CONSTITUENTS. 6 



elements potassium, magnesium, calcium, iron, and phos- 

 phorus, besides sulphur already mentioned. Iron is present 

 in only very small quantity. These five elements, though 

 forming a very small portion of the plant, are indispensable 

 to its life. Besides the elements just named, an ash will 

 generally contain sodium, silicon, and chlorine, with fre- 

 quently manganese, and perhaps minute quantities of 

 other elements. The supplementary elements just named 

 are not apparently essential to plant life, though some of 

 them discharge useful functions in the plant. 



The metals above-named occur in the plant as salts, 

 being combined with phosphoric, nitric, sulphuric, and 

 various vegetable acids, of which formic, acetic, oxalic, 

 malic, tartaric, and citric acid are the most common. 

 The metals are also frequently present as chlorides. 

 Phosphorus occurs in the form of phosphates ; silicon is 

 present as silica. Sulphur occurs partly as sulphates, 

 and partly as a constituent of albuminoids. In the ash 

 of plants the bases of the nitrates, and of the salts of 

 vegetable acids, are found in the form of carbonates. 



It is usual to speak of the combustible ingredients 

 of a plant as organic, and of the incombustible ingredients 

 as inorganic. This distinction is scarcely accurate, as 

 those ash constituents which are indispensable parts of 

 plants have, during the life of the plant, as much right to 

 be called " organic " as albumin or cellulose. 



In the following table will be found the average com- 

 position of a crop of meadow grass weighing five tons when 

 cut, and producing one and a-half ton of hay ; this will 

 illustrate what has just been said as to the constituents of 

 plants. Further information as to the composition of 

 crops will be found on pp. 48 49. 



