PLANT GROWTH. 6 



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

 phorus, besides sulphur alread}^ 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 oxalic, malic, tartaric 

 and citric acid are the most common. The metals are 

 also sometimes 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 

 nitrates, and the salts of the vegetable acids are found in 

 the form of carbonates. 



It is common to speak of the combustible ingredients 

 of a plant as "organic," and the incombustible ingredients 

 as "inorganic." This distinction is scarcely accurate, as 

 those ash constituents which are indispensable parts of 

 plants have, during the plant's life, 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 5 tons when 

 cut, and producing IJ 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 page 38. 



