92 ELEMENTARY AGRICULTURAL CHEMISTRY 



Calcium oxalate is very frequently found in plants, often in 

 the form of crystals of CaCgO^-SH^O. Acid potassium oxalate 

 also often occurs in solution in the sap of plants. 



Tannic acid is present in many plants, often associated 

 with glucose. 



Organic acids occur in the sap of the roots and root-hairs of 

 plants, and possibly aid in promoting the solubility of the 

 mineral matter of the soil. The nature of these acids has not 

 been much investigated, though it has been shown that in a 

 large number of plants the average acidity of the sap, expressed 

 in terms of hydrogen, is about '013 per cent., corresponding to 

 about 0'91 per cent, of crystallsed citric acid.* 



V. Inorganic Elements. — Many of the inorganic constituents 

 of plants, the metals in particular, occur in combination with 

 organic acids, as already stated. Others — phosphorus and 

 sulphur — are associated with complex organic compounds, e.g., 

 albuminoids. A few words may be said about the mode of 

 occurrence and functions of each of the inorganic elements 

 found in plants. 



SvlphuVy though existent in a living plant chiefly as a con- 

 stituent of albuminoids, is left in the ash as sulphate, or some- 

 times as sulphide. It is probably obtained by the plant from 

 the sulphates in the soil, and can often be detected in that form 

 in the sap. 



Phosphoi'us is undoubtedly absorbed as phosphates, and exists 

 in that form in the ash. In the living plant, however, it exists 

 partly in union with organic compounds, and appears to move 

 about in association with the albuminoids. 



Silicon is probably taken into the plant in the form of 

 alkaline silicates. There is considerable evidence that though 

 silicon is often present, generally as deposits of silica in the 

 outer walls of the stem and leaves (particularly in cereals), it 

 is not indispensable. 



Chlorine is found in all plants, but does not appear of much 

 * Vide footnote on p. 79. 



