212 THE CONSTITUENTS OF PLANTS. IX. 



3. The Gum Resins. These are mixtures of gums and 

 resins, and some are of value in the arts, e.g., ammoniacum, 

 asafoetida, gamboge, myrrh, scammonium, &c. 



V. THE INORGANIC SALTS. 



In the introductory chapter a list of the elements essential 

 to plant life was given. It now remains to discuss what is 

 known of the functions of the inorganic elements in the nutri- 

 tion of plants. 



Sulphur is a constituent of proteids and therefore may be said, 

 in a measure, to be present in the organic portions of a plant. It 

 is left in the ash chiefly as sulphate, though, as the author has 

 observed, sometimes as sulphide. The presence of sulphates 

 in solution in the sap can generally be detected. 



Phosphorus occurs in plant ashes as phosphates, and in that 

 form it is absorbed from the soil. In the living plant, however, 

 it undoubtedly exists partially as organic compounds, e.g., as 

 lecithin (v. p. 206), and it seems to move in the plants in com- 

 pany with the proteids. 



Silicon is probably taken into the plant as alkaline silicates. 

 It is always found in the ash, sometimes in large quantities, as 

 silica, SiO 2 , or silicates. Indeed in many plants, c.y., the 

 cereals and grasses, large deposits of silica occur in the outer 

 walls of the steins and leaves ; but it appears that the plant 

 derives little benefit by this and can thrive vigorously without 

 siliceous food. 



Chlorine occurs iri all plants, but it cannot be proved to be 

 essential, except perhaps in buckw r heat and mangolds. Its 

 presence seems to aid in the hydrolysis of starch. 



Potassium is absorbed as various soluble salts and apparently 

 exists in the plant in combination with organic acids, though 

 some saps contain the nitrate, chloride, and sulphate. It is 

 always most abundant in the parts of the plant where assimi- 

 lation is most active, i.e., leaves and twigs. It seems to be 

 absolutely indispensable for the production of starch, sugar, 

 &c., but what part it plays in their production is not known. 



Calcium seems to aid in the conversion of starch into sugar. 

 It probably exists in combination with organic acids ; indeed 

 crystals of calcium oxalate or carbonate are often found in the 



