206 MINERAL CONSTITUENTS 



portance in varnish making, etc., e.g., copal, dammar, elemi, sandarach, 

 amber. 



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, seammonium. 



V. THE INOKGANIC 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 nutrition 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 can 

 generally be detected in the sap. 



u i * Phosphorus occurs in plant ashes as phosphates, and in that form 

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

 doubtedly exists partially as organic compounds, e.g., as lecithin (vide 

 p. 198), and it seems to move in the plants in company with the 

 proteids. It is always most abundant in the seeds. Liberal supplies 

 of phosphates favour root development and tend to early maturity. 



Silicon is probably taken into the plant as alkaline silicates. It is 

 always found in the ash, sometimes in large quantities, as silica, Si0 2 , 

 or silicates. Indeed in many plants, e.g., the cereals and grasses, 

 large deposits of silica occur in the outer walls of the stems and 

 leaves ; but it appears that the plant derives little benefit by this and 

 can thrive vigorously without siliceous food. 



Chlorine occurs in all plants, but it cannot be proved to be essen- 

 tial, except perhaps in buckwheat 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 assimilation is most active, 

 i.e., leaves and twigs. It seems to be absolutely indispensable for the 

 production of starch, sugar, etc., but what part it plays in their pro- 

 duction 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 cells. One of its 

 functions may be to combine with, and render harmless, the vegetable 

 acids formed in the plant. It is found chiefly in the leaves. 



Magnesium is fairly uniformly distributed ; little was known of its 

 functions ; but the recent discovery by Willstatter 1 and his collabora- 

 tors that it is an essential constituent of chlorophyll, shows how im- 

 portant it is to the vegetable organism. 



Iron is usually present only in small proportion, but it is quite 



* Vide Chlorophyll, p. 216. 



