8 THE ULTIMATE CONSTITUENTS OF PLANTS 



if fused, surround particles of carbon, and effectively 

 prevent their oxidation. In cases of this trouble aris- 

 ing, it is best to char the substance before ignition at 

 as low a temperature as possible (faintly visible red 

 heat), and then to extract the major portion of the 

 alkali salts by boiling with water. The black residue 

 and the evaporated extract are separately ignited in 

 the muffle, and their weights added together, to give 

 the value for the total ash. 



Suitable substances for the above determination are wheat 

 (grain), different grasses and hays, potatoes, etc., the ash obtained 

 from which may be kept for 6, below. 



The compounds present in the ash of plants are generally not 

 present as such in the plant, but have been produced during 

 ignition from various mineral constituents. For example, the 

 metallic phosphates are produced from complex phosphorus- 

 containing bodies such as lecithin, while the carbonates always 

 present in the ash are formed by the action of heat on such com- 

 pounds as potassium oxalate, potassium malate, etc. The percent- 

 age of ash in a plant varies between certain limits, depending upon 

 climatic conditions, stage of growth, and nature of the soil. In 

 the various portions of the same plant also the percentage varies, 

 being generally less in the seeds than in the leaves or stems. 



Percentages of Ash in different Plant Products. 



6. Qualitative Examination of Ash Constituents. 



Plant ash, which contains potassium, sodium, calcium, mag- 

 nesium, iron, phosphate, carbonate, chloride, sulphate, and silica, 

 can naturally be examined by the ordinary methods of qualitative 

 analysis applicable to a mixture of any metallic salts. In order, 

 however, to avoid the somewhat troublesome separation of phos- 

 phoric acid necessary in the case of a complete qualitative analysis, 

 the following simplified method of procedure may be employed. 



