138 HOW CEOPS GEOW. 



•when we desire to make statements which may be com- 

 pared together, because, as has just been remarked, we 

 cannot always, nor often, say what sulphates or what 

 chlorides are present. 



In the paragraphs that follow, which are devoted to 

 a more particular statement of the mode of occurrence, 

 relative abundance, special function, and indispensdbility 

 of the fixed ingredients of plants, will be indicated the 

 customary and best method of defining them. 



§3. 



QUANTITY, DISTRIBUTION, AND VARIATIONS OF THE ASH- 

 INGREDIENTS. 



The ash of plants consists of the various fixed acids, 

 oxides, and salts, noticed in § 1. 



The ash-ingredients are always present in each ceU of 

 every plant. 



The ash-ingredients exist partly in the cell-wall, in- 

 crusting or imbedded in the cellulose, and partly in the 

 plasma or contents of the cell, (see p. 224.) 



One portion of the ash-ingredients is soluble in water, 

 and occuL's in the juice or sap. This is trae, in general, 

 of the salts of the alkalies, and of the sulphates and 

 chlorides of magnesium and calcium. Another portion is 

 insoluble, and exists in the tissues of the plant in the 

 solid form. Silica, the phosphates of lime, and the mag- 

 nesia compounds, are mostly insoluble. 



The ash-ingredients may be separated from the volatile 

 matter by burning or by any process of oxidation. In 

 burning, portions of sulphur, chlorine, alkalies, and phos- 

 phorus, may be lost under certain circumstances, by vola- 

 tilization. The ash remains as a skeleton of the plant, 

 and often actually retains and exhibits the microscopic 

 form of the tissues. 



The Proportion of Ash is not invariable, even in the 



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