LIME IN PLANTS. 217 



Dutch rushes, contain even more than hay or straw ; 

 and cane contains so much silica, that it appears 

 completely coated with flint. 



551. Lime is almost invariably found in the ashes 

 of all plants, and frequently constitutes a large pro- 

 portion of the earthy substances present in them. It 

 sometimes exists in combination with sulphuric acid, 

 as sulphate of lime (237), but far more commonly 

 united to phosphoric acid, or in the state of phosphate 

 of lime (242). It is likewise very frequently found 

 in considerable quantity in plants, combined with 

 organic acids, particularly as oxalate of lime ; hence 

 the ashes of plants commonly contain a portion of 

 carbonate of lime, which is formed in a similar man- 

 ner to the carbonate of potash already described 

 (494). The quantity of oxalate of lime in some 

 plants is so large, that, upon cutting them through 

 with a knife, multitudes of little crystals of this salt 

 may be seen by means of a common pocket lens. 



552. Magnesia is very often found associated with 

 lime, more especially in the state of phosphate of 

 magnesia (253), and as a double phosphate of mag- 

 nesia and ammonia ; but as this salt is decomposed 

 by heat, the ammonia being volatile, only phosphate 

 of magnesia is found in the ashes. 



553. The ashes of almost all plants contain a trace 

 of oxide of iron; and occasionally of the oxide of 

 manganese also ; and in one or tw^o rare instances a 

 minute quantity of copper has been found, especia^lly 

 in certain fungi. 



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