170 THE INTERCHANGE OF CROPS. 



in some, in the graminece for example, in the form 

 of silicates, in others, in that of tartrates, citrates, 

 acetates, or oxalates. 



When these alkalies are in combination with 

 silicic acid, the ashes obtained by the incineration 

 of the plant contain no carbonic acid ; but when 

 they are united with organic acids, the addition of 

 a mineral acid to their ashes causes an effervescence. 



A third species of plants requires phosphate of 

 lime, another, phosphate of magnesia, and several 

 do not thrive without carbonate of lime. 



Silicic acid is the first solid substance taken up 

 by plants ; it appears to be the material from which 

 the formation of the wood takes its origin, acting 

 like a grain of sand around which the first crystals 

 form in a solution of a salt which is in the act of 

 crystallizing. Silicic acid appears to perform the 

 function of woody fibre in the Equisetacece and bam- 

 boos, just as the crystalline salt, oxalate of lime, 

 does in many of the lichens. 



When we grow in the same soil for several years 

 in succession different plants, the first of which 

 leaves behind that which the second, and the 

 second that which the third may require, the soil 

 will be a fruitful one for all the three kinds of pro- 

 duce. If the first plant, for example, be wheat, 

 which consumes the greatest part of the silicate of 

 potash in a soil, whilst the plants which succeed it 

 are of such a kind as require only small quantities 

 of potash, as is the case with the Leguminosce, 



