86 THE SOIL. 



with a layer of corky substance, wliicli, being impene- 

 trable by water, affords, to the soluble matter deposited 

 within, some protection against osmotic uifluences. 



Absorption of nutriment from the soil is effected by 

 the extremities of the roots, Avhose fluid contents are 

 separated fi'om the earthy particles around them by an 

 exceeduigly thin membrane alone ; and the contact of 

 the two is the more intimate, as the root-fibre during its 

 formation exerts upon the earthy particles a pressure suf- 

 ficiently powerful, under certain ckcumstances, to push 

 them aside. The evaporation of water fi'om the leaves 

 produces a vacuum within the plant, whereby a draught 

 is created, which powerftdly assists the contact of the 

 moist earthy particles with the cell-wall. The cell and 

 the earth are pressed against each other. Between the 

 fluid contents of the cells and the nutritive substances 

 physically combined in the earthy particles, there mani- 

 festly exists a strong chemical attraction, which, with the 

 cooperation of carbonic acid and water, causes the trans- 

 ference of the incombustible matters into the system of 

 the plant. 



By the powerful chemical attraction of any body, we 

 understand its entering into a chemical combination, in 

 which it loses its original properties and acqukes new 

 ones. In the case of potash, hme, and phosphoric acid, 

 such a combination must take place immediately upon 

 their passage into the cell ; for, as already stated, the sap 

 of the roots is always slightly acid. In the sap of the 

 root-shoots of the vme, we can always detect bitartrate of 

 potash ; in that of others, oxalate or citrate of potash, or 

 tartrate of hme ; but we never find these bases combined 

 in such saps with carbonic acid, nor can phosphate of 

 lime or magnesia be detected. If the fi^esh sap of the 

 potato-tuber is mixed with ammonia, no precipitate of 



