100 THE SOIL. 



acid, wliicli, carried by raiii to the deeper layers, must 

 powerfully contribute to disintegrate and decompose the 

 earthy particles. In a dense wood, where the air is more 

 rarely renewed than in the open plain, this supply of 

 carbonic acid is important ; moreover, the thick carpet of 

 leaves protects the ground from being dried by the air, 

 and maintains it in a permanent state of moisture, par- 

 ticularly useful to fohaceous trees, which exhale from 

 their leaves larger quantities of water than the coniferous 

 plants. 



To understand the operations of agriculture, it is indis- 

 pensably necessary that the farmer should have the 

 clearest knowledge of the manner m Avhich plants derive 

 their nutriment from the soil. 



The opinion that the roots of plants extract their food 

 immediately from those portions of the soil which are in 

 direct contact with their absorbent surfaces, does not 

 imply that potasli, lime, or phosphate of lime, in the sohd, 

 undissolved state can penetrate the membrane of the 

 cells ; * nor does it imply that the nutritive substances 



* If a glass vessel is filled to the brim with Avater, in -which are a 

 few drops of hydrochloric acid, and covered closely with a piece of 

 bladder, so that the water moistens the bladder and no air is left 

 between them, and the outside of the bladder is carefiilly dried, it 

 may then be seen how a solid body, without the cooperation of a fluid 

 from the outside, can make its way through the bladder to the water in 

 the glass. For if a little chalk or finely-pulverised phosphate of Hme 

 is strewed upon the dried outer surface of the bladder, the powder will 

 disappear in the course of a few hours, and the usual reactions will 

 show the presence of lime and phosphate of lime in the fluid. 



Of course the passage of the carbonate and phosphate of lime in the 

 solid state through the bladder into the water, is only apparent. Both 

 salts are dissolved in the pores of the membrane where they come in 

 contact with the acidulated water, and as the evaporation of the water 

 from the bladder somewhat diminishes the inner pressiu'e as compared 

 to the outer, the stronger outer pressure, assisted by the solvent jjower 

 of the water, forces the solution inward. 



