142 ON THE FOOD OF PLANTS. 



stantly bojing holes, and drawing the hght sub- 

 stances around them, but that by their sHme 

 they very much quicken the decomposition ; this 

 is readily proved by leaving a piece of twine 

 on the surface of the earth a short time ; and 

 within a few days after it has been laid hold of 

 by the worms, it will be found quite rotten ; 

 their holes also afford ready channels of convey- 

 ance for the decomposed matter. 



And further, these little creatures perform 

 another important operation ; they devour and 

 cast upon the surface, the more finely divided 

 earthy substances and undecomposed matter; 

 and thus that part of the soil best adapted to 

 sustain vegetation, is collected and accumulated 

 on the surface, and within the healthful range 

 of the roots of plants. The soil is also by such 

 means, rendered more conducive to an equal 

 percolation and evaporation of water ; and it 

 may be observed, that placing dung on the sur- 

 face, favours both the existence and the working 

 of these useful cultivators ; the surface of the 

 earth, thus continually opened, also affords a free 

 ingress to the carbonic acid gas, and egress to 

 the carburetted hydrogene. 



Again, Sir Humphry says, " All green sug- 

 *' culent plants contain saccharine, or mucilagi- 

 " nous matter, with woody fibre, and readily 



