16 



ROOT. 



remains unfit for its further nourishment re- 

 turns to the spongioles, is by them thrown ofi' — 

 this substance so thrown off is unsuitable and 

 even poisonous to this species of plant, but is 

 not so to other species ; it may even be suitable 

 to them. 



Hence soil may be rendered impure, (or, as 

 we inaccurately say, worn out) for one species, 

 which will not be impure for others. 



This is the true key of the theory of rotation 

 of crops. 



This also may serve to explain in part why 

 light soil is indispensable to so many plants, 

 and heavy or tenacious soil suitable to so few : 

 for in the former case the spongioles will meet 

 with little resistance to their elongation, and 

 will consequently be continually quitting the 

 place where their excrementitious matter is de- 

 posited ; while, in the latter case, the reverse 

 will occur. 



It will also be one of the reasons why an 

 orchard planted too thickly of the same trees 

 cannot thrive, the trees by their roots soon 

 absorb all the nutriment from the earth, and 

 only those on the borders can send out their 

 roots to a distance for fresh juices, those in the 

 centre have little else to feed on but the sub- 



