[ 320 J 



turned up from pasture ; but in a few years they de* 

 generate, and require a fresh soil ; and the organiza- 

 tion of these plants is such, as to be constantly pro- 

 ducing the migration of their layers : thus the straw- 

 berry by its long shoots is constantly endeavouring to 

 occupy a new soil ; and the fibrous radicles of the 

 potatoe produce bulbs at a considerable distance from 

 the parent plant. Lands in a course of years often 

 cease to afford good cultivated grasses ; they become 

 (as it is popularly said) tired of them ; and one of the 

 probable reasons for this was stated in the last Lec- 

 ture. 



The most remarkable instances of the powers of 

 vegetables to exhaust the soil of certain principles ne- 

 cessary to their growth is found in certain funguses. 

 Mushrooms are said never to rise in two successive 

 seasons on the same spot ; and the production of the 

 phenomena called fairy rings has been ascribed by 

 Dr Wollaston to the power of the peculiar fungus 

 which forms it to exhaust the soil of the nutriment 

 'necessary for the growth of the species. The conse- 

 quence is, that the ring annually extends ; for no 

 seeds will grow where their parents grew before them ; 

 and the interior part of the circle has been exhausted 

 by preceding crops ; but where the fungus has died, 

 nourishment is supplied for grass, which usually rises 

 within the circle, coarse, and of a dark green colour. 



When cattle are fed upon land not benefitted by 

 their manure, the effect is always an exhaustion of the 

 soil ; this is particularly the case where carrying 

 horses are kept on estates ; they consume the pasture 



