LECT. V.] THE ROOT. 203 



natural limit of its longitudinal extension, it 

 throws out lateral branches ; and these extend with 

 most vigour in that direction in which the most 

 abundant supply of nourishment is to be found ; 

 a circumstance which has been very unaccountably 

 attributed by some to a sentient, or an instinctive 

 principle in plants. The fact is, that the influence 

 of a quantity of manure, or of richer earth, is 

 not confined to the immediate spot where it is de- 

 posited ; but extends to a certain distance in every 

 direction, diluted, as it were, by the poorer soil, 

 until its power is altogether dissipated : as soon, 

 therefore, as any branch of a root impinges upon 

 the limits of this circle, it obtains a supply of nu- 

 triment capable of exciting its vital energy in an 

 increased degree : and as this augments in a direct 

 ratio, as it extends, this branch in length and 

 vigour necessarily far exceeds those on the opposite 

 side of the main root, which have had a more 

 scanty supply of nourishment. Thus, also, if a 

 seed of a tree, conveyed by the wind, or other- 

 wise, as may accidentally happen, be planted on the 

 top of a wall, its roots will gradually extend until 

 they reach the ground ; whilst the upright growth 

 of the tree will be very scanty previous to the root 

 establishing itself in the soil. Sir J. E. Smith, in 

 referring to a fact of this kind, communicated to 

 him by the Rev. Dr. Walker of Edinburgh, regard- 

 ing an Ash, which grew on a wall in the Canon- 



