210 Mr. Knight on the Causes which influence 
hypothesis has been founded, and others which have occurred 
in the course of my own experience, and which are favourable 
to it; after which I shall endeavour to trace the effects observed 
to the operation of different causes. 
When a tree, which requires much moisture, has sprung 
up, or been planted, in a dry soil, in the vicinity of water, it 
has been observed, that much the largest portion of its roots 
has been directed towards the water ; and that when a tree 
of a different species, and which requires a dry soil, has been 
placed in a similar situation, it has appeared, in the direction 
given to its roots, to have avoided the water and moist soil. 
A tree growing upon a wall, at some distance from the 
ground, and consequently ill supplied with food and water, 
has also been observed to adapt its habits to its situation, and 
to make very singular and well directed efforts to reach the 
soil beneath, by means of its roots.* During the period in 
which it is making such efforts, little addition is made to its 
branches, and almost the whole powers of the plant appear 
to be directed to the growth of one or more of its principal 
roots. To these much is in consequence annually added, and 
they proceed perpendicularly towards the earth, unless made 
to deviate by some opposing body : and as soon as the roots 
have attached themselves to the soil, the branches grow with 
vigour and rapidity, and the plant assumes the ordinary habits 
of its species. 
Du Hamel caused two trenches to be made so as to in- 
tersect each other at right angles, and a tree to be planted 
at the point of intersection ; and taking up this tree some 
years afterwards, he found that the roots had almost wholly 
* Smith’s Introduction to Botany. 
