214* Mr. Knight on the Causes which Influence 
upon the bark of the root, may give ability to that organ to 
attract and employ the descending, or cortical current of sap ; 
and if this be the case, an easy explanation of all the preced- 
ing phenomena immediately presents itself. 
A tree growing upon a wall, and unconnected with the 
earth, will almost of necessity grow slowly, and as it must be 
scantily supplied with moisture during the summer, it will 
rarely produce any other leaves than those which the buds 
contained, which were formed in the preceding year. Some 
of the roots of a tree, thus circumstanced, will be less well 
supplied with moisture than others, and these will be first 
affected by drought : their points will in consequence become 
rigid and inexpansible, and they will thence generally cease 
to elongate at an early period of the summer. The descend- 
ing current of sap will be then employed in promoting the 
growth and elongation of those roots only, which are more 
favourably situated, and those, comparatively with other parts 
of the tree, will grow rapidly. Gravitation will direct these 
roots perpendicularly downwards, and the tree will appear to 
have adopted the wisest and best plan of connecting itself with 
the ground : and it will really have employed the readiest 
means of doing so, as effectively as it could have done, if it 
had possessed all the feelings and instinctive passions and 
powers of animal life. The subsequent vigorous growth of 
such a tree is the natural consequence of an improved and 
more extensive pasture. 
When the seeds of the carrot and parsnip, in the experK 
ments I have stated, were placed in a poor superficial soil, but 
which permitted the roots of the plants to pass readily through 
it, these were conducted downwards by gravitation ; whilst 
