215 
the Direction of the Growth of Roots. 
the plants grew feebly, because they received but little nutri- 
ment. The roots were in a situation analogous to that of the 
stems of trees in a crowded forest; and when the leading 
fibres of the roots came into contact with the rich mould, 
they acquired a situation correspondent to that of the leading 
branches of such trees, which are alone exposed to the light. 
The form of the roots of the plants was consequently long, 
slender, and cylindrical, like the stems of such trees. The 
roots of the one required the actual contact of proper soil and 
nutriment ; and the branches of the other required the actual 
contact of light, to promote their growth. 
When, on the contrary, the seeds of the preceding species 
of plants were placed in a rich superficial soil, their situation 
was analogous to that of a tree fully exposed, on every side, 
to the light, whose branches would be extended, in every 
direction, immediately above the surface of the ground : and 
as the fibrous roots of the plants came into contact with the 
subsoil, which was not well calculated to promote their growth, 
their situation became analogous to that of shaded branches ; 
and they consequently ceased to extend downwards. The 
fibrous roots of a tree, under similar circumstances, would 
have extended along the lower surface of the favourable soil ; 
but after these roots had much increased in bulk, they would 
be found partly compressed into the subsoil, however poor and 
unfavourable, provided it contained no ingredients actually 
noxious. In obedience to similar laws, the roots of an aquatic 
tree will not extend freely in dry soil, nor those of a tree 
which requires but little moisture in a wet soil ; and on this 
account the roots of the one will appear to have sought, and 
those of the other to have avoided, the contiguous water ; 
