the 'Direction of the Growth of Roots . 215 
zontally along the surface of the mould, and in contact with 
it ; and in a few days emitted many fibrous roots upwards 
into it : just as they would have done, if guided by the instinc- 
tive faculties and passions of animal life ; and as I concluded 
before I made the experiment that they would do, under the 
guidance of much more simple laws, whose mode of operating 
I shall endeavour to explain. 
Whatever be the machinery by which the sap of trees 
is raised to the extremities of their branches, it is obvious 
that this machinery is first put into action by the stems and 
branches, and not by the roots : for the graft or bud, when- 
ever it has become fully united to the stock, wholly regulates 
the season and temperature, in which the sap is to be put in 
motion, in perfect independence of the habits of the stock ; 
whether those be late or early. If all the branches of a tree, 
exclusive of one, be much shaded by contiguous trees,* or 
other objects, the branch which is exposed to the light attracts 
to itself a large portion of the ascending sap, which it employs 
in the formation of leaves and vigorous annual shoots, whilst 
the shaded branches become languid and unhealthy. The 
motion of the ascending current of sap appears therefore to 
be regulated by the ability to employ it in the trunk and 
branches of the tree ; and this current passes up through the 
alburnum, from which substance the buds and leaves spring. 
But the sap which gives existence to, and feeds the root, de- 
scends through the bark and if the operation of light give 
ability to the exposed branch to attract and employ the as- 
cending or alburnous current of sap, it appears not improbable 
that the operation of proper food and moisture in the soil, 
* Phil. Trans. 1805 and 1809, p. 8. f Phil. Trans. 1809, 1st Part, p. t . 
