40 THE BOOK OF FORESTRY 
watery vapor leaving behind the mineral salts which 
were dissolved in it. Thus the leaves and twigs are 
always very rich in these salts accounting for the fact 
that forest mold composed of partially decayed leaves 
and twigs is very rich and fertile. 
Indeed a forest adds continually to the fertility of 
the land upon which it grows. The trees and shrubs 
draw up water containing salts in solution from the 
subsoil many feet beneath the surface, and a large 
amount of these salts remaining in the twigs and leaves 
is deposited upon the surface when the leaves fall. In 
addition the nitrogenous material in the leaf litter 
makes the surface very rich so that on sandy soils 
splendid crops can be raised for a few years after the 
forest has been removed. When the humus is con- 
sumed, however, the land becomes too sterile for profit- 
able agriculture, and after being tilled for a few years 
may be abandoned. This condition of affairs is found 
in parts of the South and in the Lake States. 
The course of the sap current in a tree is well known, 
but the reason for its rise is still unsolved. The raw 
Sap comes in through the thin cell walls of the root 
hairs, passes through the supporting roots and up 
through the sapwood—the outermost layer of the trunk 
—to the leaves. After being combined with water and 
oxygen in the leaves, the energy being supplied by 
sunlight, the assimilated plant food passes down 
through the cambium layer—the growing ring—and the 
inner bark or bast. Various reasons have been put 
forward to explain the course of the sap in the tree, 
among which might be mentioned “osmotic force” (the 
attraction of a stronger solution for a weaker solution 
through a permeable membrane) and root pressure, or 
the suction set up by the evaporation which is contin- 
