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- 



