graves: place of forestry among sciences 51 



confined merely to changes in their external form and growth; 

 it extends also to their internal structure. The specific gravity 

 of the wood, its composition, and the anatomical structure which 

 determines its specific gravity differ in the same species, and on 

 the same soil, and in the same chmate, according to the position 

 which the tree occupies in the stand. Thus in a 100-year-old 

 stand of spruce and fir the specific gravity of wood is greatest in 

 trees of the third crown class (intermediate trees) . The ratio of 

 the thick wall portion of the annual ring to the thin wall of the 

 spring wood is also different in trees of different crown classes. 

 The difference in the size of the tracheids in trees of different 

 crown classes may be so great that in one tracheid of a dominant 

 tree there may be placed three tracheids cells of a suppressed 

 tree. The amount of lignin per unit of weight is greater in domi- 

 nant trees than in suppressed trees. 



Forest trees in a stand are thus influenced not only by the 

 external physical geographical environment, but also by the new 

 social environment which they themselves create. For this 

 reason forest trees assimilate, grow, and bear fruit differently 

 and have a different external appearance and internal structure 

 than trees not grown in a forest. 



Forestry, unlike horticulture or agriculture, deals with wild 

 plants scarcely modified by cultivation. Trees are also long- 

 lived plants; from the origin of a forest stand to its maturity there 

 may pass more than a century. Foresters, therefore, operate 

 over long periods of time. They must also deal with vast areas; 

 the soil under the forest is as a rule unchanged by cultivation and 

 most of the cultural operations applicable in' arboriculture or 

 agriculture are entirely impracticable in forestry. Forests, there- 

 fore, are largely the product of nature, the result of the free 

 play of natural forces. Since the foresters had to deal with 

 natural plants which grew under natural conditions, they early 

 learned to study and use the natural forces affecting forest 

 growth. In nature the least change in the topography, exposure 

 or depth of soil, etc., means a change in the composition of the 

 forest, in its density, in the character of the ground cover, and 

 so on. As a result of his observations, the forester has developed 



