THE ECONOMIC IMPORTANCE OF FORESTS 505 



center part is called the " heart wood " and is often darker in 

 color and more durable than the outer, live region or " sap-wood." 

 The heart of a tree may totally decay and yet cause the tree no 

 harm other than weakening its strength, but the sap-wood is neces- 

 sary to the growth of the tree and may even keep it alive when the 

 bark has been girdled. 



Shrinkage and Warping. Fresh-cut timber contains much water 

 and the process of drying, called " seasoning," has to be thoroughly 

 accomplished before it can be used. This is because lumber 

 shrinks as it dries and no amount of nailing will hold poorly sea- 

 soned boards together. As a board dries there is a tendency for 

 the side nearest the bark to shrink fastest causing the board 

 to curve away from the center, or " warp." Unless the lumber 

 be properly piled and dried it may be rendered unfit for use. 



Hard and Soft Woods. Trees can be grouped in two classes, 

 those with broad leaves, which are shed annually (maple, oak) 

 and those with needle-shaped leaves, which are not all shed at 

 one time (pine, spruce). The former produce " hard wood" 

 lumber and the latter " soft wood," though some broad-leaved 

 trees have lumber that is very soft (basswood, willow) and some 

 pines produce " hard pine " lumber, which nevertheless, is classed 

 as a " soft wood." 



" Knots " in lumber are places where a branch has been broken 

 off and the scar covered by additional annual rings. If the wound 

 healed at once and no rot commenced, the knot is tight and does 

 not harm the lumber so much, but if the healing was incomplete, 

 a loose knot results and a knot-hole in the board is the result. 



A tree grows in height only at the tips of new branches; it grows 

 in thickness layer by layer, over all parts, hence a nail driven into 

 a tree will always remain at the same height from the ground, but 

 will be covered, in time, by the growth in thickness. 



Street Trees. In proportion to their number, trees are more 

 valuable in the city than in the forest. Shade trees add to the 

 cash value of property in the same way as do wide streets, good 

 pavements, and favorable location. A city always is proud of 

 handsome trees and shady streets, but often there is little care 



