100 RELATION OF BRANCH TO STEM 



years with the growth of the stem, but eventually they were 

 killed by the overhanging branches and in time became over- 

 grown with the annual layers of xylem. If the branch is favor- 

 ably situated so that it continues to live then a cone-like struc- 

 ture is developed (Fig. 64, a) since the branch increases in girth 

 each year in the same manner as the stem. These branches run- 

 ning through the xylem are the cause of knots that appear in 

 lumber. If the branch is living the tissues of stem and branch 



T"lG. 64. Section through the trunk of basswood showing relation of 

 ^branches to main stem. In the upper portion of the figure are three branches 

 that were Icilled after a few years' growth by shading and that have been 

 •overgrown by the annual rings of the wood. The branch shown at a has 

 remained alive and increased in size after the manner of the main stem. 



are closely bound together and we have a solid knot, but it not 

 infrequently happens that branches after living several years die 

 and begin to decay before they are overgrown. Such a limb will 

 produce in the lumber a knot that is often black and shaky or it 

 may fall out, forming a knot hole. The lower branches of trees 

 growing in a forest are quickly pruned off by the shade of the 

 upper branches. This causes straight, clean trunks to develop 

 and the lumber is free of knots. 



It follows from what has been said above that in pruning 

 trees the limbs should be cut off close to the trunk. Painting 

 the cut surface does not promote the healing of the wound 

 though it may exclude organisms causing decay. When a wound 



