94 



ORIGIN OF LEAVES AND BRANCHES 



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 

 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 



Fig. 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 killed 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. 



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 and the trunks 

 should be painted to promote the healing of the wound and to 

 exclude organisms that promote decay. When a wound is made 

 that extends across the cambium as in the cutting off of a limb 

 or by a cut into the trunk that removes the tissues as far as the 

 xylem it will be noticed that the exposed cambium is stimulated 



