NATURE OF PLANTS 85 



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

 ture is developed (Fig. 63, 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 w r e 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 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 



A. 



FIG. 64. Section of a portion of a stem showing the healing of a wound 

 caused by the cutting off of a lateral branch : A, formation of the callus, 

 cl, owing to the renewed activity of the living cells exposed by the wound ; 

 c, cortex ; cm, cambium ; x, region of the xylem. B, a similar stem three 

 years later c, cortex ; cm, cambium ; x' , xylem added since the wound 

 was made ; o, position of the cambium at the time the branch was cut off ; 

 x, original xylem of the stem. C, trunk from which three branches have 

 been removed, showing the gradual covering of the wounds by new tissue. 



