176 APPLIED BIOLOGY 



51) is placed in contact with the cambium of another tree, and 

 after the bud grows into a stem the original stem is cut away. 



The growth of new tissue which takes place in the healing 

 of wounds, as after pruning or breaking of limbs by winds, is 

 essentially the same as in grafting. Examine trees from 

 which limbs have been cut and notice the ring of new tissue 

 (callus) forming at the bark and tending to grow over the 

 entire wound. Look for cases where the wound is almost 

 overgrown, and others which are completely so. Look for 

 cases where limbs have been broken or cut off some distance 

 from the main stem, leaving a stub ; and compare the healing 

 in such cases with those in which the branch has been cut off 

 near the main branch or trunk. How should branches be 

 cut when pruning trees? Numerous illustrations of these 

 points may be seen in orchards and in shade trees. 



In cases where very large limbs have been cut off, there is 

 a tendency of the wood to decay before the cambium has been 

 able to form enough new tissue to cover the wound completely. 

 In old and neglected orchard trees many holes are formed by 

 decay thus started, and which, after many years, extends into 

 the very heart of the tree, making it unable to withstand the 

 force of the wind. The remedy is (1) prevention of decay 

 by painting or tarring all injured places so as to keep out the 

 bacteria and molds which cause decay; and (2) if decay has 

 already started, the softened wood should be cut out and the 

 cavity filled with tar or portland cement. 



167. Pruning is the cutting out of unnecessary and in- 

 jured branches of trees, shrubs, and vines. It is necessary 

 in orchards because trees standing alone develop too many 

 branches, and grow too high for convenience in spraying 

 against fungi and insects or in picking fruit. Also, too many 

 branches result in small and imperfect fruit. 



Pruning has been developed so well that the gardener can 

 now make a young tree take almost any desired shape by 

 simply removing buds and branches which are not wanted, 



