68 



STRUCTURE OF THE STEM 



As the bud develops into a branch, the few bundles which it 



originally possessed increase greatly in number, and at length, 

 as the branch grows, form a cylinder of 

 wood which cuts across the annual rings, 

 as shown in Fig. 69. This interruption to 

 the rings is a knot, such as one often sees 

 in boards and planks. If the branch dies 

 long before the tree does, the knot may 

 be buried under many rings of wood. What 

 is known as "clear" lum- 

 ber is obtained from trees 

 that have grown in a 

 dense forest, so that the 

 lower branches of the 

 larger trees were killed 

 by the shade many years 

 before the tree was felled. 

 In pruning fruit trees 

 or shade trees the 

 branches which are re- 

 moved should be cut 

 close to the trunk. If 

 this is done, the growth 



of the trunk will bury the scar before decay 



sets in. 



83. Grafting. When the cambium layer 



of any vigorously growing stem is brought in FIG. 70. Grafting 



contact with the same layer in another stem At the left scion and 



of the same kind or a closely similar kind of 



plant, the two may grow together to form a 



single stem or branch. This process is called 



grafting, and is much resorted to in order to 



secure apples, pears, etc., of any desired kind 



(Fig. 70). A twig known as the scion from a plant of the chosen 



variety may be grafted upon another individual of similar kind 



FIG. 69. Formation of 

 a knot in a tree trunk 



R, cut-off end of stick, 

 showing annual rings ; 

 K, knot formed by 

 growth of a branch. 

 After Roth 



stock are shown 

 ready to be united ; 

 at the right they are 

 joined and ready to 

 cover with grafting 

 wax. After Perci- 

 val 



