102 



FOUNDATIONS OF BOTANY 



the vessels. Examine the smbothly planed surface of a billet of red 

 oak that has been split through the middle of the tree (quartered 

 oak), and note the large shining plates formed 

 by the medullary rays. 



Look at another stick that has been planed 

 away from the outside until a good-sized flat 

 surface is shown, and flee how the medullary 

 rays are here represented only by their 

 edges. 



112. Interruption of Annual Rings by 

 Branches ; Knots. — When a leaf-bud is 

 formed on the trunk or branch of a 

 dicotyledonous tree, it Ls connected with 

 the wood by fibro-vascular bundles. 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. 74. 

 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 lumber 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 removed should be cut close to the trunk. If 

 this is done, the grpwth of the trunk will bury the scar 

 before decay sets in. 



Fig. 74. — Formation of 

 a Knot in a Tree- 

 Trnnli. 



R, cut-off end of stick, 

 showing annual rings ; 

 Kj knot, formed by 

 growth of a branch. » 



