102 FOUNDATIONS OF BOTANY 
the vessels. Examine the smoothly 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 see 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 is 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 
Fic. 74. — Formation of : f 
a Knot in a Tree- Cylinder of wood which cuts across the 
‘eae annual rings, as shown in Fig. 74. 
gee ras ns This interruption to the rings is a knot, 
K, knot, formed by such as one often sees in boards and 
pee ee 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 growth of the trunk will bury the scar 
before decay sets in. 
