STRUCTURE OF WOODY STEMS 203 
as primary rays; and (2) those reaching only part way through 
the wood and known as secondary rays. (Fig. 180.) The medul- 
lary rays are composed of thin-walled living cells, which function 
in food storage and in the transportation of materials laterally 
through wood and bark. They are narrow plates of cells, one or 
only a few cells in thickness, and extend up and down through 
the stem only very short distances as may be ascertained in 
Figure 180. 
Both annual rings and medullary rays have an economic im- 
portance in connection with lumber, where they form the beauti- 
ful figures on the surface of cabinet woods. When lumber is 
quarter sawed, that is, sawed so that the broad surface of the board 
is parallel with the medullary rays, then its beauty is due to the 
medullary rays which form the smooth-looking blotches as shown 
on the edge of the board in Figure 181. When plain sawed, that 
is, sawed at right angles to the medullary rays, the beauty of the 
board is due to the figures formed by the annual rings as shown 
on the broad surface of the board in Figure 181. 
In summarizing, corky bark, annual rings, and prominent 
medullary rays may be stated as the distinguishing features of 
woody stems. Like herbaceous dicotyledonous stems, they are 
characterized by the circular arrangement of vascular bundles 
and presence of cambium — features which distinguish them 
from monocotyledonous stems where the vascular bundles have 
the scattered arrangement and cambium is absent. 
