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 hving 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 radial surface B 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 tangential surface A 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. 



