8o THE MEDULLARY RAYS 



the rays are arranged in a lens shaped group (Fig. 57, mt). The 

 medullary rays are of great importance in the distribution and often 

 also in the storage of foods. Their structure and arrangement in 

 the stem are admirably adapted for this work. Their thin walls 

 can readily withdraw from the ascending current in the xylem 

 as much water as is needed for the phloem and cortex. Tracheids 

 are often associated with the medullary rays to further increase 

 their absorbing power (Fig. 58). Particularly is this true in 

 the spring wood where the ray cells also often -become greatly 

 enlarged in order to absorb the large volume of water required 

 at this time of year. In the same way the carbohydrates and 

 albuminous substances transported through the phloem are with- 

 drawn through the medullary cells and conveyed to the active 

 cambium and the growing cells of xylem. 



The value of oak and other woods for interior decoration is 

 materially increased by the medullary rays. The trees are sawed 

 into timber in such a way as to expose the medullary to the best 

 advantage, a process called quartering (Fig. 59). Consequently 



A 



FIG. 59. Diagram showing a common method of sawing in producing 

 quartered oak. The log is first squared to remove the sap wood and then 

 a number of boards, depending upon the size and grain of the tree, are 

 removed at a and b. These are the best boards obtained from the log, 

 being large and approximately radial and therefore showing the medullary 

 rays to good advantage. The remaining portions of the log are cut 

 radially, as shown at c. One of the ends of quartered lumber is oblique 

 and must be squared,' as shown in the diagram. 



the rays appear as flecks and bands upon the surface of the board 

 and vary greatly in character owing to the angle of cutting and 

 to the irregularities' of the trunk. The medullary rays can 

 always be detected in the grain of wood because they run across 

 or at right angles to the xylem cells (Fig. 57, mr). As a rule 



