80 THE MEDULLARY RAYS 



the rays are arranged in a lens shaped group < bi.u'. 57. mi >. The 

 medullary rays are of great importance in the distribution and often 

 alsi> in the storage of foods. Their structure and arrangement in 



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



can readily withdraw from the ascending current in the xylem 

 a> much water -a^ 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 greatrj 

 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 <»ak 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 medullar)' to the best 



advantage, a process called quartering 1 Fig. 59). Consequently 



I i'. ;■; Diagram showing a common method of sawing in producing 

 quartered oak. The log i-- first squared ti> remove the sap \\"<>d and then 

 a number of boards, depending upon the size and .urain of tin- inc. are 

 removed at a and />. These arc tin- best boards obtained from the log, 

 being large and approximately radial and tin rehire showing the medullary 

 I .'.Mid advantage. The remaining portions of the log are cut 

 radially, a- shown at .. One of the ends of quartered lumber is oblique 

 and must he squared, as shown in the diagram. 



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

 and vary greatl) in character owing to tin- angle of cutting and 

 to th<- irregularities of the trunk. 'The medullary rays can 

 always l»- detected in the grain of wood because the) run a< 

 or at righl angles to the xylem cells < Fig. 57. mr). \& a rule 



