NATURE OF PLANTS 



95 



tex. Tracheids are often associated with the medullary rays to 

 further increase their absorbing power (Fig. 59, A, mf). Par- 

 ticularly 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 the phloem are withdrawn through the medullary cells and con- 

 veyed 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 rays to the 

 best advantage, a process called quartering (Fig. 60). Conse- 



FIG. 60. 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 broad and 

 approximately radial and therefore showing the medullary rays to good ad- 

 vantage. 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. 



quently 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 w r ood because they 

 run across or at right angles to the xylem cells (Fig. 58, mr). 

 As a rule the w r ood cells of the stem remain alive for only a few 

 years. This fact is usually apparent in any log or stump where it 

 will be seen that the outer annular rings are of a lighter color, the 

 sap wood, and the older parts are of a darker color, the heart 

 wood. The sap wood is active in the transport of water and 

 contains many living cells. For this reason it is not so valuable 



