NATURE OF PLANTS 



Their thin walls can readily withdraw from the ascending current 

 in the xylem as much water as is needed for the phloem and cor- 

 tex. Tracheids are often associated with the medullary rays to 

 further increase their absorbing power (Fig. 59, A, mt). 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 the phloem are withdrawn through the medullary cells and con- 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. 



Fig. 59. Relation of the medullary rays to the xylem cells in pitch pine: 

 A , radial section showing the elongated cells or tracheids, t, marked by circular, 

 thin places on bordered pores, p; m, medullary ray of two cells accompanied 

 by tracheids, mt. B, tangential section showing medullary rays one cell 

 broad and three to nine high — p, bordered pores. 



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. 60). Consequently 

 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 al- 

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

 or at right angles to the xylem cells (Fig. 58, mr). As a rule 



