94 



MEDULLARY RAY 



40. The Medullary Ray.^ — Another characteristic of most 

 woody stems as seen in cross section is the series of delicate 

 lines, the medullary rays, that radiate from the center of the 

 stem (Fig. 58, mc). Some extend from near the pith through the 

 phloem and others originate in the various annual zones of the 

 xylem and extend partially through the phloem. When magni- 

 fied these medullary rays are seen to consist of rather thin- 

 walled, oblong cells and varying in width from one to a few cells 

 (Figs. 57, w; 58, mc). In a longitudinal section cut parallel 

 to the rays, called a radial section, they appear as bars of oblong 

 cells running at right angles to the elongated cells of the xylem 

 (Figs. 58, mr; 59, A), while in longitudinal sections cut at right 



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. 



angles to the rays, called a tangential section, we see that the 

 cells of the rays are arranged in a lens-shaped group (Figs. 58, mt', 

 59, B). The medullary rays are of great importance in the dis- 

 tribution 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 cro- 



