NATURE OF PLANTS 



79 



rings may be formed in one season owing to the checking of 

 the growth by fire, severe drouth, depredations of insects and 

 the subsequent recovery and renewal of growth. These annual 

 rings also reveal the life history of the tree, broad bands indi- 

 cating favorable seasons and narrow bands telling of fires, drouth, 

 transplanting, and other factors that limit the annual growth. 

 39. 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. 57, mc). Some extend from 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. 56, m; 57, mc). In a longitudinal section cut parallel 

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



Fig. 58. 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. 



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

 (Fig. 57, mr), while in longitudinal sections cut at right angles 

 to the rays, called a tangential section, we see that the cells of 



