NATURE OF PLANTS 



87 



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

 ca.ting favorable seasons and narrow bands telling of fires, drought, 

 transplanting, and other factors that limit the annual growth. 

 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 the pith through the 

 phloem and others originate in the various annual zones of the 



Fig. 56. 



Fig. 57. 



Fig. 56. Diagram of a cross-section of a stem of black oak four years old ; 

 p, pith; I, 2, 3, 4, annual rings of xylem; c, cambium cylinder; ph, phloem; 

 cr, cortex; ck, cork; m, medullary rays. 



Fig. 57. Magnified view of a portion of one of the bands of black oak in 

 Fig. 56, showing the thick-walled summer wood succeeded by the thinner- 

 walled cells and vessels. This association of cells causes the banded appear- 

 ance of the annual rings of growth, m, medullary ray; v, vessels in the spring 

 wood. 



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, m; 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 



