92 ANNUAL RINGS 



vided with thicker walls. So there is a sharp contrast between 

 the small thick-walled cells of the summer wood and the thinner 

 walls and larger cells of the spring wood (Fig. 57). This ap- 

 pears to the eye as a band, the annual ring (Fig. 56). The 

 difference in the structure of the cells of the spring and summer 

 wood is doubtless due to the seasonal changes that are charac- 

 terized by the annual spring and summer growing periods and 

 by the winter rest. At least tropical woods grown under uniform 



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, 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, ray; v, vessels in the spring wood. 



conditions do not show the annual rings. The age of a tree can 

 generally be ascertained by counting the annual rings. However, 

 two 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, 



