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 large volumes of water that are 

 transported to the rapidly growing shoots, flowers, and leaves 

 each spring. Later in the season a lessened volume of water 



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 rays; v, vessels in the spring 

 wood. 



produces firmer and denser tissues. The age of a tree can gener- 

 ally 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, 



