86 



THE ANNUAL GROWTH 



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 



Fig. 55. Cross-section of stem of castor bean three weeks older than one 

 shown in Fig. 54. Note the changes that have occurred in the stem and 

 especially the numerous cells that have been added to the xylem. ck, cork; 

 ph, phloem; cam, cambium cylinder.— H. O. Hanson. 



transported to the rapidly growing shoots, flowers, and leaves 

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

 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 



