THE WORK OF STEMS 247 



many annual rings of wood. The annual ring of wood 

 is the growth made by the cambium in one year. The 

 wood formed in autumn (or toward the end of the 

 season of growth) is much denser and harder than that 

 formed in the spring and composed of smaller cells 

 (Figs. 135 and 138) ; the difference is apparent to the 

 naked eye and the rings may be easily counted, thus 

 fixing the age of the tree (see Fig. 145) . If, on account 

 of drought, destruction of the leaves by insects, or for 

 any other reason, there are two seasons of growth in 

 a year, there are two more or. less sharply defined 

 rings. The formation of denser wood in the fall is 

 probably due partly to the unfavorable conditions of 

 growth then prevailing and partly to the "binding" of 

 the bark, which gets tighter as the expanding wood 

 stretches it, so hindering the growth and making the 

 cells smaller. Cut a section (about one -fourth of an 

 inch thick) from a twig during the growing season. 

 Slit the bark on one side and peel it off carefully. 

 Now replace the ring of bark on the twig; can you 

 make the ends meet ? What does this show in i-egard 

 to the stretched condition of the bark! Fruit-growers 

 often slit the bark of trees in the spring with the point 

 of a knife, in order to allow the wood to expand: 

 washes of soap or lye are also used to soften the bark. 

 Trees exposed to a prevailing wind grow thicker in the 

 direction in which the wind blows; this is believed to 

 be due to the stretching and loosening of the bark on 



