178 STEMS 



the tree stops work over winter. They are caused by the 

 fact that the last cells formed in the fall are much smaller 

 than the first cells formed in the spring. Just so the 

 brick wall would be marked off into distinct layers if the 

 mason laid small bricks the last thing every night and 

 large ones the first thing every morning. 



The rings are caused, then, by the difference in size be- 

 tween adjacent cells ; layers of small cells lie against layers 

 of larger cells. During freezing weather the cambium is 

 not active. When in spring the sap ascends from the 

 roots and conditions for growth are right again, the cam- 

 bium begins to form what is called spring wood, and the 

 cells of this spring wood are much larger than the last 

 cells which were formed the fall before. 



A long period of drought may cause the cambium to 

 become inactive just as winter causes it to become inactive ; 

 in other words, very dry weather has the same effect upon 

 growth that freezing has. So, if a long, summer drought is 

 followed by a warm, wet fall, two growth rings, as you can 

 readily see, are likely to be formed. In some tropical 

 regions it has been noted that alternating wet and dry 

 seasons appear to have about the same effect in producing 

 rings that winter and summer have. 



H. The Bark and Things related to It. Lenticels. — With 

 bark most boys are familiar. If you five in the country, 

 you have almost surely cut your initials in it. As you 

 cut them you noticed the difference between the outer, 

 dry, and corky bark, and the inner, moist, and fibrous 

 bark. If you were doing a good job of initial cutting, you 

 cut clear through to the wood itself. That is, you cut 

 through cork, cortex, phloem, and, last of all, the cambium. 



