[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 267 



phloem in cross section are not so evident. They may be seen in the 

 ends of a twig of basswood with the aid of a hand lens. 



The terminal stem segment contains one cylinder of wood and one of 

 phloem. In the next stem segment below the first terminal bud scale 

 scars there are two cylinders of wood and two of phloem. The cylinder 

 of wood adjacent to the cambium is the one that extends through both 

 annual stem segments. Likewise, the cylinder of phloem adjacent to the 

 cambium is the one that extends through both stem segments. 



In the stem segment below the second teiTninal bud scale scar there 

 are three cylinders of wood and three of phloem. Only the cylinder 

 of the wood and the cylinder of phloem adjacent to the cambium extend 

 through all three stem segments. Can you visualize the positions of 

 these cylinders all the way down the branch and trunk of a tree to its 

 base near the soil? 



If the tree is a century old there should be 100 annual rings of wood 

 at its base. The innermost cylinder of wood adjacent to the pith is as 

 high as the tree was when it was one year old. The next cylinder is as 

 high as the tree was when it was two years old, and so on to the 100th 

 and youngest cylinder of wood adjacent to the cambium. This cylinder 

 extends all the way from the base of the tree to the tip of the terminal 

 twig. Each lateral branch also has a similar arrangement of annual 

 cylinders of wood. 



Likewise, the innermost cylinder of phloem extends all the way from 

 the tip of the terminal twig to the base of the tree. Each branch contains 

 a cylinder of youngest phloem external to the cambium. The cylinders of 

 xylem and phloem in any stem are continuous with those of the same age 

 in each of its lateral branches. 



Furthermore, these youngest cylinders of wood and phloem also ex- 

 tend all the way down the root to near its tip. The older cylinders of 

 wood and phloem in the root, like those in the stem, are successively 

 shorter and shorter until we come to the shortest ones, which were 

 formed during the first season of growth. The pith in the stem, on the 

 other hand, develops from the apical meristem and merely elongates 

 each year and is therefore continuous from the tips of the branches 

 to the base of the trunk ( Fig. 96 ) . 



Since annual stem segments bear leaves but once, the youngest cylin- 

 der of wood is the only one that is continuous with the xylem in the 

 veins of the leaves of deciduous trees and shrubs. Likewise, the youngest 

 cylinder of phloem in the stem is the only one that is continuous with 



