202 



STEMS 



bium adds new phloem on its outside at the same time that it adds 

 new xylem within, annual rings occur in the bark as well as in 

 the wood; but in the bark where the tissues are soft and there- 

 fore crushed, the annual rings are either indistinct or obKterated. 

 In some woody stems having many annual rings, only the outer 

 annual rings which constitute the sap wood, recognizable by its 



Fig. 181. — Diagrammatic drawing of an Oak log, showing cross sections or 

 end view of log, a view of a surface (tangential) at A made by sawing off a 

 slab from the side of the log, and a view of a surface (radial) at B made by 

 sawing from peripery to center. 



light color, are active in conducting. (Fig. 179.) . Sap wood is 

 often called the living wood because, although much of it is dead, 

 the cells of the medullary rays and wood parenchyma are alive, 

 while the heart wood is practically all dead. Heart wood is usu- 

 ally recognized by its dark color due to deposits of various sub- 

 stances, principally in the cell walls. 



The medullary rays are also formed by the cambium and are 

 of two kinds: (1) those extending from pith into bark and known 



