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 obliterated. 
In some woody stems having many annual rings, only the outer 
annual rings which constitute the sap wood, recognizable by its 
aS 
as 
Fie. 181. — A piece of an oak board. v, end of the board showing medul- 
lary rays (m) and annual rings (r); u, edge of the board showing the medul- 
lary rays (m); b, broad surface of the board showing the annual rings con- 
sisting of light and dark bands. 
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 
