
THE STEM 143 
a continuous ring of cambium in the region between the xylem 
and the phloém (Fig. 139). In trees the region outside the 
cambium is known as bark, while that inside is known as wood. 
y 
F frame 
wl bs. 
Usually the bark can be readily separated from the wood, as 
- ; ‘ 
the cambium cells are soft and weak and can be easily broken. 
AY 
ee L/ : : % 
x UL CS é 2 < 
ss 
a 
SEN X\ \\ 


ras 
Pokag-] 
+ a 
+e Se) 
a 
Peas 
: 
We ter: 

Fie. 139. A cross section of a young stem of a tropical tree, ylang-ylang 
(Canangium odoratum), in which there has been some secondary thickening 
The wood is composed chiefly of wood fibers and large vessels. The bark has 
an alternation of bands of phloém and strands of thick-walled bast fibers. (x 40) 



Activity of cambium. The cambium layer consists essentially 
of a single layer of cells. These cells divide in a direction par- 
allel with the epidermis. Each time a cell of the cambium di- 
vides into two, one of the daughter cells remains meristematic, 
while the other is differentiated into permanent tissue. If 
