CHAP. II. 
EXOGENOUS STEMS. 
67 
Oak and the Cane, specimens of which can be easily obtained 
for comparison. A transverse slice of the former exhibits a 
central cellular substance or pith^ an external cellular and 
fibrous ring or bark^ an intermediate woody mass, and certain 
fine lines radiating from the pith to the bark, through the 
wood, and called medullary rays ; this is called Exogenous 
structure. In the Cane, on the contrary, neither bark, nor 
pith , nor wood, nor medullary rays, are distinguishable ; but 
the transverse section exhibits a large number of holes irregu- 
larly arranged, and caused by the section of vasiform tissue, 
and the mass of woody and cellular substance in which they 
lie imbedded. This kind of structure is named Endogenous. 
In both cases there is a cellular and vascular system distinct 
from each other ; it is only by a diversity in their respective 
arrangement that the differences above described are caused. 
In explaining in detail the peculiar structure of E:^C)genous 
and Endogenous stems, it will be more convenient to consider 
them with reference to those two systems, than to follow the 
usual method of leaving the fact of there being two distinct 
- systems out of consideration. 
The cellular system in an Exogenous stem chiefly occupies 
F 2 
