CHAP. II. 
STEM. 
85 
tular simple stems without articulations, as those of Rushes ; 
but neither of these differ in any material degree from com- 
mon stems, and the employment of either term is superfluous. 
This has already been remarked with respect to culm us by 
Link, who very justly inquires (Linnaea, ii. 235.) ‘‘cur 
Graminibus caulem denegares et culmum diceres? ” 
If a plant is apparently destitute of an aerial stem, it is 
technically called stemless {acaulis)^ a term which must not 
however be understood to be exact, because it is, from the 
nature of things, impossible that any plant can exist without 
a stem in a greater or less degree of developement. All that 
the term acaulis really means, is that the stem is very short. 
3. Of its Internal Modifications. 
The internal structure of the stems of Flowering plants, is 
subject to two principal and to several subordinate modifica- 
tions. The former are well illustrated by such plants as the 
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 hark, 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 larger number of holes irregu- 
larly arranged, and caused by the section of bothrenchy- 
matous and vascular tissue, and the mass of woody and cel- 
lular 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 Exogenous 
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. 
G 3 
