CHAP. II. 
EXOGENOUS STEMS. 
101 
and has a very thin but distinct bark, with a central pith 
surrounded by very compact woody tissue. There are neither 
zones nor medullary rays ; but the vascular system consists of 
an uniform mass of vessels and woody tissue, disposed with 
great symmetry, and of the same degree of compactness at 
the circumference as in the centre. Amongst this wood are 
interspersed, at the distance of about half a line, with great 
regularity, passages containing loose cellular tissue. These 
passages are convex at the back and rather concave in front, 
run parallel with the vessels, and do not seem to have any 
kind of communication with each other. They, no doubt, 
represent the medullary rays of the cellular system of this 
highly curious plant. It must be remarked, that the re- 
semblance borne by this stem to that of an Endogenous 
plant is more apparent than real ; for whilst, in the latter, the 
vascular system is separated into bundles surrounded by the 
cellular system, in this, on the contrary, the cellular system 
consists of tubular passages, surrounded by masses of the 
vascular system. 
It will be observed that, in all those cases of irregular 
developement, the part next the centre is but little affected ; 
and such seems to be the general rule. In the Penny Cy- 
clopaedia, article Exogens, there are figures of several cases 
of structure still more anomalous than the preceding, with 
the woody matter contorted excessively ; but even in them 
the centre is in the normal condition of exogenous wood. 
Such examples show the student that it is neither me- 
dullary rays nor concentric zones in the wood that are the 
certain indications of Exogenous growth, both the one and 
the other being sometimes absent ; but that the presence of 
a central pith, and a greater degree of hardness in the wood 
next thg centre, than in the circumference, are the signs from 
which alone any absolute evidence can be derived. 
