80 
ORGANOGRAPHY. 
BOOK I. 
None of the anomalous forms of 
-1^- ■ Exogenous stems are, however, so 
vJ^f^^^'rA'^t^j's^i^^^^ remarkable as an unknowTi Burmese 
tree (^^^.40.), for a specimen of which I 
'^^l^^!^ am indebted to my friend Dr. Wallich. 
In a section of this, the general ap- 
%'^>'^^M pearance is so much that of an Endo- 
genous stem, that without an attentive 
^^W^ examination it might be actually mis- 
taken for one. The diameter of this 
stem is two inches seven lines ; it is nearly perfectly circular, 
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. Amono-st 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. 
These examples of anomalous structure will show the stu- 
dent that it is neither medullary 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 centre than in the circumference, are 
the signs from which alone any absolute evidence can be 
derived. 
