80 



ORGANOGRAPHY. 



BOOK I. 



40 









None of the anomalous forms of 

 Exogenous stems are, however, so 

 remarkable as an unknown Burmese 

 tree (_;?(7.40.),foraspechnenof which I 

 am indebted to my friend Dr. Wallich. 

 In a section of this, the general ap- 

 pearance is so much that of an Endo- 

 genous stem, that without an attentive 

 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 viniform 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 

 intei'spersed, 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 ciu'ious 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. 



