STRUCTURE.] STRUCTURE OF PISONIA. 215 



the stem, or even less, and they anastomose in various ways ; 

 the medullary rays are unusually large, and lie across the 

 wood like parallel bars ; and, finally, the plates of which the 

 wood consists each contains but one vessel, which is situated 

 at the external edge of the plate. 



None of the anomalous forms of Exogenous stems are, 

 however, more remarkable than a Burmese Pisonia, (fig. 42.), 

 for a specimen of which I am indebted to Dr. Wallich. In 



fig- 42. 



a section of this, the general appearance is so much that of an 

 Endogenous stem, that without an attentive examination it 

 might be actually mistaken 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. 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 resem- 

 blance borne by this stem to that of an Endogenous plant is 

 more apparent than real ; for whilst, in the latter, the vascular 



