162 PROF. W. C. WILLIAMSON ON THE STRUCTURE 



vertical instead of horizontal as is common in living plants. 

 In other respects they appear to be distributed as is usual 

 amongst Coniferse^ strongly reminding us of their arrange- 

 ment in the carboniferous genus Dadoxylon. 



Thus far I have confined myself to a description of the 

 woody zone in its undisturbed form, as it appears in the 

 middle of the internodes ; but at and in the neighbourhood 

 of the nodes or articulations (fig. 2 6) a remarkable modi- 

 fication occurs. As is well known, each of these nodes is 

 represented in the common type of Calamite by a circular 

 transverse constriction; but in the living plant it was 

 merely an indentation of the pith, occasioned by a pro- 

 jecting lenticular ring of woody tissue, of which the me- 

 dullary margin was somewhat wavy, or projecting at points 

 corresponding with the longitudinal grooves into a series 

 of small irregular teeth. 



Fig. 10 represents a vertical section of this structure, as 

 seen under a very low magnifier, a indicating the pith, 

 and b the woody lenticular ring. 



Fig. 1 1 represents a more highly magnified view of one 

 of the tooth-like projections from this ring, as seen in its 

 horizontal section. The cellular clusters at its internal 

 angles (^, i) probably belonged to the pith. 



I have already called attention to the remarkable series 

 of horizontal verticils of cylindrical prolongations of the 

 pith seen in fig. i. It is well known that in many fossil 

 Calamites, at the uppermost part of each longitudinal 

 ridge of the several joints there is a small round mark or 

 cicatrix, to which various functions have been assigned. 

 The example figured indicates that wherever such marks 

 occur, the specimen bearing them is a pith, and that 

 the marks are merely the points from which what I have 

 termed the verticillate medullary radii spring. In nearly 

 every specimen hitherto found these radii have disap- 

 peared along with the woody zone which they penetrated. 



