16G PROF. W. C. WILLIAMSON ON THE STRUCTURE 



enchymatous tract {d) of the internode above. At this 

 point we have appearances that vary in different examples. 

 Sometimes, as in fig. 6 d', we have a number of long sinu- 

 ous vessels^ converging at a central point, where we find 

 a small cluster of similar but transversely divided ducts, 

 with a few cells in its centre; at others, as in fig. 15, 

 these transversely divided tubes occupy a much larger por- 

 tion of the area, whilst their open orifices radiate in some- 

 what regular lines from the central point, which consists, as 

 before, of a small cluster of transversely divided cells. 

 Around the margin of this circular area, which closely re- 

 sembles a transverse section of some coniferous branch, we 

 again find the long vessels, bending away in the plane 

 of the section, to contribute their respective shares to the 

 two vascular tracts 15 c', c'. A vertical section, made 

 in the plane of a medullary ray, enables us to interpret 

 these appearances. As the vessels of the vascular tracts 

 approach a node, instead of following the outline of the 

 pith, as in the rest of their course, they are suddenly de- 

 flected from it, bending outwards in parallel arched curves, 

 but return to their original direction after passing the 

 node. Mr. Binney found a similar arrangement in his 

 Calamodendron. Fig. 17 is a diagram designed to explain 

 these appearances. It represents a section somewhat like 

 that shown in fig. 10 ; but in order to illustrate the rela- 

 tions of the several parts to the medullary radius, the 

 diagram is made to represent an oblique section, passing 

 from fig. 15/ on the left to c' on the right of the same 

 figure. But the arched vessels (17 c) must be understood 

 to pass downwards, on each side of the medullary radius 

 (17/), and not to terminate abruptly at the latter, as the 

 diagram indicates. 



But, in addition to these arching vessels, we have at 

 the nodes numerous groups of divergent vessels (17 e), 

 varying in diameter in different parts of the plant, which 



