164 PROF. W. C. WILLIAMSON ON THE STRUCTURE 



with the furrows of the adjoining ones^ though occasionally 

 no such alternation takes place. In the former case_, one 

 of the radiating vascular tracts would^ if prolonged straight 

 upwards through the node_, run into a prosenchymatous 

 tract of the joint above. In the exceptional instances_, the 

 vessels are prolonged through the node with little dis- 

 turbance^ and continued into the corresponding vascular 

 tract of the next joint. In both cases the vessels of the 

 various articulations are shown to be continuous. This 

 alternation in the arrangement of the tracts causes pe- 

 culiar modifications in the disposition of the vessels on 

 crossing the node. 



Three disturbing elements exist at the articulation_, — ist, 

 the verticillate medullary radii ; 2nd, the verticils of vas- 

 cular bundles ; and, 3rd, the alternation of the tracts. 



Fig. 13 represents a transverse section almost in the 

 plane of the verticillate radii ; exactly so on the left of the 

 figure, but traversing their upper surface to its right : 

 a represents the pith, / the several medullary radii, and 

 c the woody wedges which separate the latter. In fig. 14 

 one of these wedges is shown more highly magnified. It 

 consists of from 30 to 40 radiating series of vessels, 

 arranged like the fibres of most recent Conifer ae ; but at its 

 two outer margins are a very few rows of larger structures 

 (c?), which we find to be sections of the outermost rows of 

 prosenchymatous cells seen in fig. 3 d, some of which, as 

 already shown in fig. 6, separate each medullary radius 

 from the nearest vascular tracts. At its inner extremity 

 each wedge is pointed, having its two sides slightly exca- 

 vated ; hence across the centre of fig. 1 3 we have a crenu- 

 lated outline of the pith, readily recognized as representing 

 a section of the exterior of an ordinary Calamite. At this 

 point of the stem the regular arrangement of the vessels 

 has undergone little disturbance, space for the passage of 

 the medullary radii being obtained at the expense of the 



