170 PROF. W. C.WILLIAMSON ON THE STRUCTURE 



getaux Fossiles, &c.^ extrait du Dictionnaire Universelle 

 d^Histoire Naturelle, 1849^ P- 5°)^ which obviously cor- 

 respond to my true medullary rays. The intermediate 

 cellular laminae consist of what Brongniart terms " woody 

 fibres" (fibres ligneuses) , but which appear to be identical 

 with my prosenchymatous tissue^ each layer having in its 

 centre what he terms one large medullary ray composed 

 of two or three vertical rows of cells. In my plant the 

 woody wedges are constructed,, like Unger^s Calamitea, of 

 elongated vessels^ separated by numerous medullary rays ; 

 only the vessels are reticulated instead of scalariform. 

 The intermediate cellular tracts also appear to resemble 

 Unger^s^ consisting^ as I have shown^ of a peculiar pros- 

 enchymatous tissue. I find here nothing like the central 

 medullary ray of linger ; but instead of it we have the 

 large verticillate medullary radius in the upper extremity 

 of each prosenchymatous tract. Thus we learn that 

 Unger's plant has the scalariform vessels of Mr. Binney's 

 type^ with the medullary rays of mine^ and constitutes, 

 in these respects, an intermediate link between the two. 



The prosenchymatous tissue is of considerable interest 

 on several grounds. When divided vertically, whether on 

 the plane of the medullary rays or tangentially, the 

 cells appear to be arranged in no special order beyond 

 what we see in many compressed forms of parenchyma. But 

 when we turn to the transverse section, we discover the re- 

 gularly linear radiating arrangement which I have described, 

 and which is so characteristic of vascular tissues, as well 

 as of the pleurenchymatous elements of Gymnospermous 

 Exogens. From their occupying the same position as the 

 cellular laminae of other Calamodendra, which are unques- 

 tionably prolongations of the pith, we might almost regard 

 them as huge medullary rays. These, however, as we have 

 seen, exist in addition to them. They so closely resemble, 

 in their linear arrangement, the pleurenchyma of conifer- 



