PRIMARY STRUCTURE OF CERTAIN PALEOZOIC STEMS. 339 



pits, with the narrow, slit-like, more or less inclined pore, very clearly. The borders of 

 the pits are sometimes beautifully shown in section, where the wood is cut tangentially, 

 as represented in PL IV. fig. 6 (b.p.), from the Williamson specimen. 



The medullary rays, which have the usual muriform arrangement of their elements 

 (fig. 4), are, for the most part, one cell only in thickness, but often become two cells 

 thick in places (see figs. 6 and 7). Some are of considerable height (up to sixteen 

 cells or more), while others are only one or two cells high (fig. 6). The outgoing leaf- 

 trace is accompanied by a considerable amount of parenchyma, especially on the upper 

 side* (fig. 7). The medullary rays in the neighbourhood of the leaf-trace are irregular, 

 and generally shorter and broader than elsewhere. 



The pits adjacent to the medullary rays are bordered only on the side towards the 

 tracheide — the usual structure in all such cases (see fig. 6, m.r.). 



The chief peculiarity of. the secondary wood is in its innermost region, near the pith, 

 where the elements have an unusual form and arrangement. The tracheides here are 

 broad and short, often with horizontal terminal walls, which thus appear in surface 

 view when seen in a transverse section (cf. figs. 2 and 3). Their course is tortuous and 

 irregular ; the maximum diameter is usually in the radial direction (see figs. 2 and 3). 

 The pits on their walls, though in more numerous series than elsewhere, are of the 

 usual form ; the arrangement of the tracheides, so far as any regularity can be traced, 

 is in radial series, and the medullary rays pass between them ; towards the exterior 

 the structure passes over rapidly into that of the normal wood. This peculiarity of the 

 inner zone of wood is common to both the specimens investigated. There seems to be 

 no doubt that the short tracheides in question belong to the secondary wood ; they 

 resemble the primary tracheides found by Mr Seward in his new genus Megaloxylon,^ 

 and may probably have served, as he believes to have been the case in that plant, 

 for the storage rather than for the conduction of water. 



The chief results arrived at from the investigation of Calamopitys fascicularis are 

 the following : — 



(1.) The small pith (2-3 mm. in diameter) is surrounded by a ring of distinct 

 primary xylem-strands, eight or nine in number, with mesarcb structure. 



(2.) These strands are the xylem-constituent of the leaf-traces ; they attain their 

 maximum diameter ("8 mm.-l mm.) when they are about to leave the pith and to pass 

 out through the secondary wood. Below this point they rapidly diminish in diameter, 

 and each unites with the adjacent strand on its kathodic side. 



(3.) The outgoing strands are arranged on five orthostichies, corresponding to a 2/5 

 phyllotaxis. In passing through the wood, each leaf-trace is represented by a single 

 strand. 



(4.) The secondary wood has the typical Araucarian or Cordaitean structure, with 



* The orientation of fig. 7 lias been determined by comparison with a transverse section of the same specimen, in 

 which the parenchyma accompanying a leaf-trace is found on the inner (= upper) side of the strand. 



t Seward, " Notes on the Binney Collection of Coal-Measure Plants." Part II. Megaloxylon- Proc. Cambridge 

 Phil. Hoc., vol. x, 1899, p. 158. 



TRANS. ROY. SOC. EDIN., VOL. XL. PART II. (NO. 17). 3 e 



