PRIMARY STRUCTURE OF CERTAIN PALEOZOIC STEMS. 355 



stems) it would be extremely rash to draw any inference as to a seasonal periodicity of 

 growth. 



At one place a bundle, no doubt a leaf- trace, is clearly shown, passing out 

 through the wood (see phot. 10, l.t.). A very definite arc of secondary wood forms 

 part of the outgoing leaf-trace, and is sharply marked off from the general wood of the 

 stem. This observation confirms the conclusion, drawn from Pitys antiqua, that in this 

 genus the leaf-traces, on leaving the pith, were single strands. 



So far as the evidence extends, there was thus a complete agreement in the primary 

 structure of the stem between Pitys Withami and P. antiqua. 



3. Pitys primseva, Witham. # 



This species, one of Witham's Tweed Mill fossils.t is a very distinct form, as shown 

 by the great width of the medullary rays, which are commonly seven cells in thickness 

 and often more, and of a decidedly broader and shorter form, in tangential section, than 

 those of P. antiqua or P. Withami (see fig. 22). The tracheides, also, are larger, and 

 the pitting slightly different, the pits of P. primseva being less crowded than those 

 of the other species. Hence the hexagonal form is less marked and a circular outline 

 more frequent in the pits of P. primseva than in those of its congeners. 



As regards the question of primary xylem-strands, the material at my disposal was 

 not favourable, as I have seen no sections passing through the pith of a main stem. 

 In one case, however, a tangential section happens to cut transversely across the base of a 

 lateral branch (PI. II. phot. 11). The pith of the branch is only partly preserved ; what 

 remains of it resembles that of P. antiqua. At two places in the pith, near the wood, 

 I observed a group of small, rather thick-walled elements, similar to the tracheides of 

 the secondary xylem. At one point the spiral band of a tracheide could be recognised. 

 The pith-cells are elongated radially around the groups in question, and the whole 

 appearance (allowing for the imperfect preservation) is in all respects similar to that of 

 the primary xylem-strands in P. antiqua. 



The same tangential section is also of interest from another point of view, for it 

 appears to throw light on the problematic fossil described by Williamson under the 

 name of Lyginodendron (?) anomalum.% In the section of P. primseva (phot. 11), 

 the medullary rays near the lateral branch have a form very different from that which 

 characterises them elsewhere (see fig. 23, and compare with fig. 22). They are shorter 

 than usual, and at the same time much wider, so as sometimes to assume a nearly 

 circular form, as seen in the tangential section. These exaggerated medullary rays 

 constitute in this region the great mass of the wood, the strands of tracheides merely 

 forming a sinuous network between them. The appearance is almost identical with 



* Sections of two specimens, one from the River Irthing, Northumberland, the other from Juniper Green, Mid- 

 lothian, were lent me by Mr Kidston for investigation. Both are from the Calciferous Sandstones. 

 t L.c, pp. 38, 71, pi. viii., figs. 4-6. 



X " Organization of Fossil Plants of Coal-Measures," Part IX., Phil. Trans., 1878, pt. ii. p. 352, pi. 25, figs. 90-92. 

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



