PRIMARY STRUCTURE OF CERTAIN PALAEOZOIC STEMS. 351 



is often rather remote from the centrifugal wood.* A similar separation is of common 

 occurrence in roots, especially those of Gymnosperms. 



Two other explanations are theoretically possible, though I believe really untenable. 

 We might suppose that the cambium was extrafascicular, the xylem-strands thus 

 representing complete vascular bundles. I have not, however, found the least indi- 

 cation of phloem in connection with them, and the preservation is sufficiently good to 

 leave little doubt that none existed, and that the xylem-strands were surrounded on all 

 sides by parenchyma only. 



The other possibility is that the xylem-strands may not have constituted the 

 primary xylem of the leaf-trace system, but may have formed a merely accessory 

 system of medullary strands, remotely comparable to the medullary bundles of 

 Encephalartos, Macrozamia, or even, as some might suggest, to the star-rings of 

 Medullosese. The absence of phloem is an obvious objection to this view also, and 

 even apart from this, the facts that the xylem-strands pass out through the secondary 

 wood, and that they are the only part of the wood where spiral elements occur, seem 

 fairly decisive for their primary nature, as part of the main leaf-trace system of the 

 plant. The very remarkable medullary xylem found by Eothert in a Conifer, which 

 he refers to Cephalotaxus Koraiana, offers a certain analogy with that of our fossil, 

 but, in the case of this recent plant, no spiral elements occur among the medullary 

 tracheides.t 



The general structure of the secondary wood of Pitys antiqua has long been 

 known,! but as Mr Kidston's specimen (No. 598) is probably the best-preserved of any 

 hitherto investigated, a short description may be given, more especially as the existing 

 accounts are inaccurate in various points. 



The chief generic character of Pitys, as at present defined — the wide medullary 

 rays — is well exhibited. The principal rays are usually much wider at their inner ends 

 than elsewhere. A ray '3 mm. in tangential width at its junction with the pith may 

 diminish to a tenth of that width in a distance of little more than a millimetre (fig. 

 14, phot. 8). The difference depends more on the width of the individual ray-cells 

 than on their number, which remains nearly constant, and is evidently due, in great 

 part, to dilatation occurring during the early stages of wood-formation. The gaps in 

 the pith, referred to above (p. 346), run out for some distance into the principal 

 medullary rays. Close to the pith, where the rays are wide, the nature of these gaps, 

 as mere tears in the tissue, is evident ; further out, as the rays become narrower, the 

 gaps assume a more regular form, and sometimes strongly suggest the resin-canals in 

 the rays of Abietineae (see PI. VI. fig. 20, g). This appearance is in all probability 



* Scott, " Anatomical Characters presented by the Peduncle of Cycadaceee," PI. XX. figs. 1-5, Ann. Bot, vol. xi., 1897. 



t Rothert, I.e. It is interesting to find (p. 285), that Rothert's "gemischte Gefasse" occur in the medullary as 

 well as in the normal wood of his Cephalotaxus, just as is the case in Pitys antiqua. 



% Witham of Lartington, Internal Structure of Fossil Vegetables, Edinburgh, 1833, pp. 25-27, 37, 38, 71, pi. iii. ; 

 pi. iv., figs. 1-7 ; pi. vii., figs. 9-12 ; pi. viii., figs. 1-3 ; pi. xvi., figs. 9, 10. Little appears to have been added 

 by later writers. 



