THE STRUCTURE AND AFFINITIES OF DIPLOLABIS ROMERI (SOLMS). 715 



very primitive type of stem and a petiole-trace which might be placed in several differ- 

 ent form-genera according to the relative position in the petiole of the section under 

 consideration. These Pettycur specimens are the first examples of this species recorded 

 from a British source, and are interesting both from a botanical standpoint and on 

 account of their geological age, since this species helps to link up more closely the flora 

 of the Calciferous Sandstone Period in Scotland with that recorded from the Culm and 

 the Devonian on the Continent. 



General Structure. 



The structure of the stem is exceedingly simple. It consists of a fair-sized stele 

 (the xylem being 3 to 5 mm. in diameter), surrounded by a cortical zone whose tissues 

 are not as a rule preserved, though its limits are indicated. A typical transverse section 

 of the stem-xylem is shown in PI. I. fig. 1 ; and in PL I. fig. 9, o.c, the cortex is 

 delimited by the lighter-coloured area surrounding the stele. The outer cortical zone 

 is rather darker than the matrix containing the specimen, and so the outer boundary of 

 the cortex is quite distinct, PL I. figs. 9 to 11, o.c. In some specimens the dark outer 

 layers of the outer cortex are preserved, but the cells do not appear to have been sclerotic. 



The xylem, however, is usually well preserved, or at least sufficiently so to be traced 

 continuously through the block. Occasionally even the xylem has become partially 

 decayed or disintegrated during petrifaction. As seen in PL I. fig. 1, the xylem of the 

 stem is circular in transverse section, and consists of an outer and an inner zone x and x' 

 respectively. The elements of the outer zone, which show a strong suggestion of radial 

 arrangement, are long, pointed tracheides without any admixture of parenchyma. 

 The inner tracheides are short, square-ended, and reticulately thickened ; they are 

 very distinct in PL I. fig. 7. The two kinds of wood can easily be distinguished, even 

 in transverse section, for the elements of the inner wood are rather smaller in diameter 

 than these of the outer xylem. They are not radially arranged, and the walls appear 

 to have been more delicate, since these are often much distorted even in cases where 

 no distortion is visible in the outer xylem (PL I. fig. l). In longitudinal section the 

 distinction is exceedingly obvious (PL I. fig. 5, x and x'). There is no secondary wood. 



Diplolabis romeri, in possessing a solid stele with two kinds of xylem elements, 

 closely resembles the Permian osmundaceous stem, Zalesskya gracilis (Eichwald), or 

 more nearly Thamnopteris schlechtendalii (Eichwald). Both of these stems have been 

 shown by Kidston and Gwynne-Vaughan (6) to possess an inner and an outer xylem. 

 In habit, however, D. romeri differs from these osmundaceous specimens, for its stem 

 was long and often branched, while appendages were given off only at long intervals 

 and not in a close spiral. Indeed, the whole appearance of the stem of this plant 

 strongly suggests that it was a rhizome. Numerous petioles of Diplolabis romeri occur 

 associated with the stems described above, and in some cases such petioles can be traced 

 into the stem. In the normal petiole the trace is X-shaped, or the two sides of the X 

 may be separated by a horizontal bar of xylem )— (, or the trace may become quite 



