XXXIV] ANTARCTICOXYLON 297 



on the Priestley Glacier south of 74 S. lat. in the course of the 

 journey of the Northern Party of Capt. Scott's second Antarctic 

 Expedition. The type-specimen is a piece of silicified stem 1 ft 

 long and 3 ins. in diameter ; there were no tissues preserved 

 external to the secondary wood and it is impossible to say what 

 proportion of the original thickness is represented. Annual 

 rings are clearly marked at least macroscopically (fig. 491, C) 

 though there is very little difference between the spring and 

 summer tracheids : the centre of the compressed stem is occupied 

 by a lighter coloured elliptical area 7x2 mm. which superficially 

 resembles a pith, but in the peripheral region it consists of portions 

 of a cylinder of spiral and scalariform tracheids, the actual pith 

 being not more than 23 mm. in breadth. The pith consists of 

 lacunar parenchyma separated by horizontal bands of dark cells 

 containing some secreted substance (fig. 491, F) : the preservation is, 

 however, not sufficiently good to enable one to describe it in detail. 

 The secondary xylem is of the pycnoxylic type ; the tracheids 

 have either a single row of contiguous and partially flattened 

 pits on their radial walls or a double row of alternate polygonal 

 pits ; the medullary rays are nearly always uniseriate (fig. 491, E) 

 and from 1 to 24 cells in depth. At the inner edge of the secondary 

 wood there was a fairly broad zone of more delicate tracheids 

 (fig. 491, A, x) characterised by spiral or scalariform bands and 

 by their relatively small diameter. This innermost zone, which 

 supplies the leaf-traces, is spoken of as the primary xylem ; it 

 appears to be endarch though this cannot be definitely determined. 

 A characteristic feature of the primary xylem in the perimedullary 

 region, as also in the leaf -traces on their way through the secondary 

 wood, is the presence of short and broad tracheids (fig. 491, D, t) 

 with reticulate thickening bands: these short elements may 

 represent centripetal xylem and are similar to the short tracheids 

 described by Scott 1 in the sheath of Mesoxylon Lomaxi and to 

 the larger elements in the stem of Megaloxylon 2 . 



An interesting feature seen in transverse sections of the 



secondary wood is the occurrence of light bands concentric with 



the rings of growth which are broadest near the long axis of the 



stem (fig. 491, C). In their narrower parts these bands are clearly 



1 Scott (12). 2 See page 175. 



