XV] LEPIDODENDRON 167 



secondary xylem, x^, composed of scalarifortn tracheae and 

 numerous medullary rays consisting of one, two, or several rows 

 of radially elongated elements with spiral and reticulate pitting. 

 In tangential sections the rays are seen to vary considerably 

 in size, some being made up of a single row of cells while others 

 are longer and broader; through the latter leaf-traces pass 

 horizontally. Portions of medullary rays are seen at mr in 

 fig. 181, C and E. 



'The leaf-traces given off from projecting ridges on the outer 

 edge of the primary xylem pass upwards for a short distance 

 and then bend outwards through a broad medullary ray; on 

 reaching the limit of the secondary xylem they again bend 

 sharply upwards, appearing in transverse section at It fig. 181, B. 

 Each leaf- trace consists at first of long tracheae accompanied by 

 numerous thin-walled spiral and reticulate parenchymatous cells 

 derived from the tissue in contact with the outer edge of the 

 primary wood. Fig. 181, B, shows a leaf-trace near the edge of 

 the secondary xylem ; it consists of a group of primary tracheae, 

 with narrower protoxylem elements, px, near the outer margin, 

 almost completely enclosed b}' radially disposed series of smaller 

 and more delicate tracheae. These secondary elements of the 

 leaf-trace are apparently added during its passage through 

 the medullary ray, but additions are also made to this tissue 

 by the meristematic zone, m, fig. 181, B and E. In contact 

 with the outermost tracheae of normal size at the edge of the 

 secondary xylem there are some smaller lignified elements, as 

 at a, fig. 181, E, and at T, fig 183; this juxtaposition of large 

 and small tracheae has been referred to in the description of 

 L. vasculare. 



Prof. Williamson ^ in his account of the Arran specimens of 

 this species, expressed the opinion that the trees probably 

 perished "in consequence of the mephitic vapours which filled 

 the atmosphere"; it maybe that in the striking difference in the 

 diameter of the conducting elements on the margin of the wood 

 we have evidence of approaching death. 



Beyond the most recently formed tracheae we have a band 



1 Williamson (96) p. 175. 



