of Coal-Measure Plants. 



161 



pits (PI. vii. fig. 9, text-figures 2 — 4m). In some places — as shown 

 in fig. 7 (PL vi.) — the large tracheids extend close to the inner edge 

 of the secondary wood, but in others we find more or less well- 

 defined groups of narrow elements occupying a peripheral position 

 in the primary stele {e.g. PI. v. figs. 3-5, It 1 — It 3, in fig. 7, also 

 PI. VII. fig. 8). The radial rows of secondary tracheids may 

 occasionally be traced into direct continuity with the narrower 

 peripheral elements of the primary xylem region, as shown in the 

 upper part of fig. 8, PL vii. ; but usually the secondary xylem is 

 succeeded internally by rather crushed thin-walled parenchyma 

 composed of polygonal or tangentially elongated cells, several of 

 which contain a dark brown or black substance. 



The primary tracheal elements assume various forms in different 

 regions of the periphery of the primary stele ; in some parts of the 

 section we find groups composed of a few tracheids, considerably 

 larger than those of the secondary xylem, traversing the parenchyma 

 of the stele ; in other regions the tracheids are more numerous, 

 and the outermost elements are characterised by their much 

 It px x 2 



mr 



Figure 1. Longitudinal Section showing the Secondary Xylem x 2 , and 

 Medullary Eays, mr ; the Xylem of the Leaf-trace It, and the Pro- 

 toxylem px. (Section I. 9, x 46.) 



smaller diameter (pas, fig. 8, PL VII.; figs. 3 and 6, PL v.), — these 

 are the spirally pitted protoxylem tracheids. In the longitudinal 

 section represented in text-figure 1, some of the protoxylem 



