212 PROTOPITYEAE [CH. 



it increases in breadth at the ends of the long axis where the 

 tracheids are intermixed with parenchyma. The primary xylem 

 and pith- tissue at the ends of the major axis of the central region 

 assume different forms at different levels, owing to the detach- 

 ment of leaf-traces and the consequent formation of foKar gaps 

 as portions of the primary xylem pass obUquely outwards into 

 the secondary xylem on the way to the distichously arranged 

 alternate leaves. The diagram, fig. 462, A, shows the inner 

 part of the secondary xylem (see also fig. 462, C, x^) which at 

 one end. It, has formed an oval group about to pass out as a leaf- 

 trace: at the opposite end the strand is detached and divided 

 into two equal branches. The two swellings of the primary 

 xylem ellipse shown at a in figs. 462, A and B, are a characteristic 

 feature: these are clearly seen after the leaf-trace has become 

 detached; at the inner edge of each of them there appears to 

 be a protoxylem strand. After the formation of a foliar gap these 

 swellings of the xylem gradually meet and so re-establish con- 

 tinuity below the outgoing leaf-trace. No protoxylem has been 

 detected in the actual trace, which is believed to be concentric. 

 The formation of the leaf-gap and the shoulders bordering it 

 constitute interesting fihcinean features, recalling corresponding 

 characters in solenostelic Ferns. At the upper end of the diagram, 

 fig. 462, B, the outgoing leaf-trace is undergoing dichotomy while 

 at the opposite end the trace has passed out of view. The 

 secondary xylem shows incomplete rings or arcs of narrower 

 elements, which at first sight give the impression of annual rings : 

 the occurrence of similar incomplete or pseudo-rings is a common 

 feature in Lepidodendron and other Palaeozoic stems. The 

 secondary tracheids (54-4ju. in tangential diameter, 68-5jU, in radial 

 diameter) have usually a single series of broadly oval bordered 

 pits on the radial walls with here and there two rows (fig. 462, D). 

 In one case only were the pits of the medullary rays recognised 

 (fig. 462, F). The rays are uniseriate, generally 1 — 2 cells deep, 

 but occasionally 3 cells in depth and very rarely deeper. The 

 cambium is of the normal type, and in some specimens secondary 

 phloem was found consisting of bands, 4 — 5 layers broad, of 

 stone-cells alternating with tubular thin-walled elements, pre- 

 sumably sieve-tubes. 



