XXXn] MEGALOXYLON 179 



448, B, U, close to the inner edge of the secondary xylem, x^, and 

 abutting internally on the contracted tissue, m, which consists 

 mainly of large and short tracheae with remains of associated 

 parenchyma. The trace includes some conjunctive parenchyma 

 interspersed with the tracheids: on its outer surface, that is on 

 the abaxial edge of the ovate xylem strand as shown at It^ in 

 fig. 447, there are six external protoxylem strands. In the peri- 

 pheral region of the primary xylem of the section reproduced 

 in fig. 447 there are several more or less well-defined leaf-traces, 

 e.g. It^, U^; these difier from that seen at It^ in their greater 

 tangential breadth and in the less compact arrangement of the 

 tracheids. As each trace is followed downwards in the primary 

 region of the stele it tends to become broader, especially in a 

 tangential direction; the spiral protoxylem strands are more 

 widely separated (fig. 448, A, px) and the elongated and com- 

 paratively narrow tracheids as they spread out fan-wise are 

 reduced in length, finally passing over into the broad and short 

 reticulately pitted cells. Each leaf-trace can be followed through 

 approximately four internodes before its individuahty is lost 

 in the general mass of metaxylem. The disposition of the peri- 

 pheral traces is such as to justify the conclusion that the phyllo- 

 taxis of the stem is f.^ 



The secondary xylem (fig. 446, C) resembles that of Lyginofteris 

 and Heterangium though it is less parenchymatous . The medullary 

 rays are numerous and vary in breadth from 1 to 5 cells, while 

 the tracheids, with multiseriate bordered pits on their radial 

 walls, form bands 1 to 8 elements in breadth. There are no 

 regular rings of growth but occasional arcs of narrow tracheids 

 interfere slightly with the otherwise uniform structure of the wood. 

 A leaf-trace in its obhque outward course through the wood 

 becomes completely enclosed by a cyhnder of secondary xylem 

 and thus appears to be concentric. Owing to the absence of any 

 tissue external to the secondary xylem of the stem, statements 

 as to the subsequent behaviour of the leaf-traces on emerging 

 from the stele are purely hypothetical. It is, however, not 

 improbable that each concentric trace lost its secondary tissue 



1 For additional figures, see Seward (99) B. 



12—2 



