CYCADOXYLEAE 485 



therefore remain an open question whether the primary 

 structure of the xylem was of the Lyginodendron type, 

 or whether the whole wood developed centrifugally, as 

 in the stem of recent Cycads or Conifers. On the 

 other hand, the large stem presents a peculiarity of 

 structure which occurs in some specimens of Lygino- 

 dendron oldhamium. It was mentioned above (p. 372) 

 that, in that plant, anomalous vascular tissues are 

 sometimes developed.' in the outer part of the pith, 

 immediately within the normal ring of bundles. These 

 tissues, when present, have inverted orientation, the 

 xylem facing outwards, towards the normal wood, and 

 the phloem inwards, towards the centre of the pith. 

 Precisely the same anomaly reappears, in a more 

 striking form, in Cycadoxylon robustum. There is a 

 distinct zone of secondary wood at the margin of the 

 pith, reaching in places a thickness of about sixteen 

 tracheides in a radial row, and on the inner side of the 

 anomalous zone, a corresponding band of phloem can 

 be recognised (Fig. 178, x?, pk s ). Thus, the medullary 

 vascular tissues had the same reversed orientation as 

 in the corresponding anomalous formations sometimes 

 found in Lyginodendron oldhamium. In Cycadoxylon 

 robustum the anomalous vascular zone is at some places 

 double, the second band of medullary wood and bast 

 having the same orientation as the first (see Fig. 178, 



x \pn 



The structure as a whole leaves little doubt that 

 the fossil Cycadoxylon robustum really possessed some 

 affinity with Lyginodendron, but, as there is no proof 

 that the primary structure of the two was similar, we no 

 longer place them in the same family, but have trans- 



