CYCADOXYLEAE 483 



the great plexus of Palaeozoic forms lying on the con- 

 fines of Ferns and Gymnosperms. 



CYCADOXYLEAE 



We have now to consider a series of fossil forms of 

 which our knowledge is still imperfect, but which are of 

 great interest, as they appear to connect the Lygino- 

 dendreae with a more distinctly Cycadean type of 

 structure. 



A portion of a large stem, Cycadoxylon robustum, 

 regarded by Williamson as belonging to Lyginodendron, 

 and subsequently named L. robustum by Professor 

 Seward, was discovered by Nield more than thirty 

 years ago, in the Lower Coal-measures of the Oldham 

 district. Although the specimens are fragmentary, 

 they show some points of considerable interest. No 

 cortical tissues are preserved, and even the wood is 

 probably incomplete, but the remaining, central part of 

 the stem has a diameter of about 14 cm., so, in the 

 natural condition, the plant must have reached the 

 dimensions of a small tree. The secondary wood, 

 which, in its present state, has a thickness of nearly 

 6 cm., resembles that of Lyginodendron oldhamium 

 (Fig. 178, x 2 ). The medullary rays are broad, and of 

 great height, so that the whole character of the wood 

 was very parenchymatous, like that of the recent 

 Cycads. The tracheides, which have a somewhat 

 sinuous course, bear, multiseriate bordered pits on their 

 radial walls. 



The pith, which is almost 3 cm. in diameter, is 

 imperfectly preserved, but there are distinct remains of 



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