MEGALOXYLEAE 475 



side of the trace, adjacent to the secondary wood, are 

 several groups of spiral elements (protoxylem), showing 

 that the xylem of the bundles of Megaloxylon was 

 centripetally developed, or exarch, as distinguished from 

 the mesarch strands of Heterangium or Lyginodendron. 



As the leaf-trace is followed downwards into the 

 stele, it is found to spread out laterally, in a fan-like 

 form, its tracheides becoming shorter and more irregular, 

 until they gradually become merged in the metaxylem. 

 The leaf-trace passes through about four internodes, at 

 the periphery of the primary wood, before losing its 

 individuality. 



The secondary wood requires little description. It 

 is of the Lyginodendron type, and is characterised by the 

 author as practically identical with that of Cycadoxylon 

 robustum, described below (cf. Fig. 178, p. 483). 



The large leaf-traces can be followed, as they pass 

 obliquely outwards through the secondary wood, en- 

 closed by a secondary zone of their own, continuous on 

 the exterior with that of the stem itself. Professor 

 Seward infers that the trace was probably concentric in 

 structure. 



The most essential points of difference from Heter- 

 angium, which are sufficient to place Megaloxylon 

 provisionally in a distinct family, are, first, the exarch 

 structure of the primary wood, and secondly, the peculiar 

 form of the short tracheides of the metaxylem, which 

 may probably have served for the storage rather than 

 for the conduction of water. 



The former character is shared by Sutcliffia among 

 the Medulloseae, with which there is otherwise nothing to 

 connect Megaloxylon. The short tracheides of the central 



