SIGILLARIA 217 



medullary rays of very variable height. The interfas- 

 cicular rays seem to have been only slightly broader 

 than those opposite the primary bundles. The second- 

 ary tracheides, as in the Lepidodendreae, are pitted on 

 their tangential as well as on their radial surfaces ; the 

 smallest elements lie towards the interior, adjacent to 

 the protoxylem of the primary strands. 



The leaf-trace bundles, so far #s their xylem is con- 

 cerned, were given off from the outer, concave part of 

 the primary strands, and passed obliquely through the 

 secondary wood ; the preservation only allows of their 

 being traced for a short distance beyond the woody 

 zone. M. Renault finds that these leaf-trace bundles 

 are " diploxylic," each consisting of an inner centripetal 

 primary strand, and an outer centrifugal secondary 

 portion continuous with the secondary wood of the 

 stele. This character was long regarded as peculiar 

 to Sigillaria, as distinguished from Lepidodendron, but 

 the distinction no longer holds good, for it is not 

 constant in Sigillaria, while, as already mentioned in 

 Chapter V., Prof. Seward and Mr. Hill found typical 

 " diploxylic " leaf-traces in a fine specimen of Lepido- 

 dendron Wunschianum, a plant which may belong more 

 properly to Lepidophloios, but is, at any rate, not a 

 Sigillaria. This structure is shown in Fig. 57, p. 

 142. 



The outer cortex, which is alone preserved, consists 

 of two zones ; the more internal is composed of uniform, 

 radially arranged tissue, and no doubt represents a 

 secondary formation, of the nature of periderm, such as 

 we so constantly find in Lepidodendron. The outer 

 zone", bearing the leaf-cushions, is simply the primary 



