266 LYCOPODIALES [CH- 



base areas. It is not impossible that in the surface-features of 

 Omphalophloios we have both leaf and rootlet scars represented. 



General considerations. 



The solid xylem core characteristic of the stele of some 

 species of Palaeozoic Lycopodiales (e.g. Lepidodendron esnostense 

 and L. rhodumnense) may probably, as Tansley and Chick' point 

 out, be regarded as the lineal descendant of a primitive axial 

 strand of water-conducting elements. In the course of evolu- 

 tion the centre of the tracheal column became partially converted 

 into parenchymatous tissue, as in Lepidodendron vasculare. 

 The arrangement of the short cells in regular vertical series is 

 reminiscent of an early stage in the development of tracheae : 

 instead of forming tubular conducting elements the central 

 part of the stelar meristem acquired the short-celled form ; 

 some of the cells became lignified as isodiametric storage tracheae 

 while others persisted as thin-walled parenchyma. 



The production of secondary xylem and an increase in the 

 girth of the whole stem led to reduction in the amount of centri- 

 petally developed conducting channels. Some of these assumed a 

 new r61e and a shape in harmony with their functions. A later 

 stage is represented by a further encroachment of the central 

 parenchyma on the cylinder of centripetal xylem, as seen in 

 Lepidodendron Harcourtii and other species. The next stage 

 is afforded by ribless species of Sigillaria in which the 

 primary xylem is broken up into separate conducting strands. 

 As Kidston'' reminds us, it is in the geologically more recent 

 species of Sigillaria, such as *Si. Brardi, which persist into the 

 Permian era, that this more extreme case of reduction occurs. 

 The older genus Lepidodendron seems to have retained to 

 the last the complete cylinder of primary xylem. In the stele 

 of Stigmaria, the rhizome of Sigillaria and of Lepidodendron, 

 reduction of the centripetal xylem has passed beyond the stage 

 represented by the broken cylinder of the ribless Sigillarias. 

 With the exception of the examples described by Kenault' and 

 by Weiss*, Stigmaria is characterised by little or no centripetal 



1 Tansley and Chick (01) p. 36. ^ Kidston (05) p. 547. 



» Renault (96) A. * Weiss, P. E. (08). 



