208 CLADOXYLEAE [CH. 



Volkelia. Solms-Laubach. 



Volkelia refracta (Goeppert). The generic name Volkelia 1 was 

 proposed by Solms-Laubach 2 as a substitute for Sphenopteris 3 in 

 the case of some petrified stems or petioles associated with frag- 

 mentary impressions of fronds from Lower Carboniferous rocks in 

 Silesia. Both leaf -impressions and petrifactions were included 

 in the genus Sphenopteris : Solms, while retaining Goeppert's 

 designation for the leaf fragments, proposed a new generic name 

 for the petrifactions on the ground that there is insufficient 

 evidence of their connexion with the leaves. The short account 

 of Goeppert's petrified specimen given by Graf Solms-Laubach 

 in his 'Fossil Botany 4 ' is supplemented by a fuller description in 

 a later paper. The fragments of highly compound fronds are 

 characterised by very small filiform ultimate segments, but the 

 specimens are too imperfect to afford a clear idea of the habit of 

 the leaf. The 'stem' bears a close superficial resemblance to 

 that described by linger as Cladoxylon dubium (fig. 459, C, D) 

 and was regarded by him as an example of that species : it contains 

 several radially placed steles represented by fairly well-preserved 

 xylem, but no phloem has been recognised. The steles vary in 

 size and shape: five reach almost to the centre (fig. 461, A) and 

 smaller xylem groups occupy a peripheral position. Each stele 

 is excentric in structure and consists of (i) an outer zone of 

 secondary tracheids of horse-shoe form in transverse section, but 

 the apparent gap in the secondary xylem on the outer edge of 

 each stele is due to the crushing of the tracheal tissue and to its 

 smaller breadth in the distal part of each group ; this is shown 

 in fig. 461, B, where the apparent gap is seen to be occupied by 

 distorted and crushed tracheids, a, identical with those which 

 form the rest of the outer zone (fig. 461, B, 6); (ii) a zone of 

 tracheal tissue continuous with and originally identical in appear- 

 ance except that the elements are rather narrower with the 

 outer secondary xylem; (iii) an excentrically situated island 

 composed of tracheids enclosing a small central area occupied 

 by thin-walled parenchyma. This third region, represented by 



1 After the late Herr Volkel of Neurode. 



2 Solms-Laubach (10), PL m. figs. 1 4. 



3 Goeppert (52) PL xn. 4 Solms-Laubach (91) A. p. 164. 



