XXXIl] 



VOLKELIA 



209 



black patches in fig. 461, A, in all probability represents the 

 primary part of each stele to which the rest of the tissue has been 

 added by the cambium. A striking feature of the secondary 

 xylem is the absence of medullary rays : the tracheids resemble 

 those of Cladoxylon and Protopitys in the transverse elongation 

 of the pits (fig. 461, D) which form either a single row or several 

 irregularly distributed rows. The primary xylem consists in the 

 peripheral region of tracheids with very narrow scalariform 

 pitting which at first sight suggest close spiral bands (fig. 461, C), 



FIG. 461. Volkelia refracta. A. Transverse section of a specimen in the Breslau 

 Museum. B. Portion of a stele; a, the crushed xylem on the outer side; 

 b, the inner side. C. Longitudinal section showing the median protoxylem. 

 D. Tracheid from the secondary xylem. (After Solms-Laubach.) 



while the inner tracheids are either annular or reticulate and 

 associated with elongated parenchyma. The imperfectly preserved 

 ground-tissue appears to consist of homogeneous parenchyma 

 with radially disposed bands of stereome in the outer cortex. 



Our knowledge of Volkelia, though far from complete, justifies 

 its generic separation from Cladoxylon from which it differs in 

 the lack of medullary rays and in the structure of the primary 

 portion of each stele. In the form and arrangement of the pits 

 in the secondary tracheids Volkelia differs from Medullosa and 



s. m 



14 



