X] PETZHOLDT AND UNGER. 299 



correlate the patches in which no tissue has been preserved 

 with the broad medullary rays, which separated the wedcre- 

 shaped groups of xylem elements; the latter being more 

 resistant were converted into a black coaly substance, while the 

 cells of the medullary rays left little or no trace in the 

 sandstone matrix. The thin black line, which forms the limit 



Fig. 70. Transverse section of a Calamite stem, showing carbonised remnants 

 of secondary wood. From a specimen (no. 40934), presented to the British 

 Museum by Dr Petzholdt from Plauenscher Grund, Dresden, h nat. size. 



of the drawing in fig. 70, external to the carbonised wood, no 

 doubt marks the limit of the cortex, and the appendage indicated 

 in the lower part of the figure may possibly be an adventitious 

 root. It is interesting to note that linger^ in 1844 expressed 

 the opinion, which we now know to be correct, that the coaly 

 mass in the specimens described by Petzholdt represented the 

 wood, and that there was no proof of the existence of canals in 

 the cortex as Petzholdt believed. 



Turning to Brongniart's later work^ we find an importiint 



> Unger (44). '* Brongniart (49), p. 49. 



