XXXIIl] DADOXYLON 251 



Coal Measures and the Lower Carboniferous of Scotland which 

 he referred to Dadoxylon but without any specific name. These 

 include the Coalbrookdale stem in which he had previously 

 demonstrated the connexion between Artisia and Dadoxylon. 

 The structure of the xylem is like that in D. Brandlingii and the 

 specimens may belong to that species. The most interesting 

 fact recorded by Williamson is the occurrence of double leaf- 

 traces, a feature which led him to suspect a remote generic affinity 

 to Ginkgo. This double trace may be an important diagnostic 

 feature but unfortunately the majority of descriptions of species 

 of Dadoxylon throw no light on the character of the foliar bundles. 



Thomson and Allin 1 have recently pointed out that a double 

 leaf-trace occurs in a stem from the Permian of Kansas described 

 by Penhallow 2 as Pityoxylon chasense and referred to that genus 

 because of the supposed occurrence of resin-canals in some of 

 the medullary rays : the canals are apparently leaf -traces traversing 

 broad rays in the secondary wood. 



The primary xylem of Cordaites is in direct continuity with 

 the secondary tracheids and does not form mesarch strands as 

 in Mesoxylon. The pith is usually discoid. The pitting on the 

 tracheids is a character of special importance: while it is true 

 to say that as a rule the number of pits on the radial walls of a 

 single tracheid is larger than in the Araucarineae, this is not always 

 the case. In Araucaria there are occasionally as many as five 

 rows of alternate polygonal pits (fig. 691, A) and in some Palaeo- 

 zoic Dadoxylons there are only one 3 or two rows. The very 

 broad zone of transitional elements at the inner edge of the xylem- 

 cylinder is a characteristic feature shared by the Araucarineae 4 ; 

 the spiral protoxylem-tracheids are succeeded by scalariform 

 elements and these, by the gradual anastomosing of the transverse 

 bars, pass into tracheids with multiseriate pitting. In this broad 

 zone we probably have a primitive feature, an epitome in a 

 single stem of the course of development of multiseriate from 

 scalariform pitting. In some Palaeozoic species with wood of 

 the pycnoxylic type and agreeing generally with typical Cordaites 



1 Thomson and Allin (12); Thomson (13) p. 14. 



3 Penhallow (00) p. 76. 3 White (08) B. p. 583. 



4 Seward and Ford (06) B. p. 341. 



