C. J. Hylander — Mid-Devonian Callixylon. 319 



In the transverse section, the wood rays appear unusu- 

 ally wide, compared with the tracheids. They are also 

 of considerable depth, varying from ten to fully twenty 

 cells deep. In only one instance, in the radial longi- 

 tudinal section, could the end of a ray cell be distin- 

 guished, and that is shown in fig. 1. The ray cells appear 

 to be about the same size as those of Callixylox Oweni. 

 The cell-width of the rays is not visible in the imperfect 

 tangential section, but the rays must be in part two-cells 

 wide, as in C. Oweni. "Wieland (6, p. 123) notes that in 

 Araucarioxylon stems the wood rays are never more than 

 two cells wide, although rays three and four cells wide 

 are not infrequent in the older Cordaites. 



The growth ring (fig. 5), as also in C. Oweni (fig. 3), is 

 a significant feature of this wood, which may have been 

 widespread in the mid-Devonian. Such growth rings are 

 not present in the Russian Callixylon; nevertheless it is 

 likely that the feature is more or less general in Cal- 

 lixylon, and it occurs in various other typical American 

 Cordaites. Miss Goldring finds it in a Carboniferous 

 Cordaite as far south as southern Texas (4). These 

 ancient rings are not thought to indicate as sharp a 

 seasonal change as the rings in Dicotyledonous plants. 

 But on the other hand, little attention has been given to 

 the fact that in old and simpler types of wood, growth 

 rings do not become a feature of the wood structure. 

 The simpler type of growth ring occurs in both Mesozoic 

 and recent Cycads, and Chamberlain (1) has now 

 observed it in a Monocotyl. Accentuation of growth ring- 

 is mainly correlated with the more marked tracheidal and 

 ray differention of mid to later Mesozoic time. 



Explanation of Figures 1-6. (All figures enlarged 100.) 

 Callixylon Marshii, sp. no v. 



Fig. 1. Eadial long, section showing approximate height of wood ray. 



Fig. 2. Eadial long, section showing the aligned grouping of the radial 

 pits. 



Fig. 5. Transverse section showing growth ring and average appearance 

 of tracheids in one of the less compressed areas. 



Fig. 6. Transverse section showing typical wood and conservation. 



Callixylon Oweni, Elkins & Wieland. 



Fig. 3. Transverse section, showing growth ring and larger and well 

 preserved tracheids. [Tracheid ends probably aligned.] 



Fig. 4. Eadial long, section, showing radial grouping of pits, for com- 

 parison with fig. 2. 



