CORDAITALES 



171 



Elkins and Wieland'*'^ figured distinct growth-rings in Calli- 

 xylon oweni, from the upper Devonian of Indiana. While these rings 

 are too distinct to be overlooked, there might be rings a little less 

 distinct, which might escape notice, just as growth-rings in some of 

 the woody monocotyls escaped for so long. We suspect that there 



Fig. 181. — Mesoxylon mtdtirame: transverse section of stem, showing large pith, 

 scanty wood, large cortex, and 13 of the 16 leaf traces which may be found in a section. 

 The leaf traces are numbered from within outward. — After Scott.s-'-' 



are seasonal differences in some of the big cacti. Some of the beauti- 

 fully silicified pieces of trunks of Callixylon oweni were more than 5 

 meters in length and nearly a meter in diameter, and the entire trees 

 may have been from 30 to 40 meters in height. 



Cordaites, the dominant genus, has an endarch siphonostele, with 

 spirally marked tracheids bordering on the pith. The first formed 

 protoxylem cells are small and are followed by cells of greater diam- 



