68 AN INTRODUCTION TO THE STUDY OF FOSSILS 



In leaf structure and in certain primitive features of the 

 seeds, Cordaites shows relationship to the Cycadofilicales, in 

 stem and root anatomy it inclines toward the conifers (Fig. 23), 

 while its male catkins resemble those of the Ginkgo. From 

 these and other characters it is evident that the Cordaitales 



Fig. 23. — Photomicrograph of a radial section of one of the most ancient of known 

 woods, Callixylon {Cordaites) oweni Wieland, from the Upper Devonian Black 

 Shale of Indiana. The preservation of this wood is so perfect that the thin sections 

 may be freely studied at 600 diameters. At the 100 diameters shown here, the 

 bordered pits of the radial surfaces of the wood cells or tracheids are seen to have a 

 radially grouped arrangement ; this radial grouping is rare in the I'aleozoic. (After 

 Wieland.) 



were a highly generalized group of seed-fern afifinity near the 

 base of all the later gymnosperm lines, originating probably in 



