CORDAITALES 



165 



VASCULAR ANATOMY 



A transverse section of the stem of Cordaitales shows a combina- 

 tion of cycad and conifer characters (figs. 196, 197). The relatively 

 large pith, sometimes reaching nearly 10 cm. in diameter, is a cycadean 

 feature, and is peculiar in being discoid; while the thick vascular 

 cylinder is as distinctly coniferous. The cylinder is collateral endarch 

 in the forms originally described, in contrast with the mesarch con- 

 dition of such a type as Lyginodendron (Cycadolilicales) ; but Scott 

 (25) has found occasional 

 centripetal xylem in certain 

 stems. There is no dis- 

 tinction evident in trans- 

 verse section between 

 primary and secondary 

 wood, but radial sections 

 show a more extensive 

 transition region than usual, 

 from the narrow spiral 

 vessels of the protoxylem 

 to the pitted tracheids that 

 form the bulk of the second- 

 ary wood. 



The secondary wood is really indistinguishable from that of Arau- 

 caria (20, 21), the large proportion of paleozoic wood referred to 

 Araucarioxylon and Dadoxylon having been found to be that of Cor- 

 daitales. Such wood is recognized by the fact that the bordered pits, 

 restricted to the radial walls, are densely crowded in two or more 

 rows (multiseriate), becoming hexagonal in outline. While in general 

 "annual rings" may not be recognizable, in certain specimens the 

 occurrence of such rings is strongly suggested (fig. 197). The medul- 

 lary rays are narrow, usually one or two cells thick, or the principal 

 rays may become three cells thick. 



The leaf traces are usually, perhaps always, double, a feature 

 characteristic of the Cycadofilicales, and plainly seen in Lyginoden- 

 dron, in which form, however, the two strands unite before joining 

 the central cylinder (fig. 198). The collateral endarch leaf trace 

 becomes collateral mesarch in the leaves, a well-known cycadean 



Fig. iq6. — Cordaites Sittcliffii: 

 section of stem; m, pith; x, xylem; 

 /, double leaf traces; c, cortex; natural size. 

 From photograph of section made by Lomax. 



transverse 

 p, phloem; 



