. Morphology of gymnosperms. Gymnosperms; Plant morphology. CORDAITALES I6S. 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 (Cycadofilicales); but Scott (25) has found occasi
. Morphology of gymnosperms. Gymnosperms; Plant morphology. CORDAITALES I6S. 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 (Cycadofilicales); 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. 196.—Cordaites Sutclijfli: transverse section of stem; m, pith; x, xylem; p, phloe
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