CYCADOFILICALES 13 



solid and concentric. Near the periphery of the included xylem there 

 occur small strands of ])rotoxylem, which determine the mesarch 

 character of the bundle. Thus far, the structure is distincdy fernlike; 

 but in addition there is a narrow zone of secondary wood about the 

 primary xylem cylinder. The elements of this secondary wood are ra- 

 dially arranged, with numerous (multiseriate) bordered pits, and the 

 zone is traversed by pith rays. The large leaf traces are collateral at 

 their junction with the vascular cylinder, but they become concentric 

 before entering the petiole. Although the leaf trace is single in H. 

 Grievii, in other species of the genus it becomes double. 



Lyginodendron.' — One of the most common stems preserved 

 in the British Coal-measures is Lyginodendron Oldhamium, fully 

 described by Williamson and Scott (18), and possibly including 

 a group of species. The stems are long, slender (3 mm. to 4 cm. in 

 diameter), and branching. The vascular axis is a siphonostele, 

 without internal phloem (fig. 11), as in Osmunda (fig. 4), the cylinder 

 inclosing a large pith. A variable number (5-9) of isolated primary 

 xylem strands occur next to the pith, being distinctly mesarch, with 

 the larger part of the metaxylem centripetal (fig. 12). While most of 

 the tracheids of the metaxylem have bordered pits, those next to 

 the spiral elements are usually scalariform (87). The main part of 

 the vascular cylinder, however, consists of a broad zone of secondary 

 wood, whose tracheids, with multiseriate bordered pits, occur in regular 

 radial arrangement, and which is traversed by pith rays. Very 

 evident gaps in the vascular cylinder are associated with the leaf 

 traces, so that the siphonostele is phyllosiphonic and is thus definitely 

 related to the siphonostele of Filicales. The leaf traces near the cen- 

 tral cylinder are single and collateral, but soon they fork and gradually 

 become concentric, so that in the petiole (once called Rachiopteris 

 aspera) a pair of concentric bundles occurs. The association of col- 

 lateral bundles in the stem with concentric bundles in the leaves is 

 a distinct Osmunda character, and is met also among the cycads. 

 Occasionally in L. Oldhamium there is a development of vascular 

 bundles with reversed orientation (xylem next to that of the primary 

 bundle) in the pith. Outside the cambium there is also evident a 

 zone of phloem, broken up by the pith rays (fig. 13). 



I The generic name somelimcs used is Lyginopteris Potonie. 



