CYCADOFILICALES 



17 



group as a whole shows the complete or nearly complete elimina- 

 tion of centripetal xylem in the primary strands. The other notable 

 difference is the development of regular zones of secondary xylem 

 between the primary cylinder and the pith (fig. 14). These centripe- 

 tally developed zones have reversed orientation, and two or more 

 successive ones may appear. It will be recalled that this feature of 



Fig. 14. — Cycadoxylon robustum: part of transverse section of stele showing 

 inner portion of normal wood, with two anomalous zones of wood and bast; sc', nor- 

 mal secondary wood; x3, phi, iSrst inverted band of xylem and phloem; x*, ph*, 

 second band; p, pith; X 12.— After Scott (87). 



Cycadoxylon appears in certain specimens of Lyginodendron Old- 

 hamium, the two genera evidently being closely related. 



Medullosa. —The stems of this important type are found through- 

 out the Carboniferous and Permian deposits, culminating in the 

 latter and then disappearing, and it seems to have been one of the 

 most abundant of the Cycadofilicales. Some of the species were 

 large trees of fernlike habit, and others had long slender stems. In 

 every case the stems were clothed by the sheathing bases of leaves. 



