BENNETTITALES 71 



arrangement of the separate steles of Medullosa, accompanied by 

 the elimination of internal vascular tissues out to the protoxylem. 

 In any event, the type of cylinder displayed by Lyginodendron, which 

 according to this view was formed in the same way, would be the 

 immediate progenitor of the cycadean cylinder, the only change 

 necessary being a transition from the mesarch to the endarch condi- 

 tion, a transition completely or at least very nearly attained by Cyca- 

 doxylon, and seen in the seedling of Ceratozamia. It was in this con- 

 nection that WoRSDELL (23) extended the very useful group named 

 Cycadophyta, suggested by Nathorst, to include the Cycadofilicales, 

 Bennettitales, and Cycadales, Nathorst's " cycadophy tes " including 

 the mesozoic plexus of cycadean forms and the living cycads. 



The leaf traces, at first endarch near the vascular cylinder, soon 

 become mesarch, as in cycads, and this structure characterizes the 

 foliar bundles. It is altogether probable that the mesarch condition 

 passes into the exarch in the ultimate foliar branches of the vascular 

 system, for this is evident among cycads as the bundles become sim- 

 pler; but the prevaihng mesarch type of leaf trace and leaf bundle 

 is an important reminder of fern connections. It is in connection 

 with the leaf traces, however, that a striking contrast between Bennet- 

 titales and Cycadales appears. In the latter group, several vascular 

 strands leave the cylinder and enter a single leaf, two of them follow- 

 ing a more or less devious route through the cortex, resulting in the 

 so-called "girdles." Among Bennettitales a single bundle leaves 

 the vascular cylinder, and in passing directly through the cortex 

 toward its leaf breaks up into numerous smaller bundles that enter 

 the leaf. The "girdle" habit will be discussed under Cycadales, 

 but at this point it is important to note that the direct course of the 

 leaf trace, characteristic of the mature stems of Bennettitales, is found 

 in the vascular connections of the sporophylls and cotyledons of 

 Cycadales. 



THE LEAF 



The leaves, arising in a crown from the top of the tuberous or colum- 

 nar trunks, closely resemble those of Dioon, Zamia, etc., both in form 

 and in structure. In fact, the abundant detached leaves found in 

 certain mesozoic deposits have suggested the establishment of such 

 leaf genera as Zamites, Dioonites, and Cycadites; while Otozamites and 



