390 



BENNETTITALES 



[CH. 



into several collateral strands and these form a simple pattern 

 as seen in a tangential section of the leaf -bases (fig. 517, A, C), 

 the central bundles on the upper side forming a U-shaped involu- 

 tion. There is no indication of any girdUng of the foUar bundles. 

 The arrangement of the petiolar bundles is simpler and more 

 Fern-hke than in the majority of existing Cycads. Each vascular 

 bundle consists in the petiole-base mainly of centrifugal secondary 

 xylem and medullary-ray tissue (fig. 519, B) ; a group of centripetal 

 tracheids is usually recognisable internal to the protoxylem : the 



A B 



Fig. 519. Cycadeoidea Oibsoniana. A, transverse section of the vascular cylinder 

 of stem, with leaf -trace. It ; c, cambium, px, protoxylem. B, vascular bundle 

 of petiole. (British Museum.) 



structure is of the mesarch type with a variable amount of cen- 

 tripetal xylem, and agrees generally with that in recent Cycads. 



The comparatively narrow cortex is composed of parenchyma 

 with scattered secretory sacs (fig. 518, B, s), and occasionally an 

 elliptical vascular strand (fig. 511) is present which is probably 

 cauUne as are the similar strands in Cycas. The persistent petiole- 

 bases are covered with ramental scales which form a dense inter- 

 fohar packing : these frequently consist of a single layer of fairly 

 thick-walled cells or, as seen in transverse section, of spindle- 

 Hke scales one-cell thick at the edges and 2 — 3 cells broad in the 

 middle (fig. 517, B). 



