XXVIIl] 



ANATOMICAL TBATUEES 



31 



subdivision give rise to the numerous collateral strands of the rachis. 

 A leaf-trace in its passage to the leaf is Uke that of a Conifer in 

 having the protoxylem on its inner edge, whereas in the petiole 

 and elsewhere in the frond it is characterised by an arrangement 

 of the xylem that has usually been described as mesarch. A typical 

 vascular bundle from a cycadean frond is seen in fig. 399, C ; by 

 far the greater part of the xylem is centripetal, the centrifugal 

 xylem being confined to an arc of scattered tracheids or a small 

 strand separated by a few parenchymatous cells from the 

 protoxylem. 



Fig. 399. Sketches iEustrating the changes in the structure of Cycadean vascular 

 bundles in their course from stem to leaf : cp, cj, centripetal and centrifugal 

 xylem; p, phloem; px, protoxylem. (After Marsh.) 



As considerable stress has been laid on the anatomical features 

 of the cycadean foliar bundles in discussions on the affinities and 

 phylogeny of certain Palaeozoic genera, it is important to consider 

 the facts more closely^. French anatomists described the cycadean 

 bundle as diploxylic on the ground that the centripetal and 

 centrifugal xylems are distinctly different things, the centripetal 

 xylem being primary — a relic of a former organisation — and 

 the centrifugal xylem secondary and homologous with the normal 



1 Carano (04); le Goo (14); Marsh (14). 



