306 MORPHOLOGY OF ANGIOSPERMS 



conservatism in the structure and course of the bundles in the 

 cones of Equisetum. 



Leaf -traces are likewise extremely conservative in structure, 

 for where cenogenetic modifications are present in the ordinary 

 cauline strands, the primitive type of fibrovascular bundle is 

 often retained in the leaf-traces, as well as in the reproductive 

 axis and in the seedling. Ancestral features are retained more- 

 over in the leaf-traces, especially those of the cotyledons, long 

 after they have disappeared elsewhere. Hence it is assumed 

 that the mesarch structure of the foliar bundles of the Cycads 

 supplies a further argument for their derivation from ances- 

 tors like Lyginodendron. 



The fact that cycadean leaf-traces are often concentric in 

 the lower part of their course has been used as an argument 

 by Worsdell 16 in favor of the hypothesis that the cauline bun- 

 dles of the ancestors of the Cycads were originally concentric. 

 This argument seems to have the same force as the similar argu- 

 ment in the case of the mesarch collateral bundles ; and the fact 

 that concentric strands are comparatively rarely present in the 

 living Cycads is probably due to the concentric condition being 

 further in the phylogenetic background. The structure of the 

 conservative trachea ry strands of the leaves and peduncles of 

 the Cycads would seem to point to a more immediate ancestry 

 with the general organization of Lyginodendron, derived in 

 the remoter past from forms like Medullosa. 



BENNETTITALES 



The external vegetative features and the reproductive organs 

 of this interesting group have already been dealt with in the 

 companion volume treating of Gymnosperms (p. 142). Al- 

 though their reproductive organs differ very strikingly from 

 those of any living Cycads, the fibrovascular anatomy of the 

 Bennettitales is strikingly cycadean (Scott 15 ). They possessed 

 a large cycadean pith penetrated by mucilage canals and bound- 

 ed by a thin fibrovascular ring. The continuity of the fibro- 

 vascular zone was broken at intervals opposite the large leaf- 

 traces, which separated in the cortex into arcs of bundles pass- 

 ing directly into the leaves. The direct course of the foliar 

 bundles is to be compared with that present in the cones only 

 of living Cycads. This condition is probably to be regarded 



