BENNETTITEAE 565 



have nothing to do with the vascular system, and 

 consist merely of bands of internal periderm, an 

 abnormal condition which is often met with in the 

 stems of recent Cycads. 



We see, then, that the structure of the Bennettiteae, 

 so far as it is at present known, was a simple monostelic 

 one, resembling in its main features that of the less 

 complex Cycads now living, but differing from any exist- 

 ing Cycads in the simpler course of the bundles supplying 

 the leaves. In this latter point, the Bennettitean stem 

 has been compared by Solms-Laubach with the peduncle 

 of recent Cycads, an organ which in other points also 

 appears to show a more primitive anatomy than that of 

 the vegetative stem. 1 



In Cycadeoidea J enneyana, one of the Dakota trunks, 

 there is some evidence for the presence of numerous 

 successive zones of wood and bast, as in the recent 

 Cycas and Macrozamia, but the point is not yet settled." 



In B. Gibsonianus, the vascular bundles of the leaf- 

 bases are beautifully preserved, and show essentially 

 the same structure as the foliar bundles of the recent 

 Cycadaceae. They are of the collateral mesarch type, 

 whereas those of the stem are endarch ; the centripetal 

 wood forms a mass of large elements, with a band of 

 the radially arranged tracheae of the centrifugal wood 

 on the outside. The same structure is found in various 

 American species, but in Cycadeoidea micromyela, a 

 French species probably of Liassic age, centripetal 

 wood appears to be entirely absent. 3 The detailed 



1 See Chapter X. p. 366. 



2 Wieland, American Fossil Cycads, p. 79. 



2 Lignier, "Vegetaux foss. de Normandie, III. Cycadeoidea micro- 

 myela," Mem. Soc. Linn, de Normandie, t. xx. 1901. 



