XXXIIl] CORDAITES 261 



and the Cordaitales, but nearer to the former. This conclusion 

 is, however, based on insufficient evidence, as nothing is known 

 of the reproductive organs. 



Roots. 



In 1871 WiUiamsoni gave an account of a petrified plant from 

 the Lancashire Coal Measures which he named Dictyoxylon radicans, 

 but he afterwards came to the conclusion that the specimens so 

 named were portions of the subterranean axis of some other 

 plant, possibly Aster o-phyllites, and proposed a new generic term 

 Amyelon^. In 1874 he brought forward fresh evidence in support 

 of connecting Amyelon radicans with Asterophyllites or Spheno- 

 phyllum, genera which WiUiamson believed to be very closely 

 related. It has since been recognised that Amyelon is the root 

 of Cordaites or of some closely aUied member of the Cordaitales. 

 Our knowledge of Cordaitean roots is based chiefly on the work 

 of WilHamson and Renault^, and more recently Osborne* has 

 added new facts of considerable interest. In the larger roots 

 the primary xylem may be diarch or there may be as many as 

 four or five protoxylem groups (fig. 477). The primary tracheids 

 are spiral or scalariform and the space, s, separating them from 

 the surrounding secondary xylem seen in fig. 477, B, was no 

 doubt originally occupied by conjunctive parenchyma. The 

 secondary wood is composed of tracheids, with contiguous bordered 

 pits identical with those in the xylem of the stem, and narrow 

 medullary rays. The section, 4 mm. in diameter, represented in 

 fig. 477, A, shows a tetrarch primary xylem strand enclosed by 

 secondary wood composed of rather thin-walled elements succeeded 

 by a zone of phloem including some secretory sacs, and beyond 

 this is a cylinder of periderm, p. In a section of a root figured 

 by Renault from Autun the periderm is separated from the stele 

 by a broad band of parenchyma which appears to be cortical, 

 but in the British specimens the deep-seated origin of the periderm 

 is clearly shown : Osborne states that it arises in a layer imme- 

 diately outside the endodermis. In one of the specimens figured 



1 Williamson (722). 2 Williamson (74) A. p. 67. 



» Renault (79) B. p. 294, PI. xv. figs. 13—17. See also Scott (09) B. p. 531, 

 fig. 191. 



* Osborne (09). 



