510 STUDIES IN FOSSIL BOTANY 



were justified in referring them to the genus Poroxylon. 

 The rootlets, in particular, are perfectly preserved, and 

 their anatomy has been fully investigated. The 

 structure is, as a rule, diarch ; in a few cases a 

 tetrarch cylinder has been observed. The anatomy 

 of the roots and rootlets is, in all respects, such as 

 characterises Gymnospermous roots at the present 

 day. In the usual case of diarch organisation, the 

 secondary wood and bast form two large masses, one 

 on either side of the primary xylem-plate, while a 

 broad ray corresponds to each of the two protoxylem- 

 groups at the ends of the plate. In the details of 

 wood and phloem the roots agree precisely with the 

 stem. The whole cortex was thrown off at an early 

 stage by the formation of pericyclic periderm. Thus 

 the outer surface of the root was formed by a layer 

 of cork, just as in the roots of Medullosa anglica, 

 described in the last chapter, or in those of recent 

 Gymnosperms. 



MM. Bertrand and Renault were able to observe 

 the mode of insertion of the rootlets on the main root, 

 and to determine that the plane of the diarch xylem- 

 plate coincided in the two organs. This is a point of 

 interest, for, as is well known from the researches of 

 Professor van Tieghem and others, this arrangement 

 is characteristic of the roots of Gymnosperms and 

 Phanerogams generally, while, in the case of the 

 diarch roots of Vascular Cryptogams, the plane of 

 the xylem-plate of the rootlet lies at right angles to 

 that of the parent root. 



Our summary has so far been based on the species 

 discovered by Bertrand and Renault ; the anatomy 



