XXXIIl] POROXYLON 217 



bundles is produced, the concave side facing the upper surface 

 of the thick lamina (fig. 464, A). Further reference is made to 

 the structure of the leaves in the description of Poroxylon 

 stephanense. The specimens of roots so far described are charac- 

 terised by a diarch plate of primary xylem and two masses of 

 secondary vascular tissue separated by two medullary rays 

 opposite the protoxylems. Bertrand mentions the occurrence of 

 roots of P. stephanense with more than two protoxylem strands. 

 The phellogen was produced in the pericycle as in the roots of 

 recent Gymnosperms. It is suggested by Lignier 1 that some 

 silicified rootlets from Grand' Croix (Loire) described by him as 

 Radiculites reticulatus and at first compared with roots of Sequoia 

 may belong to some Cordaitalean plant, possibly Poroxylon. 



Poroxylon Edwardsii Renault. 



This species 2 affords a good illustration of the generic characters 

 already summarised. The strap-like leaves are fleshy and the 

 occasionally forked, parallel or slightly divergent, veins are 

 embedded in a homogeneous mesophyll with hypodermal strands 

 of mechanical tissue. The pith consists of parenchyma in vertical 

 series with scattered secretory sacs and differs from that of 

 Cordaites and Mesoxylon in the absence of transverse discs. There 

 are 13 primary-xylem strands close to the inner edge of the second- 

 ary wood: the centripetal tracheids are scalariform or have 

 multiseriate pitting like that in the secondary xylem. The 

 structure of the leaf -traces is clearly shown in fig. 464 : the double 

 trace seen in fig. 464, C, has two protoxylem-strands accompanied 

 by some parenchyma, and these are almost enveloped by the 

 metaxylem tracheids which abut on the secondary wood. At 

 this stage in its course, that is just before bending outwards, the 

 centripetal xylem reaches its maximum development and the 

 trace forms a prominent and broad twin-strand in striking con- 

 trast to the two narrower and tangentially extended strands 

 shown in fig. 464, E, D. Each of these strands with a single 

 protoxylem-group would at a higher level assume the broader 

 and more compact form and contain two protoxylems as in fig. 

 464, C. The tracheids of the secondary xylem have 4 7 alternate 



1 Lignier (II 2 ). 2 Renault (80). 



