182 



RHETINANGIEAE 



[CH. 



from the peripheral region of the primary xylem go to form a 

 single leaf -trace: these U-shaped strands of xylem destined for 

 a leaf are connected laterally by parenchyma and form an irregu- 

 larly corrugated band. Fig. 450 shows a petiole-trace still enclosed 

 on each side by the secondary xylem cylinder x^. 



Fig. 449. Rhetinangium Arberi Gord. Transverse section of stem with large 

 leaf -base ; cr, outer cortex of stem ; p, petiole. (After Gordon ; x ] J . ) 



There are several protoxylem groups in a leaf -trace, one on 

 the outer face of each xylem-strand. In the petiole the xylem 

 groups are more intimately connected and the trace has the form 

 of a flat band with abaxial protoxylem. There is no indication 

 that a leaf-trace undergoes division into separate strands. The 

 roots are described as tetrarch with well-developed secondary xylem. 



Fig. 450. Transverse section of leaf-trace of Rhetinangium Arberi showing the 

 exarch structure; r'^, secondary xylem. (After Gordon.) 



The primary xylem of Rhetinangium. agrees in its exarch 

 structure with the Palaeozoic genera Sutcliffia, Megaloxylon and 

 Stenomyelon, also with Lygodium and some other recent Ferns: 

 the secondary wood is of the manoxylic tj'pe like that of Lygino- 

 fteris, Heterangium and other genera. In the general structure 

 of the stele Rhetinangium agrees with Sutcliffia and, except in 

 the exarch structure of the primary xylem, with the steles 

 of Heterangium and Medullosa anglica^; but the structure 

 and origin of the leaf-traces are characters which mark it off 



' Page 90. 



