330 FI'LICALES 



[CH. 



walls and more irregular shape. The protoxyleni, px, is repre- 

 sented by groups of narrower elements rather deeply immersed 

 in the peripheral part of the metaxylem. A many-layered 

 pericycle, per, and traces of an endodermis, en, succeed the 

 phloem, ph., which is characterised by several rows of large 

 contiguous sieve-tubes ; beyond the endodermis is a broad thin- 

 walled inner cortex. The leaf-traces arise as in Zalesskya, 

 but the protoxylem in Thamnopteris is at first central ; as the 



per en l^Jt jiX 



a 



Fig. 250. Thamnopteris Schlechtendalii (Eioh.). Part of stele : a, outer xylem; 

 b, inner xylem. (After Kidston and Gwynne-Vaughan. x 13.) 



trace passes outwards a group of parenchyma appears immedi- 

 ately internal to the protoxylem elements and gradually 

 assumes the form of a bay of thin-walled tissue on the inner 

 concave face of the curved xylem. The next stage is the 

 repeated division of the protoxylem strand until, in the sclerotic 

 outer cortex, the traces acquire the Osmundaceous structure 

 (fig. 312, A, p. 453). The petiole bases have stipular wings as 

 in Todea and Osmunda. 



