328 



FILICALES 



[CH. 



the xylem and separated from it by a parenchymatous sheath 

 is a ring of phloem, ph, composed of large sieve-tubes and 

 parenchyma separated from the inner cortex by a pericycle 

 4 to 5 layers in breadth. The occurrence of a few sclerotic 

 cells beyond the broad inner cortex points to the former 

 existence of a thick-walled outer cortex. The leaf-traces are 



ph 



Fig. 249. Zalesskya diploxylon. Kidston and Gwynne-Vaughan. Transverse 

 section of stem, ph, phloem. (After Kidston and Gwynne-Vaughan. 

 x24.) 



given off as mesarch strands from the edge of the xylem ; they 

 begin as prominences opposite the protoxylem and become 

 gradually detached as xylem bundles, at first oblong in trans- 

 verse section, then assuming a slightly crescentic and reniform 

 shape, while the mesarch protoxylem strand takes up an 

 endarch position. As a trace passes further out the curvature 



