XXj OPHIOGLOSSALES 323 



strands (fig. 246). In Botrychium the stele is a cylinder of 

 xylem surrounded externally by phloem. This genus affords 

 the only instance among ferns of a plant in w^hich the addition 

 of secondary tracheae occurs on a scale large enough to produce 

 a well-defined cylinder of secondary xylem traversed by radial 

 rows of medullary-ray cells^ (fig. 247). The unsatisfactory 

 nature of the evidence in regard to the past history of the 

 Ophioglossales renders superfluous a fuller treatment of the 

 recent species. 



1 Jeffrey (98). For an account of the anatomy of Helminthostachys, see 

 Farmer and Freeman (99). 



