PTERIDOPHYTA 



265 



ium and Helminthostachys the stem is an ectophloic siphonostele, while in 

 Ophioglossum, because of the presence of very large, overlapping leaf gaps, 

 the bundles are widely separated and a dictyostele is formed. In all 

 genera the relation of the xylem to the phloem is collateral. The devel- 

 opment of the xylem is mesarch in Helminthostachys and endarch in the 

 two other genera. A notable feature of Botrychium is the presence 

 of a stelar cambium and of marked secondary thickening (Fig. 221). 



-^J 



Fig. 221. Cross section of the central portion of the rhizome of Botrychium virginianum, 

 an ectophloic siphonostele, X 72. Most of the xylem is of secondary origin. 



Although a considerable amount of secondary xylem is formed, there is 

 httle or no secondary phloem. The vascular rays are one layer of cells in 

 width. An advanced feature is the occurrence, in Botrychium and Hel- 

 minthostachys, of large tracheids with bordered pits instead of the sca- 

 lariform markings found in other ferns. In Ophioglossum the tracheids 

 are reticulate. 



Sporangium. All the Ophioglossales are homosporous and eusporan- 

 giate. The most distinctive feature of the group is the presence of a 

 "fertile spike," a sporangium-bearing stalk that arises from the basal por- 

 tion of the leaf as a specialized leaf segment. In Ophioglossum the fertile 

 segment is relatively simple, being cylindrical and unbranched, and bear- 

 ing two lateral rows of sunken sporangia (Fig. 217). Each sporangium 

 does not seem to arise from a single initial cell but from a small group of 

 initials. 



