138 THE MORPHOLOGY OF PTERIDOPHYTES 



later ones are reticulate (some being ob-reticulate) but 

 scalariform tracheids are absent. ^^ A pronounced feature of 

 all three genera is the distinctly bordered circular pits in the 

 metaxylem tracheids, but early accounts of the universal 

 presence of a torus in the pit closing membrane appear to be 

 incorrect. Bierhorst^^ records them only in Botrychium 

 dissectum and states that even in this species they are not a 

 constant feature. 



The sporangia in all three genera are 'marginal' in origin. 

 In Botrychium, they are borne in two rows along the 

 ultimate pinnules of the fertile spike (Fig. 20B) and each 

 receives its own separate vascular supply from a vein running 

 into the pinnule (Fig. 20C). In Helminthostachys, the axis of 

 the fertile spike bears numerous 'sporangiophores' in several 

 rows, each bearing several sporangia and a few tiny green 

 lobes at the tip. The spike of Ophioglossum bears two rows 

 of sporangia fused together, beyond which the axis projects 

 as a sterile process (Fig. 20G). A number of vascular bundles 

 run longitudinally up the middle, anastomosing occasionally 

 and giving off lateral branches to the sporangia (Fig. 20I). 



Early stages of development of the sporangium are similar 

 to those in the Marattiales ; a single initial cell undergoes a 

 perichnal division, the inner half giving rise ultimately to 

 the archesporial tissue, while the outer half goes to form 

 part of the sporangium wall. Adjacent cells contribute 

 further to the wall, which is very massive and several cells 

 thick at maturity. A tapetum of several layers of cells is 

 formed from the inner regions of the sporangium wall, 

 which break down to form a continuous Plasmodium in 

 which the spores develop. As in Marattiales, there are 

 stomata in the sporangium wall. 



Dehiscence of the sporangium is transverse in Botrychium 

 and Ophioglossum (Figs. 20B and 20H), but longitudinal in 

 Helmifithostachys, and large numbers of spores are released 

 (more than 2,000 in Botrychium and as many as 15,000 in 

 Ophioglossum). 



