XIIl] 



OPENING MECHANISMS 



255 



comparison of allied genera and species. The dehiscence consequently be- 

 comes not oblique as in the Gradatae (Fig. 250, e), but transverse, as it is in 

 Dryopteris {¥\g. ly). Here, nevertheless, as in other Ferns with a vertical 

 annulus and transverse dehiscence, it is still possible to distinguish the phy- 

 letically distal from the phyletically proximal sides of the sporangium by the 

 segmentation. Thus in Fig. 16, 4a presents its distal or peripheral face to the 

 observer, while ^b presents its proximal or basal face. Professor von Goebel 

 has suggested the distinction of the "longicidal" from "brevicidal" sporangia. 

 These terms may be found useful in a descriptive sense ; but the one type 

 merges so frequently into the other that it cannot be held to mark any real 

 distinction of race. The facts which run parallel with those relating to other 

 criteria of comparison suggest that there has been in a plurality of phyletic 



Fig. 252. A = Dennstaedtia apiifolia. Hook. Sorus showing basipetal suc- 

 cession throughout. C = dehiscent sporangium of the same, showing very 

 slightly oblique annulus. B — Dennstaedtia rtibiginosa, Kaulf. Sorus in 

 vertical section showing that it has been at first basipetal, but a mixed 

 character has supervened. Z> = dehiscent sporangium of the same, seen 

 from the base, showing that the annulus stops short on either side of the 

 insertion of the stalk (j-/.). All x loo. j, j'/ = sporangia; z«^= indusium. 



lines an actual swing of the annulus from the primitively transverse position 

 to the oblique, and finally to the vertical position, with the natural consequence 

 that the dehiscence has changed from the longitudinal to the transverse plane. 

 The case of Loxsoma, where half of the annulus, though indicated by the 

 cell-arrangement, is not indurated, and hence is mechanically ineffective, 

 suggests that when circumstances prevent its mechanical function being 

 performed the annulus may be partially abortive (Fig. 250, d). In the Hydro- 

 pterideae, which shed their spores in water, it may be wholly abortive. In 

 the Salviniaceae there is no structural evidence of an annulus. But in the 

 Marsiliaceae, which in many characters show affinity with the Schizaeaceae, 

 Campbell has noted in Pilularia aniericana a vestigial annulus, with thin 



