256 THE SPORE-PRODUCING ORGANS [CH. 



cell-walls. The form of the microsporangium and the arrangement of the 

 cells round its apex so clearly resemble those in Schizaea or Anemia that 

 they may be held as the vestigial equivalent of the annulus seen in these 

 genera (Fig. 253). 



As the specialisation of the 

 annulus increases so does that 

 o{ the stomiitin. In Eusporan- 

 giate Ferns the dehiscence is 

 defined by two parallel rows of 

 cells, as in Opiiioglossiim or Bo- 

 trychium (Fig. 249, E). In the 

 Schizaeaceae and Gleicheni- 

 aceae the condition is but little 



more advanced (Fig. 249, C, D), Fig. 253. Sporangium of Pilularia americana, after Camp- 



and even in the Hymenophyl- bell, l^^ .^^^^cki.aea type, but with the annulus 

 laceae and Loxsonia there is no 



constantly organised group of cells which localises the dehiscence (Fig. 206); 

 but in Dicksonia and Plagiogyria, and many other Gradatae, a more definite 

 group, belonging to the series of the annulus, may be recognised as a stomium. 

 Though the number of the thin-walled cells adjoining that group may 

 often be indefinite in relatively primitive Ferns, the stomium settles down 

 to a structure usually composed of four sister-cells (Fig. 250, c), a condition 

 which is maintained through most of the advanced Leptosporangiates, though 

 sometimes the number may be only two. As this definition of the stomium 

 progresses it swings round from a median to a lateral position, with the effect 

 that the slit of dehiscence passes from the median to an oblique, and finally 

 where the annulus is vertical to a transverse plane. This last, which is strictly 

 maintained by the Mixtae, is obviously in relation to the closely packed 

 sorus, in which the apex of the sporangium alone is exposed so as to allow 

 free movement to the distal arch of the annulus. It thus appears from com- 

 parison of the opening mechanisms of Ferns that in the Eusporangiate 

 types the annulus was ill-defined and not a precise mechanism ; that in the 

 Leptosporangiate Ferns the annulus became definite, as a single ring of 

 cells : that it changed its position from being almost transverse to the axis 

 of the sporangium {Schizaea), through an oblique position (Gradatae) to 

 vertical (Mixtae). The stomium, originally ill-defined, became a definite 

 group of cells, while the slit of dehiscence shifted from a median to an 

 obliquely-lateral, and finally to a transverse position. 



Professor von Goebel {Organographies II. Aufl. II. Teil, p. 1180) has discussed the 

 question whether or not a change of position of the annulus has actually occurred in indi- 

 vidual groups of Ferns. He admits that such a change might appear where form is already 

 differentiated, and a change of function has occurred. But he points out that often 



