XIIl] 



OPENING MECHANISMS 



253 



distinguished as the proxitnal or basal region of the wall. The indurated 

 annulus separates these two regions of thin-walled tabular cells from one 

 another. These two parts of the sporangial wall are constantly present 

 throughout the Leptosporangiate Ferns. The external differences which 

 their sporangia show depend chiefly upon their varying proportion, and upon 

 the position of the annulus which separates them relatively to the sporangial 

 stalk. 



In the case of Lygodium and of Schizaea the distal wall is extremely 

 small in area, being represented only by a single cell ; but the proximal 

 region is distended with the result that the sporangium appears ovoid, with 

 the annulus as a cap at its apical end (Fig. 250, a). The dehiscence is by a 

 median longitudinal slit (Fig. 249, A, C). But if the distal region were en- 



Fig. ■249. y/ = S]jorangium o{ Lygodium lanceolatum, Desv. ( x 50), 

 showing annulus of more than a single row of cells ; B = Kidstonia 

 he7-aclee>isis, Zeiller, lateral view of sporangium (X50); 

 C= Schizaea, apex of sporangium with distal wall ("Platte") of 

 only one cell; D = Anemia, apex of sporangium with "Platte" 

 of many cells; j5" = line of dehiscence of sporangium in Ophio- 

 glossH/n reticulation (x 100). [A, B, after Zeiller; C, D, from 

 Engler and Prantl.) 



larged so as to form a considerable convex area, and the annulus were dilated 

 so as to form a zone running obliquely round the body of the sporangium, 

 the type of Gleichenia would result, though still with the same constituent 

 parts, and with the dehiscence in a median longitudinal plane (Fig. 250, b). 

 A slight difference in proportion, but with only partial induration of the 

 annulus, gives the sporangium of Loxsoma, the only known Fern with a 

 gradate sorus which retains the median longitudinal dehiscence (Fig. 250, d). 

 In all other Gradatae and in all of the Mixtae the dehiscence is lateral. In 

 the least modified of these the annulus still remains as a complete ring. 

 The lateral dehiscence which they show results from a change in develop- 

 ment of certain of its cells. For instance, in Lophosoria, Dicksonia, Hynieno- 

 phylliim, or Plagiogyria the annulus is oblique and complete, and the stomium 



