344 



MOSSES AND FERNS 



CHAP. 



the annulus arises from the upper or cap segment of the spo- 

 rangium wall, and extends from the stomium over the top of 

 the sporangium, and joins the part of the annulus upon the 

 other side. The walls of all the cells are at first alike, but those 

 of the annulus begin to thicken, this being confined to their 

 inner and radial walls, the outer walls remaining thin. In most 

 species the cells of the annulus are the same for the whole ex- 

 tent, but in PolypMum falcatum (Fig. 191), which is figured 

 here, the cells of the annulus immediately above the stomium 



are larger and thinner- 

 T walled. The stomium 

 cells are more extended 

 laterally than the other 

 cells of the annulus, and 

 between them the spo- 

 rangium opens by a wide 

 horizontal cleft 



Atkinson ((3), p. 68) 

 describes the process 

 thus for the Polypodi- 

 acese. "While the open- 

 ing of the stomium be- 

 tween the lip cells is aid- 

 ed by their peculiar form, 

 it seems possible that at 

 maturity the line of un- 

 ion is less firm than be- 

 tween the other cells. 

 The fissure once started 

 proceeds across the lat- 

 eral walls of the sporan- 

 g i u m , usually in a 

 straight line, thus split- 

 ting in half the cells of the middle row, their frailty favouring 

 this. The drying of the annulus brings about the unequal ten- 

 sion of its cell walls. During this process it slowly straight- 

 ens, carrying between the distal portion of the lateral walls 

 of the sporangium, which remain attached to the free extrem- 

 ity, the greater part of the spores. When straight, it continues 

 to evert, and this usually proceeds until the two ends of the 

 annulus nearly or quite meet, when with a sudden snap it 



Fie. 191. Surface view of a nearly ripe sporan- 

 gium of Polypodium falcatum, Xi7SJ st, 

 stomium; r, annulus. 



