336 



MOSSES AND FERNS 



CHAP. 



special cell of the last-formed lateral segment. The segment 

 on the opposite side (next older but one) shows a quite similar 

 arrangement of cells, and, according to Muller,^ the cell corre- 

 sponding to the stomium by two transverse walls forms the 

 first segment of the annulus. The cells immediately below also 

 divide similarly, and give rise to a second section. The rest of 

 the annulus arises from the upper or cap segment of the 

 sporangium 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 re- 

 maining thin. In most 

 species the cells of the 

 annulus are the same for 

 8t the whole extent, but in 

 Polypodiuni falcatuni (Fig.. 

 1 68), which is figured 

 here, the cells of the an- 

 nulus immediately above 

 the stomium are larger 

 and thinner-walled. The 

 stomium cells are more 

 extended laterally than 

 the other cells of the an- 

 nulus, and between theim 

 the sporangium opens by^ 

 a wide horizontal cleft. 

 Atkinson - describes the process thus for the Polypodiacea;. 

 " While the opening of the stomium between the lip cells is 

 aided by their peculiar form, it seems possible that at maturity 

 the line of union is less firm than between the other cells. 

 The fissure once started proceeds across the lateral v/alls of 

 the sporangium, usually in a straight line, thus splitting in 

 half the cells of the middle row, their frailty favouring this. 

 The drying of the annulus brings about the unequal tension 

 1 Muller, C. (2). 2 Atkinson (3), p. 68. 



Fig. 168. — Surface view of a nearly ripe sporangium of 

 Polypodimn falcatum, X 175 ; st, stomium ; r, 

 annulus. 



