210 



MOSSES AND FERNS 



CHAP. 



gonium, much less, relatively, than in any of the forms hitherto 

 described. Before the final division of the spores it divides 

 more or less completely into two layers. The cells resulting 

 from this last division are the spore mother cells, which separate 

 soon after their formation and lie free in the space betw^een the 

 inner and outer spore-sacs, where each one divides into four 

 tetrahedral spores. 



In the operculum, as the capsule approaches maturity, the 

 differentiation of annulus and peristome takes place. The 

 annulus consists of five or six rows of cells that occupy the 



B 



Fig. III. — A, Longitudinal sections of a nearly ripe capsule of F. hygrometrica, X260; 

 per, peristome; r, annulus; t, thickened cells forming the margin of the theca; B, 

 the sporogenous cells shortly before the final divisions; i, inner; 0, outer spore- 

 sac, X525- 



periphery of the broadest part of the operculum. The upper 

 rows of cells are very much compressed vertically, but are 

 greatly extended radially and have their walls thicker than those 

 of the neighbouring cells. These thickened annulus cells form 

 the rim of the loosened operculum. The two lower rows of 

 annulus cells — the annulus proper — have thin walls and finally 

 become extremely turgescent. It is the destruction of these 



