Mosses and Lichens 



one of two things happens: The walls of a zone of cells of the 

 spore-case wall, in the exterior layer, separates from the adjoining 

 walls when the spore-case is mature, or a zone of cells consisting 

 of one or more rows has the cell-walls modified so that when 

 they are distended by absorbed moisture, the zone of cells is dis- 

 placed as a ring or annulus and so frees the outer layer of cells 

 in the upper part of the spore-case to form a lid. 



Lid. 



Spore-case with lid. 



Mnium affitte. 



Lid. 



Spore-case without lid, 



Polytrichum commune. 



Tooth. 



Spore-case 

 with lid. 



Spore-case 



without lid. 



Bryum argenteum. 



Lid. 



Trematodon ambiguum. Vertical section of 

 a single peristome, (See page 53). 



Lid 



Ulotacrispa, Double row of teeth. 

 (See page 53). 



Spore-case with lid. Spore-case without lid. 

 Polytrichum sexangulare. 



