THE GKEAT GEOUPS CF BBYOTHYTES 127 



center of the capsule to the lid-like operculum (Figs. 108, 

 109). When the operculum falls off the capsule is left like 

 an urn full of spores, aud at the mouth of the urn there is 

 usually displayed a set of slender, often very beautiful teeth 

 (Fig. 110), converging from the circumference toward the 

 center, and called the peristome, meaning " about the 

 mouth." These teeth are hygroscopic, and by bending 

 inward and outward help to discharge the spores. 



