224 MOSSES. CHAP. II* 



which protuberance is denominated the Apo- 

 physis. 

 Opercu- The mouth of the capsule is externally covered 



lumorlid. . , . 



with an operculum or lid, assuming in different 

 species a variety of different forms, and detaching 

 itself horizontally when ripe. Sometimes it is flat, 

 as in Polytrichum alpinum ; and sometimes it is 

 hemispherical, as in Polytrichum undulatum ; but 

 for the most part it is conical and acute. In its 

 position it is erect, or oblique, or bent, or crooked ; 

 in its surface it is smooth or streaked ; in its colour 

 it is brown, red, or scarlet, when ripe. 



If this lid is stripped off, or detaches itself spon- 

 taneously, the mouth of the capsule is then found 

 for the most part to be internally furnished with a 

 circular and double row of fine and tooth-shaped 

 Peristo- substances, called the peristomium or fringe, some- 

 fringe.^ times united into one set, and sometimes divided 

 into several sets. In some genera, however, it con- 

 sists of only a single row of teeth, and in others 

 it is altogether wanting, as in the genus Sphagnum. 

 The number of the teeth is also variable in different 

 genera, though generally uniform in the same ; 

 consisting in the genus Tetraphis of only four ; in 

 Dicranum of sixteen ; in Trichostomum of thirty- 

 two; and in some of the Polytricha. of even 

 double that number. In Encalypta the teeth are 

 erect ; in Tortula they are spirally twisted ; and in 

 Dicranum they are inflected at the top, and cloven 

 half way down, In their ripened state they assume 



