EXAMINATION OF A MOSS. 



191 



341. The slender scape-like stalk which rises above 

 the leaves is technically called the seta, or bristle ; in the 

 left-hand part of the figure (c) the upper end of the seta 

 is covered by a hairy cap, the 

 calyptra. In the right-hand por- 

 tion the calyptra has been re- 

 moved, disclosing a little pod, 

 variously spoken of as the theca, 

 or urn, or capsule, or sporangium. 

 Fig. 241 is an enlarged view. 

 This capsule is closed at the top 

 by a circular lid, the operculum, 

 which falls away when the cap- 

 sule is mature, thus allowing the 

 escape of the spores, which are, 

 produced in it. The spores are 

 developed upon the surface of a 

 central column which rises from 

 the bottom of the capsule, and 

 which is known as the columella. 

 The opening through which the 

 spores escape is called the stoma, 

 and a good lens reveals the fact 

 that around the stoma there is a 

 circle (sometimes two) of minute 

 teeth,- known collectively as the 

 peristome. In the Moss now be- 

 fore us the peristome consists of sixty-four teeth. In 

 other Mosses the number varies, being always, however, 

 seme power of 2 ; either 4, or 8, or 16, or 32, or 64. 

 Occasionally the teeth are altogether absent. 



Fig. 239. Two fertile stems of a Moss (Potytrichum commune) of the 

 natural size ; at c the calyptra is seen enveloping the capsule. (Wood and 

 Steelel 



Fig. 239. 



