DEVELOPMENT. 25 



irregularly, or rots away, and in its decay liberates the 

 spores. 



The lid or operculum varies in form, being sometimes 

 convex, as in many of the Bryums, or conical (fig. 15, 3), as 

 in Physcomitrium pyriforme, Tetraphis pellucida, etc. ; or it 

 may be rostrate (beaked) (fig. 12, 2), as in Dicranella hetero- 

 malla, etc. 



FIG. 13. Andreaa alpina. i, plant natural size. 2, nerveless leaf magnified, 

 z a, apex of same, to show dot-like, thick walled upper cells. 



A ndrecea nivalis. 3, plant natural size. 4, nerved leaf enlarged ; 4 a, apex of same 

 to show areolation. 5, capsule bursting (dehiscing) by four valves. 



When the lid is removed, or has been cast off naturally, 

 the inner structure of the capsule may be seen, and in some 

 mosses, such as Pottia truncata, the mouth will be found to 

 be naked, but in many other cases it will be seen to be 

 surrounded by a delicate, fringe-like appendage, called the 

 peristome (fig. 15, 2), (Gr. peri, around, and stoma, a mouth). 

 This fringe consists of minute tooth-like processes, which 



