SECT. I 



CRYPTOGAMS 



437 



The peristome of Mnium hornum (Fig. 398) is double ; the outer peristonie 

 is formed of 16 pointed, transversely striped teeth inserted on the inner margin 

 of the wall of the capsule. The inner peristome lies just within the outer, and 

 consists of cilia-like appendages, which are ribbed on the inner side and thus 

 appear transversely striped ; they coalesce at their base into a continuous membrane. 

 Two cilia of the inner peristome are always situated between each two teeth of 

 the outer row. The cilia facilitate the dissemination of the spores by their 

 hygroscopic movements. 



The teeth and cilia of the peristome are formed in this instance of thickened 



ap 



FIG. 396. Mnium hornum. Median .longi- 

 tudinal section of aihalf-ripelsporogonium. 

 o Operculum ; p, peristome ; c, columella ; 

 3, spore-sac containing the spores ; i, air- 

 space ; ap, apophysis ; gt, stomata. ( x 18.) 



Fio. 397. Mnium hornum. Transverse section 

 through the wall of the capsule in the region 

 of the ring, a, Cells of the ring ; 1-k, succes- 

 sive cell layers with the thickened masses of 

 the peristome, d', d"; d'", transverse pro- 

 jecting ribs ; c, the .coalesced cilia, (x 

 240.) 



portions of the opposite walls of a single layer of cells next to the operculum (Fig. 

 397), the teeth from portions of the external wall, and the cilia from portions of 

 the internal walls of the same layer. On the opening of the capsule the un- 

 thickened portions of this layer break away and the teeth and cilia split apart. 

 The transversely ribbed markings on their surface indicate the position of the 

 former transverse walls. 



The structure of the peristome varies greatly within the Bryinae. By its 

 peculiar form and hygroscopic movements the peristome causes a gradual dis- 

 semination of the spores from the capsule. 



Variations in the form of the capsule, peristome, operculum, and calyptra afford 

 the most important means of distinguishing the different genera. The Bryinae 



