DIV. I 



BRYOPHYTA 



495 



a fringe, the PERISTOME, consisting usually of tooth-like appendages, but in others 

 this is wanting. 



The peristome of Mnium hornum (Fig. 460), which will serve as an example, 

 is double ; the outer peristome is formed of 16 pointed, transversely striped teeth 

 inserted on the inuer 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 



ap 



FIG. 458. Mnium hornum. Median longi- 

 tudinal section of a half-ripe sporogonium. 

 o, Operculum ; p, peristome ; c, columella ; 

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

 space; _/), apophysis ; st, stomata. (x 18. 

 After STRASBTRGER.) 



FIG. 459. Mnium hornum. Transverse section 

 through the wall of the capsule in the region 

 of the ring, a, Cells of the ring ; 1-4, succes- 

 sive cell layers with the thickened masses of 

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

 jecting ribs ; c, the coalesced cilia. ( x 240. 

 After STRASBURGER.) 



a continuous membrane. Two cilia of the inner peristome are always situated 

 between each two teeth of the outer row. 



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

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

 459), 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- 

 thickeued 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. 



In the Polytrichaceae the origin of the peristome teeth follows a peculiar type ; 

 they are composed of a number of elongated entire cells. 



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



