GROUP II. BRYOPHYTA: MUSCI. 



369 



or layers of the amphithecial cells, tbe entire walls, or only portions of the 

 longitudinal and transverse walls, of larger or smaller plates of these cells, be- 

 come thickened, cuticularised, and 

 coloured yellow or brown. The 

 unthickened cell-walls, or the un- 

 tbickened portions of them, break 

 away as the capsule becomes ripe, 

 leaving only the thickened portions 

 forming, as it were, a skeleton at- 

 tached to the urn just below the 

 level of the annulus. The following 

 are the principal varieties of peri- 

 stome-formation. In the Georgia- 

 ceae (e.g. Tetraphis) tbe peristome is 

 developed from the two layers of 

 cells beneath the epidermis which 

 forms the operculum : the walls of 

 these cells all become thickened; 

 when the operculum falls off this 

 plate of tissue splits from the centre 

 into four equal segments which arc 

 the teeth of the peristome. In most 

 Mosses the peristome is formed from 

 a single layer of cells, and consists 

 of two rows of teeth, an inner and 

 an outer. This double peristome is 

 dependent upon the distribution of 

 the cuticularisation of the walls : 



both the internal and the external walls of the peristomial cell-layer become 

 cuticularised, but the lateral and part of the transverse walls joining them 

 remain unaltered and eventually break away, leaving the thickened internal 

 and external walls as separate strips, which may be further divided longitudin- 

 ally into teeth. The teeth of the outer peristome are generally larger than 



those of the inner which are sometimes dis- 

 tinguished as cilia ; their number is a power 

 of two (4-8-16-32-64.) There is consider- 

 able variety in the structure and form of the 

 peristome ; this affords a means of classifica- 

 tion. The genus Polytrichum, for instance, 

 is characterised by the fact that the teeth of 

 the peristome consist of strands of thick- 

 walled fibres, the tips of which are not free, 

 as is usually the case, but are connected by 

 a membrane stretched over the aperture of 

 the urn, termed the epiphragm. 



A peristome is not present in several genera 

 (e.g. Gymnostomum, Hymenostomum, Schis- 

 tostega, etc.) ; nor in some species (e.g. 



V. S. B. B B 



FIG. 251. Funaria hygrometrica. A An adult 

 shoot (g), bearing a calyptra (c). B A plant 

 (g) bearing a nearly ripe sporogonium ; s its 

 seta; /the capsule ; c the calyptra. C Median 

 longitudinal section of the capsule : d oper- 

 culum ; a annulus ; p peristome ; c c' columella ; 

 7i air-cavity ; s the archesporium. 



Fi&. 252. Mouth of the theca of 

 Fontinalis antipyretica. ap Outer 

 peristome ; ip inner perietome. ( x 

 60.) 



