344 



PART IV. CLASSIFICATION. 



operculum : the walls of these cells all become thickened ; when the oper- 

 culum falls off this plate of tissue splits from the centre into four equal 

 segments which are the teeth of the 

 peristome. In most Mosses the peri- 

 stome 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 dis- 

 tribution of the cuticul arisation of the 

 walls : both the internal and the ex- 

 ternal 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 longitudinally into 

 teeth. The teeth of the outer peri- 

 stome 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 

 considerable variety in the structure 

 and form of the peristome ; this affords 

 a means of classification. The genus 

 Polytrichum, for instance, is charac- 

 terised 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 , 

 H3 r menostomum, Schistostega, etc.); nor 

 in some species (e.g. species of Pottia and 

 Encalypta, etc.) belonging to genera in 

 which a peristome is usually present. 



As the capsule matures, the cells(except 

 the spores) lose their cell-contents ; and 

 those whose walls have not become 

 thickened and cuticularised, dry up and 

 shrink, the shrinkage being necessarily 

 accompanied by the tearing of the thin 

 walls in various parts. The persistent 

 cuticularised walls are highly hygro- 



FiG. 209. -Mouth of thethecaof { d k j j consequence of the 



Fontmalis antipyretic.', ap Outer 



peristome ; ip inner peristome ( x tensions set up by the unequal stretching 

 60.J and contraction of these walls, due to 



FIG. 208. 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 operculum ; a annulus ; p peristome; 

 cc'columella; h air-cavity ; s the arche- 

 sporium. 



