BEYOPHYTA— MUSCI 119 



the innermost amphithecial layer, situated just above the 

 termination of the cylindrical air-chamber, midergo thickening 

 and cuticularisation on their external and internal faces. The 

 lateral and part of the transverse %valls connecting the others 

 remain michanged. These, as the peristome dries, break away 

 from the cuticularised walls, and the latter separate longitudinally 

 into several strips {fig. 878). There are thus two rows of cuti- 

 cularised membranous teeth formed, which, so long as the oper- 

 culum is in position, curve over and meet at a point in the centre. 

 The outer ones are known as the teeth of the peristome, the inner 

 ones, which are less strongly thickened, are called cilia. The 

 number of teeth varies from 4 to G4, according to the number of 

 longitudinal divisions of the cells. When the operculum falls 

 off, these teeth uncoil in virtue of their elasticity and remain as 

 a kind of fringe round the opening. 



The base of the capsule below the 

 air-space is slightly dilated, forming a Fig. 878. 



mass of cells known as the apoplujsis. /p 



This is furnished with stomata, and con- 

 tains a large amount of assimilating 

 tissue. 



There is a good deal of "''ariety in 

 the formation of the peristome in diffe- 

 rent genera. In some, as Gymnostomuvi, 

 none is developed. In Tetrapliis the /-v,,. grs. Surface view of 

 layer giving rise to it is two cells in peristome.i^.of Fuuaria. 

 thickness, and all their walls become 



thickened, forming a plate of tissue. When this is released it 

 splits into four valves or segments. In Polj^trichum the develop- 

 ment is different. The teeth are formed not of simple pieces of 

 membrane, but of bimdles of thick prosenchymatous cells. A 

 plate of cells unites the top of the teeth and remains after de- 

 hiscence of the theca as a structure called the epiphragm. 



As the spores mature the rest of the contents of the capsule 

 dry up and shrink ; when they are ripe nearly everything else has 

 disappeared, so that the spores lie loose in the cavity of the theca. 



Some species show important differences of structure from 

 the type of Funaria. In Spliagnum the archesporium which 

 is developed from the inner layer of the amphithecium is a 

 hemispherical or dome-shaped band extending across the top 

 of the capsule and covering the columella. In Polytrichum a 

 second air-chamber is formed in the endothecium on the inner 

 side of the archesporium. In a few mosses there is no operculum. 



