148 



MUSCINE^ 



layer of cells beneath the epiderm ; but in Polytrichum the teeth are 

 composed of bundles of thickened prosenchymatous cells. The exterior 

 peristome may have two distinct forms. Either the teeth have a double 

 outer and a single inner series of plates {Diplolepidd), or the exterior 

 series is simple (Aplolepida), and then the inner series is nearly always 

 double. The Aplolepidae never have a double peristome ; and in the 

 Diplolepidse the inner peristome is occasionally wanting in particular 

 families or genera. In much the larger number of genera {Arthrodontea) 

 the teeth are septated by transverse walls ; in a much smaller number 



Fig. 113. — Hypnum popttleum S\v. (natural size). 



Fig. 114. — TetrapkispellucitUi 

 Hedw. a (slightly magnified), 

 with open sporange ; b, ditto 

 with gemma : c, sporange with 

 calypter (greatly magnified) ; 

 dj open sporange, showing 

 peristome. 



Fig. lis. — Bryum krgenteum L. Fig. ii6.—Sptachmim am- 

 (natural size). pullaceutn L. (natural size). 



{NematodontecB) the transverse septa are wanting. The cilia, when 

 present, are usually shorter and less developed than the teeth; they 

 are also composed of two layers of plates, often marked on the surface 

 by a beautiful network ; their divisions correspond to those of the teeth. 

 The number of both teeth and cilia is always a multiple of 4, the most 

 common numbers being 8, 16, 32, and 64. The most perfect type 

 of peristome is seen in the Encalypteae, from which all the less perfect 

 forms are, according to Philibert, derived by degradation. In addition 

 to the presence of an epiphragm, the genus Polytrichum presents the 

 peculiarity, in most species, of the seta being swollen beneath the spo- 

 range, forming an annular cushion known as the apopkyse (see fig. i r 2). 



