488 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



peristome is apparently double (Fig. 106, e.), but Wilson shows 

 that the outer peristome, which consists, like the peristome of 

 Tetraphis, of irregular cells, is an excrescence from the more 

 external layers of the tissue of the sporangial membrane, of 

 which, he says, we find analogous though less evident examples 

 in Polytriclium and Orthotrichum ; the leaves are few in 

 number, nerveless, fringed, and broadly reticulate. In JDiphy- 

 scium, on the contrary, there is a tuft of nerved spreading 

 leaves ; the sporangium is almost sessile, the outer peristome 

 almost rudimentary, and the inner consisting of a plicate coni- 

 cal membrane, thickened at the sixteen salient angles. The 

 nearest affinity is with Polytrichum, but the habit of Diphy- 

 scium is that of Fhascum, and its duration scarcely more 

 than annual. There is a second species of Buxbaumia, which 

 has not yet been found in Great Britain ; another appears in 

 Java and Australia. A species of Diphyscium is found at 

 Myrong. 



18. PoLYTRiCHEi, Br. & Sc, Mont. 



Mouth of sporangium mostly closed by a flat membrane ; 

 calyptra rough vidth dependent hairs, rarely naked. 



542. This is an important tribe of acrocarpous mosses, con- 

 taining some of the very finest species, which are remarkable 

 not only for beauty, but for structure. The spore-sac is some- 

 times separated from the columella as well as from the sporan- 

 gial wall, and waved, so that the sporangium when cut across 

 looks like a real capsule. The sporangium is often quadrate, 

 furnished with an apophysis below ; the calyptra is rough, 

 with silky hairs, which are a sort of paraphyses, but distinct 

 from the true attendants on the archegonia ; they are deve- 

 loped after impregnation, and arise partly from the vaginula, 

 partly from the walls of the archegonium. The peristome 

 consists of 32 — 64 teeth, united above to the membranous 

 drum-like top of the columella. Polytrichum is divided into 

 several subgenera, which it is not necessary to distinguish here. 

 Dawsonia differs from Polytrichum, in having an oblique 

 sporangium like Buxbaumia, and in the numerous cilia of 

 the peristome, which are either free or partly connected vsdth 

 the top of the columella. Lyellia has a similarly shaped 



