334 



MUSCINEm. 



which forms the peristome ; its walls {a\ which face outwards, are strongly thickened, 

 and of a bright red colour ; the thickening is continued also partially along the septa ; 

 the longitudinal walls which lie on the axial side of the same layer of cells [i) are also 

 coloured, but less strongly thickened. In Fig. 251 is shown further a part of the 

 transverse section through the basal part of the operculum ; r r are the epidermal cells 

 placed immediately above the annulus, forming the lower edge of the operculum; a 

 and i the thickened parts of the layer of cells concentric with the operculum, which 

 form the peristome. A section near the apex of the operculum would show, instead 

 of the broad thickening-masses /*, z", i" , only the middle part of the inner wall, but 

 more strongly thickened. If now it is supposed that when the theca is ripe the annulus 

 and the operculum fall off, the cells p and those which he between a and e (Fig. 250) 

 disappear, and that the thin pieces of wall between a, a, a", and between i, i', i", in Fig. 251, 

 are also]^ destroyed, then the thick red pieces of wall alone remain, forming sixteen pairs 

 of tooth-like lobes pointed above, crowning the edge of the theca in two concentric 

 circles. The outer row are termed Teeth, the inner row Cilia. The thickened cells at 

 t, Fig. 250, unite the base of the teeth with the edge of the theca. According as 

 the layer of cells which forms the peristome consists, in transverse section, of a larger 

 or smaller number, and according as one or two thickened cells are formed within each 

 one of these cells, the number of teeth and cilia varies ; it is always however a multiple 

 of four, generally 16 or 32. In many cases the thickening at i is wanting; the peri- 

 stome is then simple, and formed only of the teeth of the outer row. The thickenings 

 at a are very commonly much stronger than is the case in Funaria, and the teeth there- 

 fore stouter. The thickened parts of the wall may also partially or entirely coalesce 

 laterally with one another ; and then the parts of the peristome either above or below 

 form a membrane ; in this case the teeth appear split from one another above, and the 

 endostome (the inner peristome) is composed of a lattice-work of longitudinal or trans- 

 verse ridges instead of cilia (Fig. 246). A great variety is met with here, which may 

 easily be understood by the beginner when he has obtained a clear idea of the principle. 

 The inner and outer sides of the teeth of the peristome are hygroscopic to a different 

 degree ; hence, as the amount of moisture in the air varies they bend inwards or out- 

 wards, or sometimes in a spiral whorl, as in Barbula. 



The genus Polytrichum, to which the largest and most highly developed Mosses 

 belong, differs from the other genera in several points in the structure of its theca. 

 The teeth of the peristome are composed not simply of single pieces of membrane, but 

 of bundles of thickened prosenchymatous cells ; these bundles are horseshoe-shaped ; 

 the branches of two adjoining bundles directed upwards form together one of the 32-64 

 teeth. A layer of cells uniting the points of the teeth (Fig. 252, ep) remains, after the 

 casting off of the operculum and the drying up of the adjoining cells, as an epiphragm 

 stretched across the theca. The spore-sac is, in some species {e. g. P. piliferum), separated 

 from the columella by an air-cavity, which is penetrated, like the outer air-cavity, by 

 conferva-like rows of cells. In most species the seta is swollen beneath the theca, 

 forming the Apophysis, a phenomenon which is repeated in a somewhat different manner 

 in the genus Splachnum, where this part is sometimes expanded transversely as a flat 

 disc. 



