GROUP II. BRYOPHYTA : MUSCI. 343 



the archesporium, which forms a hollow cylinder round the columella, 

 but does not extend over the top of it : an air-chamber is developed in the 

 amphithecium round the spore-sac, and is generally traversed by strands 

 of cells (containing chloroplastids) stretching from the wall of the capsule 

 to the spore-sac. In the lower forms the capsule is either indehiscent, its 

 wall becoming eventually disorganised, or it ruptures irregularly ; in the 

 higher forms, the apical portion of the wall is thrown off as an oper- 

 culum, and a peristome is generally developed round the aperture thus 

 formed. In all cases a portion of the calyptra is carried up as a cap on 

 the top of the developing sporogonium. 



The Bryineae are classified as follows : 



Tribe I. Cleistocarpee. Tribe II. Stegocarpae. 



Tribe I. CLEISTOCARP^E. The adult shoots are generally minute, un- 

 branched, annual, and always acrocarpous; there is generally a central 

 strand in the stem, and a mid-rib in the leaf. 



With regard to the sporogonium, the seta is generally short, sometimes 

 expanded at the base into a false (epibasal) foot (e.g. Phascum, Ephe- 

 merum), without any central strand in some forms. The capsule does 

 not open by means of an operculum, nor has it any peristome : it either 

 ruptures irregularly, or the wall simply 

 decays. 



Tribe II. STEGOCARPAE. The character- 

 istics of this tribe are to be found in 

 the sporogonium, which is distinguished 

 by the formation of an operculum and, 



generally, of a peristome. 



FIG, 207. a Ephemerum, sernituui 

 The operculum is developed from the (x 3) . b 8hoot of AndreaM nira ii, f 



apical portion of the capsule, either from with (K) capsule (nat. size). 

 the epidermis alone (e.g. Georgiaceae), or 



from it and one or more of the subjacent amphithecial layers. The cell- 

 walls become cuticularised and assume a yellow or brown colour. The 

 outline of the operculum is circular; its form cap-like, more or less 

 flattened in some cases, more or less conical in others, sometimes apiculate. 



The limit between the developing operculum and the rest of the capsule 

 (urn) is generally marked by a slightly prominent zone, consisting of one 

 or more rows of rather large epidermal cells, with cuticularised outer 

 walls, termed the ring or annulus : its position is just above the level of 

 the top of the spore-sac and of the air-chamber. 



The peristome is developed within the operculum, from the innermost 

 layer or layers of the amphithecial cells; the entire walls, or only portions 

 of the longitudinal and transverse walls, of larger or smaller plates of 

 these cells, become thickened, cuticularised, and coloured yellow or brown. 

 The unthickened cell-walls, or the unthickened portions of them, break 

 a way as the capsule becomes ripe, leaving only the thickened portions 

 forming, as it were, a skeleton attached to the urn just below the level of 

 the annulus. The following are the principal varieties of peristome- 

 formation. In the Georgiaceae (e.g. Tetraphis) the peristome is developed 

 from the two layers of cells beneath the epidermis which forms the 



