GROUP II. BKYOPHYTA : MUSCt. 361 



Archidium there is no archesporial layer, bat the sporogenous cells 

 are scattered throughout the endotheciura. The cells which bound 

 the archesporium on each side constitute the spore-sac. The endo- 

 thecial tissue which lies internally to the archesporium constitutes 

 the columella. In the Sphagnaceae and Andreaeaceae the arche- 

 sporium is a hollow hemisphere covering the top of the columella 

 like a cap ; in the Bryinese the archesporium is a , hollow cylinder 

 surrounding the columella which extends to the apex of the cap- 

 sule. In Archidium and in the Bryineae a large intercellular space 

 is developed in the amphithecium, between its outer and its two 

 inner layers ; in most Polytrichaceae a similar intercellular space is 

 developed in the endothecium internally to the spore-sac, between 

 it and the central portion of the columella. 



At maturity the internal cells of the capsule become dry and dis- 

 organised, so that it simply contains the spores which now lie loose 

 in its cavity. It dehisces by the throwing off of its apical portion 

 as a lid or operculum in Sphagnaceae and the higher Bryineae (Stego- 

 carpse) ; or by longitudinal fissures, as in Andreseaceas ; or it rup- 

 tures irregularly or simply decays, as in Archidium and the lower 

 Bryinece (Cleistocarpae). In the higher Bryineae the mouth of the 

 dehisced capsule bears a fringe, the peristome, the development and 

 structure of which will be described subsequently (p. 368) . 



The basal portion of the capsule, where it joins the seta, is 

 termed the neck. In the Polytrichaceae the neck is considerably 

 dilated, as also in various species of Splachnum. (e.g. S.sphcericum, 

 vasculosum, etc.) whilst in Splachnum luteum and rubrum it grows 

 out into an umbrella-shaped structure. When the neck is thus 

 markedly developed it is termed the apophysis. 



The histological differentiation of the sporogonium is well- 

 marked. There is a well-defined epidermis, in which, on the cap- 

 sule, stomata of various forms are generally present (absent in 

 Andreaeaceae, Archidium, and some Bryinese) ; either scattered all 

 over, as in the Sphagnaceae ; or confined to a particular region of 

 the capsule, generally the neck or the apophysis, in the Bryinere. 

 The operculum and the peristome (Bryineae) show considerable 

 complexity of structure. The structure of the seta in the higher 

 Bryineae, where alone it is elongated, very much resembles that of 

 .the stem: in many forms, even in such in the stem of which no 

 central strand is present, there is a central strand in the seta, 

 marked off from the ground-tissue by one or two layers of sheath- 

 cells. In the Bryineae also, the structure of the neck (or apo- 



