286 SPOROGONIUM OF MOSSES 



divides to form four spores (Fig. 156, C), Between the spore- 

 sack so formed and the capsule-wall is a conspicuous air-space (S.) 

 bridged by occasional fine threads of green cells ; in a few 

 Mosses {e.g. Polytrichtm) a similar air-space separates the spore- 

 sack from the columella. The capsule is protected by a thick- 

 walled epidermis {ep.) beneath which are several layers of cells 

 containing chloroplasts. The features just described are equally 

 recognisable in a transverse section through the middle . of the 

 capsule. 



Just below the constriction (Fig. 156, A and B) marking the 

 commencement of the lid, a diaphragm (d.) composed of thickened 

 cells, and having the form of a circular ledge perforated by the 

 central thin-walled tissue, stretches inwards from the epider- 

 mis {ep.) of the capsule. Arising from the inner edge of this 

 diaphragm is a dome-shaped layer of cells (the peristome, p.) 

 characterised by marked thickening of the tangential {i.e. inner 

 and outer) walls {i. and 0.), and of the adjoining portions of 

 the horizontal walls, although the radial {i.e. vertical) walls 

 remain thin. At the lower edge of the lid the thick-walled 

 epidermis is interrupted by one or two rings of larger thin-walled 

 cells which constitute the so-called annulns {an.). Dehiscence 

 eventually takes place along this line, as a result of the drying 

 of the capsule, and soon after the lid is blown off. The epidermal 

 cells above the annulus are usually markedly flattened, and form 

 the lower edge of the lid (Fig. 156, B). 



At maturity all the thin-walled tissues of the capsule shrivel, 

 leaving, apart from the spores, only the epidermis, the diaphragm, 

 and the thickened walls of the peristome. Each of the rows of 

 cells forming the latter necessarily tapers from base to apex, 

 and, as a result of the breaking down of the thin radial {i.e. ver- 

 tical) walls between some or all of the rows, a number of separate 

 plate-like pcristoine-teeth (Fig. 156, D), attached below to the 

 edge of the diaphragm, are formed. When both inner and 

 outer tangential walls of the peristome are thickened, a double 

 set of teeth {0. and i.) of course arises as the thin portions 

 of the horizontal walls shrivel. The peristome varies greatly 

 in different Mosses, and, together with the characters of the 

 leaves, constitutes one of the chief means for distinguishing the 

 different genera and species. 



