BRYOPHYTES 



119 



tissues between the epidermal layers dry up, and the spores are free in the large 

 cavity. 



Operculum and peristome. The development of the operculum is complicated 

 (fig. 264). It is sometimes early differentiated from the capsule by a shallow de- 

 pression where a narrow zone of cells forms a 

 plane of cleavage. Above and below this 

 cleavage plane the tissue grows more rapidly, 

 resulting in two evident rings ; the upper one 

 is the annulus, the lower one the rim. The 

 rim is the more or less thickened top of the 

 urnlike capsule; but the annulus is a definite 

 ring which often becomes detached. The 

 sterile apex is at first solid, the center, occu- 

 pied by endothecial tissue, being a continua- 

 tion of the columella. The sporogenous 

 tissue and the region of cavities end just 

 opposite the cleavage plane, so that they are 

 not represented at the apex. The amphi- 

 thecial region of the apex develops in a 

 peculiar way. The outer walls of one of the 

 inner layers of cells (usually the innermost) 

 become much thickened ; this layer is an- 

 chored to the rim below by a plate of thick- 

 walled cells. When the tissues of the capsule 

 dry out, all the tissues within the operculum, 





262 



FIGS. 261, 262. True moss: 261, longitudinal 

 section of tip of sporophyte showing (below) the 

 several wall layers (amphithecium), the sporoge- 

 nous layer (shaded) cut off from the endothe- 

 xrium, and (above) the beginning of the operculum; 

 262, cross-section of same, showing (beginning at 

 center) columella, single layer of sporogenous 

 cells, and eight or nine wall layers developed from 

 the amphithecium-. After CHAMBERLAIN. 



HI in X/^/J/fcfli^Sv ^ 



260 



FIGS. 259, 260. True moss: 

 259, longitudinal section of tip of 

 young sporophyte, showing differen- 

 tiation into amphithecium (three 

 layers of cells below) and endothe- 

 cium; 260, cross section of the same, 

 the cells of the endothecium shaded. 

 After CHAMBERLAIN. 



except this heavy layer of walls, 

 disappear, and the operculum 

 slips off like a cap, leaving these 

 heavy walls in a conical group of 

 tooth like projections (peristome) 

 anchored below to the rim. Some- 

 times there are two sets of peri- 

 stome teeth, in which case both 

 the inner and the outer walls of 

 the peristome-forming cells be- 

 come thickened. There are 

 many variations in peristomes 

 in detail of development and in 

 pattern. 



A pophysis. The lower 

 part of the capsule does not 

 always develop sporogenous 

 tissue; it is then character- 

 ized by a greater display of 

 chlorophyll tissue and stomata 



