THE BRYOPHYTA: MUSCI, THE MOSSES 425 



from a spore mother cell, whose nucleus divides into four, during which 

 reduction division takes place, so that before separation the four spores 

 form a tetrad. Each is marked with a triradiate ridge caused by their close 

 packing in the tetrads. As the spores develop, the tissue around the spore 

 layer breaks down, and the loose tissue outside it withers away so that the 

 spores are distributed over the whole of the space or spore sac between the 

 columella and the wall of the capsule. At this stage the individual spores 

 each contain chloroplasts and a considerable amount of reserve material in 

 the form of oil. There is a very thin inner wall and a conspicuous cuticularized 



outer wall. 



We must now consider the complex structures which constitute the cone 

 of tissue, called the operculum, at the top of the capsule. In this tissue 

 there arises a structure which is termed the peristome. The peristome 

 consists of an inner and outer series of teeth, each series containing some 

 sixteen teeth, composed of strips of specialized cell wall. These teeth are 

 built up from a dome-shaped layer of cells whose outer and inner walls are 

 cutinised. The radial walls are of unaltered cellulose (Fig. 410). When 

 mature the cellulose walls and the cell contents disappear and the two 

 cutinized walls alone are left, which split longitudinally into the outer and 

 inner rings of teeth respectively (Fig. 411). These peristome teeth are 

 hygroscopic, that is, they react to the influence of moisture. In damp 

 weather the outer peristome teeth bend inwards, covering the inner peristome 

 and efi'ectively preventing the spores from escaping. In dry weather all the 

 teeth bend outwards thus enabling spore discharge to take place (Fig. 412). 

 The spores themselves are shaken out of the capsule by the action of the 

 wind, for when mature the base of the capsule bends over so that the apical 

 end is downwards, and the spores drop out readily when the peristome 

 teeth open and allow them to do so. The mature seta is also hygroscopic, 

 and its twisting movements assist in spore dispersal. 



The peristome is covered during development by the surface layers of 

 the operculum. This covering finally detaches itself from the peristome 

 teeth and from the top of the theca and falls off, revealing the rings of teeth. 

 Its dehiscence is assisted by the presence of a ring of large thin-walled cells, 

 the annulus, at its base, the swelling of which causes its detachment from 

 the rim of the theca, which is formed of several layers of very thick-walled 

 cells, to which the peristome teeth are attached (Figs. 413 and 414). 



The central tissue of the operculum, together with the top of the 

 columella, wither away when the capsule is ripe, so that the dropping ofi" of 

 the lid leaves the capsule open at the top, except for the rings of teeth. 



We see then that the top of the capsule has three protections. Firstly 

 there is the calyptra, which is shed at an early stage, then there is the operculum, 

 which remains until the peristome is mature, and finally, after this too has 

 dropped off, there are the teeth of the peristome, which regulate the discharge 

 of the spores according to the condition of the weather. 



From a comparison of the capsule of the Moss with the corresponding 

 structure in the Liverwort, we realize that the former is considerably more 



