THE BRYOPHYTA: MUSCI, THE MOSSES 



423 



of the archegonium is ruptured, at its base, all the upper part is carried 

 upwards, covering the capsule with a cap called the calyptra. This 

 remains as a covering to the capsule for only a comparatively short while. 

 It turns brown, and before it is finally thrown off it splits along one of its 

 tapering sides. 



The structure of the capsule is somewhat complicated (Fig. 407). The 

 body is shaped like a barrel, with a cone at either end. The seta is thin, 



PERISTOME LAYER 



OPERCULUM 



APICAL CELL 

 COLUMELLA 



RIM 



ARCHESPORIUM 



Fig. 406. — Mniiim hormim. Development of sporogonium from oospore. A to D, Early 

 stages in segmentation. E, Spindle-shaped embryo with apical cell at each end enclosed 

 bv calyptra formed from wall of archegonium. F, Upper half of embryo in more 

 advanced stage showing differentiation of capsule wall and central tissue. G, Operculum 

 and upper part of theca showing principal tissues differentiated. {From various sources.) 



with highly thickened outer layers of cells and a central strand very like that 

 of the gametophyte axis. At the base of the capsule the seta enlarges cone- 

 wise into the apophysis, a mass of tissue containing chloroplasts, which is 

 attached to the lower end of the barrel or theca and marked off from it by a 

 slight furrow. The apophysis contains an axial strand of conducting cells 

 continued upwards from the seta. It also possesses numerous stomata 

 (Figs. 408 and 409), very much like those of the Angiosperms (see Chapter 

 XXII), and a rudimentary palisade layer of chlorophyllous cells, by means 

 of which it can carry on photosynthesis. In the centre of the theca is a 

 column of large-celled tissue termed the columella, and around the 

 columella lies a layer of archesporium, from the cells of which the spores 

 are formed. Outside this is a cavity containing loosely attached cells, with 

 chloroplasts, partly filamentous in form, which wither away when the capsule 

 is ripe. The spores are liberated into the space thus left. The outer wall 



