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A TEXTBOOK OF THEORETICAL BOTANY 



there is no well-marked annulus, though the theca has the usual thickened 

 rim. The peristome teeth are short and stout, formed of groups of sclerotic 

 cells, not of mere strips of cuticle as in Mmum (Fig. 425). They do not 

 move hygroscopically but form a ring of rigid teeth when the operculum has 

 fallen off. The top of the columella is expanded into a flat disc, the 



EPIPHRAGM 



ROSTRUM 



OPERCULUM WALL 



PERISTOME 



INNER 

 AERENCHYMA 



ARCHESPORIUM 



OUTER 

 AERENCHYMA 



CAPSULE WALL 



Fig. 424. — Polytrichtmi commune. Diagram of structure of operculum and 

 upper part of theca. (After Lorch.) 



epiphragm, which fills the space inside the ring of peristome teeth and 

 is attached to their tips (Fig. 426). The openings between the teeth thus 

 form a ring of pores like those round the top of a poppy capsule, and the 

 spores are dispersed through these pores by the force of the wind shaking 

 the sporogonium. The pores are left open by the drying and subsequent 

 shrinkage of the interior columella tissue. 



The spores on liberation germinate to produce a protonema, which is 

 similar in appearance to that of Mnium, and the new Moss plants arise as 

 vegetative buds from its branches in the usual way. The older filaments 

 of the protonema in Polytrichum sometimes become twisted together into 

 cable-like strands, as in the mycelia of some Fungi (Fig. 427). 



