MOSSES 75 



will, however, soon convince us that this is not 

 so, and that, on the contrary, special means have 

 been taken to meet special circumstances, in the 

 shape of a very delicate round membrane re- 

 sembling a diminutive drum-head, and probably 

 from this resemblance called the tympanum (the 

 Latin name for a tambourine or drum) ; this in 

 reality forms the end of a small rod or column, 

 known as the columella, which occupies the centre 

 of the capsule, and round which the spores are 

 formed. This delicate membrane, then, fills up 

 the space which is left uncovered by the peri- 

 stome teeth, and thus the mouth of the capsule 

 is effectually closed, the spores, when ripe, being 

 allowed to escape through the spaces between the 

 teeth. This is illustrated by figs. 10 and 14 of 

 Plate IV.b, while at Plate V. fig. 4 the tympanum 

 is in position, and the peristome teeth are closing 

 round it; fig. 10 of the same plate gives the 

 larger tympanum of the Common Hair-moss 

 (Polytrichum commune), and the notches in its 

 edge show where the peristome teeth have been 

 attached. Such is yet another instance of Nature's 

 care for the welfare of her children. 



In many mosses the peristome, instead of being 

 formed of teeth, consists of a number still always 

 a multiple of four of long and extremely fine 

 hair-like bodies, which are twisted into a beautiful 

 coil or spiral. At fig. 10 of Plate II. is a drawing 

 of the capsule of the Wall Screw-moss (Tortula 



