MOSSES 69 



its characteristic features ; a remark, by the way, 

 which applies with greater or less force to practi- 

 cally every part of a moss. The teeth of the 

 peristome, like other portions of the plant that we 

 have met with, are very sensitive to the presence 

 or absence of moisture in the atmosphere, and 

 open or close according to the influence which 

 is thus exerted upon them. Its function is sup- 

 posed to be to control the escape of the spores 

 from the capsule, either by closing down, and so 

 shutting them in, should the state of the sur- 

 rounding air be unfavourable to their germination, 

 or by standing erect, when outside conditions are 

 such as to promote their growth. Plate V. fig. 1 

 shows, on a somewhat large scale, the peristome 

 of the beautiful capsule of the Swan-neck Thyme 

 Thread-moss (Mnium hornum), and fig. 9 of 

 Plate IV.b gives the entire capsule. In both 

 cases the teeth are standing erect, while in 

 fig. 6 of Plate V. the peristome of the Dwarf 

 Swan-neck Moss (Campylopus pyriformis), they 

 are even bent back (recurved). On the other 

 hand, in fig. 2 of the same plate, which gives 

 the peristome of the Velvet Feather- moss (Brachy- 

 thecium velutinum), they have closed down over 

 the capsule mouth. 



The number of teeth of which the peristome is 

 composed is always either four or a multiple of four. 

 There are only two British species in which we 

 find so few as four ; and though one of these, the 



