MOSSES 79 



colour, in this instance, is a peculiarly rich crimson. 

 It will be noticed that the tooth is pierced by two 

 tiny holes, so small, indeed, as only to be visible 

 when a fairly high magnifying power is used. 

 This feature, though so minute, is practically 

 constant, and may sometimes form a useful aid in 

 identification. In such a case the teeth are said 

 to be cribrose. Pig. 12 of the same plate shows a 

 few teeth of both the outer and inner peristome 

 of another member of the Bryum family (JBryum 

 murale), and will give some idea of the form that 

 the inner peristome commonly takes. 



I have before referred to the fact that the teeth 

 of the peristome are very sensitive to changes in 

 the atmosphere, being easily affected by the 

 presence or absence of moisture ; as a rule they 

 open when the air is damp, and close when it is 

 dry, as, indeed, we might not unnaturally expect 

 them to do, judging from the moisture-loving 

 nature of these little plants. Not unfrequently, 

 however, we come across a case precisely the 

 reverse of this, and in which, as soon as a dry 

 capsule is moistened, the teeth of the peristome, 

 which but a moment before were standing erect, 

 will close down tightly over the capsule mouth, 

 as if to effectually shut in the spores. Plate IV. 

 fig. 12 is a capsule of the Sessile Grimmia 

 (Grimmia apocarpa), a moss which grows in dry- 

 looking tufts of an olive-green colour, on walls and 

 rocks, often choosing the uppermost stones of some 



