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SCIENCE PROGRESS 



two rows of them) and they are inserted around the rim of the 

 bowl-like spore receptacle which forms the moss capsule. The 

 teeth in many cases exhibit active and rather complex move- 

 ments but the common moss, Bryum argenteum, provides a fairly 

 simple example. If the ripe capsule of this moss be examined 

 after the operculum (which covers the teeth like a candle 

 extinguisher) has been removed, the mass of spores will be seen 

 to be imprisoned by the infolded triangular teeth of the peristome 

 curving inwardly towards a common centre above the spores. 

 If the air be dry the teeth will presently curve outwards and 

 downwards from the rim, leaving the spores fully exposed. The 

 moisture of the breath is sufficient, however, to cause them 

 instantly to bend upwards and centrally, so as again to cover 



B c 



Fig. 2.— Fissidens, sp. 



A, capsule ; b, upper part of capsule with operculum removed, showing the peristome teeth in the damp 

 position ; c, peristome teeth in dry position ; d, longitudinal section of peristome tooth, showing the 

 different arrangements of the striae on the inner (/) and outer (o) surfaces. 



the spores. The movements in this moss are very rapid and 

 render it a favourable species for experiment. When one of the 

 teeth is examined it is at once seen to consist of a membrane 

 only and it is obvious that there are differences between the 

 inner and outer surfaces ; these are striated but whereas the 

 striation runs longitudinally on the inner face, it is transverse 

 on the outer surface. In some mosses these relations are 

 reversed but the same corresponding difference is generally to 

 be observed. As in the Rose of Jericho, the pits, so here the 

 striation affords a criterion as to the direction of greatest 

 absorption and loss of water and of the elongation or shortening 

 of one surface of the peristome tooth ; the circumstance that the 

 striations differ in the manner indicated renders hygroscopic 

 movements inevitable. The exact character of the movements 

 in a particular case will vary according to the structure of the 



