EXTERNAL APPEARANCE. 21 



with wliich we are familiar : for this we might be prepared 

 by the simphcity of their structure, and we find that in 

 decaying, there is often a degree of roughness produced on 

 the stems by the midrib surviving the decay of the expanded 

 portion of the leaf. 



A nerve or supporting midrib is very common in the 

 leaves of Mosses, and sometimes there are two arising from 

 a common centre. They vary much in length, thickness, 

 direction, etc., and important specific characters are afforded 

 by these variations. 



No external apertures resembling the stomata or breath- 

 ing apparatus of more perfect plants have been observed 

 on the leaves, though doubtless some such provision must 

 exist to enable them to absorb moisture so easily as they 

 do, after being dried. This property of reviving under the 

 application of moisture, is one of much importance to the 

 collector, who may not be able to examine liis specimens as 

 soon as they are gathered, or may receive them from a 

 distance, for though months and years may elapse, they wiU 

 at once, when moistened, assume their original form, though 

 the tints in most are apt to fade considerably"^. 



* It is proper to meution here, that such as are submitted to very great 

 pressure in order to make fine specimens, do not so readily revive. 



