GRIMMIACEAE 113 



especially when the plants are well fruited and the 

 operculum shows bright red at the tips of the branches. 

 The stems are more branched than any of the acrocarp- 

 ous mosses except Rhacomitrium (p. 114) and the Ortho- 

 trichaceae (p. 116), but the fruit is always terminal, 

 which shows the plants are not pleurocarpous mosses. 

 The common species of Orthotrichum (p. 117) and 

 Ulota (p. 119), that grow on rocks and most closely 

 resemble Grimmia, have the capsule more exserted, but 

 if the plants are not in fruit the looser, darker growth 

 of Grimmia, with stems usually longer and more slen- 

 der, are the only distinguishing characteristics. In the 

 pleurocarpous mosses, Hedwigia (p. 172) grows on 

 rocks and has the capsule almost concealed by leaves, 

 but it is lateral instead of terminal as in Grimmia, and 

 the plants usually occur in larger patches, are more 

 branched, and are a grayer green. See Andreaea 

 (P- 75). 



20. RHACOMITRIUM Brid. (Rha-co-mi-tri-um) 



A large cosmopolitan genus. Named from the Greek 

 for a '' frayed cap," referring to the calyptra, which is 

 often ragged at the base. 



Rhacomitrium aciculare (L.) Brid. is the common 

 species of this genus, and is found on rocks along 

 streams, especially in mountainous or hilly regions. 

 The fruit is not common. 



