vi INTRODUCTION. 



sule, and then the peristome precisely agrees with that 

 of Neckera, to which perhaps the genus ought to be 

 united. In those mosses which make yearly shoots, 

 these sometimes arise so near the point of insertion of 

 the fructification as to make the fruitstalk appear late- 

 ral, which is especially the case in the genus Bartramia. 

 -Even the calyptra of some mosses seems to be inter- 

 mediate, having so slight a fissure, that we are doubtful 

 which we should call that of Cmclidotus and of Splach- 

 num: sometimes in Triclwstomum^ besides the short 

 fissure at the base, we see in Tr. microcarpon a single 

 longitudinal cleft reaching three-fourths of the way up, 

 making it appear a truly dimidiate calyptra. Such top 

 is the case with the Tr.funale of Schwaegrichen, which 

 gave him occasion to say of it " Calyptrre forma ab affi- 

 nibus Trichostojnis etiam recedit et rursus, calyptram 

 ad definitiones genericas adhibendam non esse, demon- 

 strat." In this, and indeed in all the previously men- 

 tioned cases, the question is to be decided by the habit 

 of the plant which thus has its share of influence in the 

 formation of Genera. 



As to what regards the species, although very constant 

 in their minute characters, they, as well as other plants., 

 vary according to exposure, soil, humidity, elevation at 

 which they grow, and a variety of other circumstances. 

 Jt is not therefore surprising that these varieties should 

 be raised to the rank of species by those who have not 

 had it in their power to devote the time and attention 

 necessary to the observing them abroad in their various 

 places of growth, and in the closet to microscopical re-- 

 searches. Frequent leisure, various journeys made pur.- 

 posely through various parts of our happy islands, an,d 

 ^specially in the more alpine parts of Scotland and Ire- 



