xxiv INTRODUCTION. 



reaching three-fourths of the way up, making it appear a truly 

 dimidiate calyptra. Such, too, is the case with the T. funale of 

 Schwaegrichen, which gave him occasion to say of it, " Calyptrse 

 forma ab affinibus Trichostomis etiam recedit et rursus calyptram 

 ad definitiones genericas adhibendum non esse, demonstrat." In 

 this, and indeed in all the previously mentioned cases, the ques- 

 tion is to be decided by the habit of the plant, which has thus 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, and elevation at which they grow, and a 

 variety of other circumstances. It is not, therefore, surprising 

 that these varieties should be raised to the rank of species by those 

 who have not had it in tfieir power to devote the time and 

 attention necessary to the observing them, abroad, in their different 

 places of growth, and, in the closet, to microscopical researches. 

 Frequent leisure, various journeys, made purposely through most 

 parts of our happy islands, and especially in the more alpine dis- 

 tricts of Scotland and Ireland ; added to a constant use of the 

 microscope at home, in the examining of our own collections, and 

 references to the descriptions of others, have, we hope, in many 

 instances, enabled us to correct errors in preceding authors, to 

 separate species from varieties, and to detect marks and characters 

 indicative of species in what had before been undecided, or only 

 considered as varieties of well known individuals. On the form 

 of the leaf undoubtedly much stress is to be laid ; and in its ser- 

 ratures, and particularly in the absence or presence, the length, 

 the breadth, and various conformation of the nerves, so much 

 insisted on by Mohr, characters will frequently be found when 

 they fail in almost every other part of the plant. 



But it is not solely on our own investigations that we wish to 

 rely for many of the facts brought forward in these sheets. 

 Several friends, both at home and abroad, have kindly contributed 



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