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PREFACE 



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A^ The study of any group of plants, with a view to the true 



relations of the species brought together in it, and their real his- 



^ tory, requires the investigation of a great number of individual 



^ states, and this often through a series of years. And there is, 



-"S perhaps, no family of plants in which extensive and continuous 



^ observation is more important than in the Lichenes. The present 



work is prepared, therefore, only as an Index and Introduction 



3-to the history of the plants described in it ; and it has been 



^ my object rather to state results of foreign study, than to offer 



the necessarily imperfect conclusions of my own limited re- 



\5" search. Use has constantly been made of the Lichenographia 



" Europma Reformata of Fries, whose characters of the sections 



■^ and genera I have, for the most part, adopted entire, or with 



\— only occasional amplification ; and the profound descriptions of 



the Lichenographia, as here applied to our species, constitute 



almost the whole descriptive value of the present enumeration. 



The exceptions to this will, I hope, meet with the indulgence 



which the extreme difficulty of any inquiries in a field as much 



neglected as this has been, among us, seems to demand.* 



* The later works of Fries have also been consulted, and, beside these, prin- 

 cipally those of Turner & I3orrcr, Eschweiler, and Schsrer, and, for American 



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