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PREFACE. 



This work is primarily intended as a text-book for the use of 

 students in colleges and universities, but will also be found useful to 

 the specialist. Although the title designates it as a text-book of 

 general lichenology, the discussion of the Basidiolichenes and more 

 or less problematical Gasterolichenes has been omitted, since these 

 orders are not represented in the northeastern United States. 



The systematic arrangement of the families and genera here pro- 

 posed is by no means perfect, yet it is hoped that some progress has 

 been made toward establishing a natural system. The fact that 

 most arrangements heretofore proposed had little scientific basis is 

 due to the lack of knowledge of the morphology and physiology of 

 these plants. Even now our methods of physiological investigation 

 are too imperfect to allow us to obtain any satisfactory data in regard 

 to the life-history of the individual lichen. Until our methods of 

 investigation are more perfect, all systems of classification must of 

 necessity be more or less artificial and subject to changes. 



In lichenology the conception of genus and species is vague and 

 uncertain. This is the chief reason why every lichenographer has 

 introduced or sought to introduce a new system, and as a result we 

 have about as many systems as there are authors. Within compara- 

 tively recent years various investigators have studied lichens from a 

 scientific point of view, especially from the standpoint of morphol- 

 ogy. Of these morphologists may be mentioned Hedlund, Lindau, 

 Minks, Reinke, Schwendener, Zahlbruckne and Zukal. Among 

 those who have studied lichens from a physiological standpoint Bon- 

 nier and Jumelle deserve special mention. The arrangement of 

 families and genera here adopted is based upon morphological data 

 and is in accordance with the results of recent investigations. 



In agreement with Reinke, lichens are treated as a distinct class, 

 coequal in systematic importance with the fungi and algae. The 

 present conception of the class Lichenes is, however, essentially differ- 



