CHAPTER II. 



DESCRIPTIONS OF THE FAMILIES AND GENERA OC- 

 CURRING IN THE NORTHEASTERN UNITED 

 STATES. 



INTRODUCTION. 



An attempt has been made to make keys as convenient aids for 

 the use of beginners in the study of the families and genera. Al- 

 though they are supposed to indicate natural arrangements of the 

 several groups it must be kept in mind that such an arrangement is 

 as yet very unsatisfactory, especially as regards many of the lower 

 genera. 



The occurrence of exceptions and intermediate forms constitutes 

 the great stumbling block in the preparation as well as the use of any 

 key. This difficulty gradually diminishes as the student becomes 

 more and more familiar with his subject. Since many of the higher 

 lichens never produce apothecia it is evident that the divisions of the 

 key based upon the apothecial characters can not be made use of in 

 their determination. A superficial general knowledge of lichens 

 will, however, enable the student to give such sterile forms their 

 generic position. It is urgently advised to begin with the study of 

 fertile forms and to omit the consideration of sterile ones until a fair 

 understanding of the limitations of the genera has been obtained. 



The student of lichenology is absolutely dependent upon the use 

 of a good compound microscope ; no key or description can be made 

 which will make it possible to avoid the use of this aid. It is also 

 necessary to make good hand-sections of the thallus (inclusive of sub- 

 stratum when possible) and apothecium ; these sections can be most 

 satisfactorily studied when mounted in water. 



Before attempting to use keys the student is advised fo study the 

 descriptions of the. families and genera. Since the descriptions are 

 simply the expression of our knowledge of the subject, they are ac- 

 cordingly perfect or imperfect as the case may be. 



It is necessary to call attention to the extreme variability of 

 lichen-structures, not only in size but also in form and coloration. 

 In the same species different sections will show that the variation in 



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