INTRODUCTION xlx 



HOW FERNS ARE CLASSIFIED. 



The ferns are classified in groups chiefly by the 

 shape and position of the sori and indiisia, so it is 

 necessary, in most cases, for the beginner to ob- 

 tain a fertile frond in order to identify the species 

 with certainty. In some species the indusium is 

 quite evanescent, and the specimen should be ob- 

 tained before the indusia have withered. Since the 

 different species of ferns mature at various times 

 throughout the season, the student must be watch- 

 ful from early summer until fall in order to secure 

 specimens when in the proper stage. The begin- 

 ner will doubtless make some mistakes at first. 

 The young fronds of some of the large species will 

 be examined until the absence of any fruiting por- 

 tion discloses their identity and one recognizes an 

 old acquaintance. Moreover, some species are ex- 

 ceeding variable in the cutting of their fronds. 

 In fact, constancy can hardly be said to be the rule 

 among the ferns, and it is a matter of much obser- 

 vation and study before one can expect to recog- 

 nize at once the different species in their various 

 forms. 



In the study of the ferns a good pocket lens is 

 indispensable. The specific differences of some of 

 the species are often quite minute. 



The nomenclature adopted in this book is that 

 of Gray's " Manual." The territory covered is, 

 in general, the eastern half of the United States 

 north of the Gulf States and the adjacent portion 

 of Canada. 



