PREFACE. 
In the Lichen Flora of Southern California, the first of the papers 
of this volume, the author, Dr. Hermann Edward Hasse, has brought 
together in synoptical form the results of many years of study of 
the lichens of that region. During the course of his investigations 
Doctor Hasse has been fortunate in having the cooperation and 
assistance of several distinguished European lichenists, upon whom 
he has relied mainly for the determination and description of new 
species. The method of classification followed is adapted from that 
proposed by Dr. A. Zahlbruckner in the Natiirlichen Pflanzenfamilien 
of Engler and Prantl. 
With respect to the use of generic names the situation at present is 
essentially as noted in connection with Professor Fink’s work on the 
Lichens of Minnesota (published in volume 14 of the Contributions, 
1910), since the principles which must govern a fundamental revision 
of these names remain unsettled. For several reasons it has seemed 
advisable in the present work, also, as there, to limit citations to 
the names of authors, except for species which have been recently 
described or renamed. In these instances full citations are given, 
mostly verified from the original sources. 
The second paper, by Mr. William R. Maxon, Associate Cura- 
tor of the National Herbarium, is the fourth of his series on tropi- 
cal American ferns, and presents brief revisions of several small 
groups which have been much misunderstood. Attention is called 
also to the confusion which exists similarly in many, if not in most, 
groups of tropical American Pteridophyta, and to the practicability 
of meeting this condition by the publication of numerous short 
synoptical reviews, dealing with the species by genera, subgenera, 
or even smaller groups. Such a practice, if seriously and generally 
undertaken, could not fail to yield beneficial results. 
During recent years the United States National Herbarium has 
received large collections of grasses from various parts of North 
America. Some of the larger genera have been revised and their 
nomenclature adjusted upon a type basis. It is planned to continue 
these revisions until all the North American species have been elabo- 
rated in thig manner. Since revisions ought to include, as a part of 
the usual taxonomic study, an examination of type specimens, 
several years must elapse before the task is completed. To meet the 
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