

PREFACE. 



The present volume, containing an enumeration and description of all 

 the Ferns collected during the voyage of the United States Exploring 

 Expedition, under Captain Charles Wilkes, in the years 1838 to 1842, 

 inclusive, was commenced by me, at the request of the Joint Library 

 Committee of Congress, as far back as the year 1846; but, owing to 

 other business claiming my attention, it was not ready for the press 

 until near the end of 1848. Circumstances wholly beyond my con- 

 trol having delayed the printing for several years, I have had the op- 

 portunity of revising portions of the manuscript by the aid of some 

 works recently published. There are, doubtless, many other works, 

 or scattered memoirs, which should have been consulted, but which 

 were not within my reach. 



In this undertaking I have had to encounter many difficulties, 

 owing in the first place to the absence of a good botanical library in 

 Washington ; and in the second place, to the want of a collection of 

 authenticated species of exotic Ferns in this country ; a want which 

 must have been felt by every one who has had to study any extensive 

 collection of this family of plants. Ferns are so exceedingly diversi- 

 fied in the outline and division of their fronds, and many species of a 

 genus, and even of two distinct genera, so closely resemble each other 

 in their general aspect, that they often baffle the most sagacious bota- 

 nist, when he endeavours to determine the specimens before him by the 

 aid of descriptions only, or mainly, and especially by the short specific 



