GEOGRAPHICAL LIMITS, AND DISTRIBUTION.--The southern limit of the territory covered by the present work is the same as in the later previous editions, viz. the southern boundary of Virginia and Kentucky. This coincides better than any other geographical line with the natural division between the cooler-temperate and the warm-temperate vegetation of the Atlantic States. The rapid increase of population west of the Mississippi River, and the growing need of a Manual covering the flora of that section, have seemed a sufficient reason for the extension of the limits of the work westward to the 100th meridian, thus connecting with the _Manual of the Flora of the Rocky Mountain Region_ by Prof. Coulter. These limits, as well as that upon the north, have been in general strictly observed, very few species being admitted that are not known with some degree of certainty to occur within them. The extreme western flora is no doubt imperfectly represented. The distribution of the individual species is indicated somewhat more definitely than heretofore in many cases, so far as it could be satisfactorily ascertained. The extralimital range is also sometimes given, but the terms "northward," "southward," and "westward" are more frequently employed, signifying an indefinite range in those directions beyond the limits of the Manual. Where no definite habitat is specified, the species may be understood as found more or less generally throughout the whole area, or at least to near the western limits. NOMENCLATURE, ACCENTUATION OF NAMES, etc.--In case of question respecting the proper name to be adopted for any species, Dr. Gray's known and expressed views have been followed, it is believed, throughout the work. While reasonable regard has been paid to the claims of priority, the purpose has been to avoid unnecessary changes, in the belief that such changes are in most cases an unmitigated evil. Synonyms are rarely given except where changes have been made. As a guide to correct pronunciation, the long sound of the accented vowel (modified often in personal names) is indicated, as heretofore, by the grave accent (`), and the short sound by the acute (´). In regard to the derivations of generic names, many valuable suggestions have been due to W. R. Gerard, Esq., of New York.