PROMINENT CHARACTERS are indicated by the use of _Italic type_ for the leading distinctions of the Orders, and generally in the specific descriptions for those points by which two or more nearly allied species may be most readily distinguished. The ready discrimination of the genera is provided for by a Synopsis of their leading characters under each order. Whenever a genus comprises several species, pains have been taken to render important differences conspicuous by proper grouping, and when needed by a series of subordinate divisions and subdivisions. The headings of these various groups are to be considered as belonging to and forming a part of the specific characters of the several species under them,--a fact which the student should always bear in mind. ARRANGEMENT OF THE ORDERS.--The Natural Orders are disposed in very close accordance with the method followed by Bentham and Hooker in the _Genera Plantarum_, the principles of which are concisely shown in the Synopsis of Orders which precedes the Analytical Key. The _Gymnospermæ_ are retained as a Subclass following the Angiospermous Dicotyledons, with which they have an obvious relationship, in preference to placing them, as some authorities would do, next before the Pteridophytes, to which their affinity, if no less certain, is nevertheless obscure. A more natural arrangement than either would be the withdrawal of the Endogens, placing them at the beginning, in perhaps an inverse order. ANALYTICAL KEY TO THE ORDERS.--As stated in Dr. Gray's Preface to the last edition, this is designed to enable the student to refer readily to its proper Order any of our plants, upon taking the pains to ascertain the structure of its flowers, and sometimes of the fruit, and by following out a series of easy steps in the analysis. It is founded upon the most obvious distinctions which will answer the purpose, and is so contrived as to provide for all or nearly all exceptional instances and variant cases. Referring to the Order which the Key leads him to, the student will find its most distinctive points brought together and printed in Italics in the first sentence of the ordinal description, and thus can verify his results. The Synopsis which follows will then lead him to the genus, to be verified in turn by the full generic description in its place; and the progress thence to the species is facilitated, when there are several to choose from, by the arrangement under divisions and subdivisions, as already explained.