This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
A general Key of the Orders and Families has been prepared by Dr. Britton according to the method followed in the Keys to the genera and species. This general Key has been elaborated on the natural method, dividing the two subkingdoms of plants described in the work into Classes, Subclasses, Orders and Families successively. The Orders are not described in the work itself, but their principal distinguishing characters are given in this key. The natural method adopted necessitates a considerable number of exceptions to statements, owing to the varying degree of development of floral organs in the derivation . of plants from their ancestors; these exceptions are either noted under the headings or indicated by cross-references.
In using this key, or any of the keys to genera or to species, the student will often find, in the analysis of a plant that it does not provide all the information necessary for its determination; this is generally owing to the incomplete condition of the specimen collected; it may be in flower, while the characteristic differences between it and others are only to be found in the fruit, or vice versa; or the species may be dioecious, or polygamous, when its other organs, perchance the characteristic ones, must be sought on another individual, and there are various other causes for incompleteness. It is therefore earnestly recommended that collections be carefully made, seeking to reduce as far as possible this more or less necessary incompleteness. Where satisfactory material can not be obtained, it will usually be found possible to reach the desired analysis by following out two or more lines of the key, and by comparing the results reached with the descriptions to determine the family, genus or species. The illustrations provide an almost indispensable aid in such cases.