The names of genera and species used in this work are in general accordance with the Code of Nomenclature recommended by the Nomenclature Commission of the Botanical Club of the American Association for the Advancement of Science, published in Bulletin of the Torrey Botanical Club 34: 167-178, 1907, to which reference is made. The synonyms given under each species in this work include the recent current names, and thus avoid any difficulty in identification.

The necessity for rules of nomenclature arose from the great confusion that has existed through the many different botanical names for the same species or_genera. Some species have had from 10 to 50 different names, and, worse still, different plants have often had the same name. For about 200,000 known species of plants there are not fewer than 700,000 recorded names. Such a chaotic condition of nomenclature is not only extremely unscientific, burdensome and confusing in itself, but the difficulty and uncertainty of identification which it causes in the comparative study of plants made it a serious and constant obstruction in the path of botanical inquiry.

The need of reform, and of finding some simple and fixed system of stable nomenclature, has long been recognized. This was clearly stated in 1813 by A. P. DeCandolle in his " Theorie Elementaire de la Botanique " (pp. 228-250), where he declares priority to be the fundamental law of nomenclature. Most systematists have acknowledged the validity of this rule. Dr. Asa Gray, in his " Structural Botany," says (p. 348): " For each plant or group there can be only one valid name, and that always the most ancient, if it is tenable; consequently no new name should be given to an old plant or group, except for necessity."

This principle was applied to Zoology in the "Stricklandian Code," adopted in 1842 as Rules of the British Association, and revised in 1860 and 1865 by a committee embracing the most eminent English authorities, such as Darwin, Henslow, Wallace, Clayton, Balfour, Huxley. Bentham and Hooker. In American Zoology the same difficulties were met and satisfactorily overcome by a rigid system of rules analogous to those here followed and now generally accepted by zoologists and palaeontologists.

At an International Botanical Congress held at Paris in 1867, A. DeCandolle presented a system of rules which, with modifications, were adopted, and are the foundation of the present rules of the botanists of the American Association. These rules were in part adopted also by the International Botanical Congress held at Genoa in 1892, and by the Austro-German botanists at their meeting in September, 1004.

The Botanical Club of the American Association for the Advancement of Science adopted rules for Nomenclature at meetings held in 1892 and 1893, which were followed in our first edition. An International Botanical Congress assembled at Vienna in 1905, and materially modified the Paris rules of 1867, and another Congress was held at Brussels in 1910. In the present edition the Code of Nomenclature recommended by the American Commission in 1907, is closely followed, as above stated.