The artificial keys herewith presented proceed on the same plan and are to be taken, as such keys always are, not as definitive in any case, but simply as an aid to help the student more speedily to reach a probably satisfactory description. FOOTNOTES: [1] _The North American Slime Moulds_, 1899. [2] Schrader, _Nova Plantarum Genera_, 1797, pp. vi-vii. [3] Cf. Edgar W. Olive, _Monograph of the Acrasieae_; Boston, 1902. PREFACE TO THE SECOND EDITION The first edition of this little book having been exhausted long ago, the writer in this second issue takes opportunity to correct sundry errata, typographical and other, and at the same time to incorporate such new information in reference to individual species and to the subject entire as the researches of more recent years may afford. To Miss Gulielma Lister, of London, the writer expresses his sense of deep obligation for much assistance in settling difficult matters of nomenclature and identification; it will be found as a result that in most instances the same thing in the two volumes, English and American, appears under the same name. There are still differences; these result in most cases from different points of view, different estimates or emphasis of characteristics in these ever elusive objects. To Professor Torrend, formerly of Lisbon, the writer is indebted for a set of European types, and to Professor Bethel, pathologist of Denver, for rich material from the fertile mountains of Colorado and California. To Professor Morton Peck, of Oregon, we are indebted for many notes of the color of plasmodia and for collections of Pacific coast forms. Mr. Bilgram, of Philadelphia, read the manuscript of the genus _Physarum_ and has contributed many rare species. To Dr. Sturgis, of Massachusetts, we are indebted for material from both east and west. The present volume is intended especially for American readers and is accordingly particularly devoted to a discussion of species so far reported on the western continent; nevertheless it has seemed wise to include a brief description of some other forms as well, and reference to many extra-limital species now generally recognized will be found here and there in connection with the more extended treatment of related American forms. _February twenty-eight, 1921._ At the last moment, nearly all plates and drawings of the first edition disappeared! necessitating a quick renewal of drawings and plates. This may in part explain lack of uniformity, and various minor irregularities sure to grieve the intelligent student. BIBLIOGRAPHY The following are the principal works consulted in the prosecution of the investigations here recorded:-- 1763. Adanson, M. Familles des Plantes. 1805. Albertini--see under Schweinitz. 1841. Annals and Magazine of Natural History. London, various volumes: 1841, Ser. I., vol. vi.; 1850, Ser. II., vol. v. 1887. Annals of Botany, vols. i-xxxi.