In the preparation of both the first edition and of the second we have had valued cooperation from many botanists, which is here gratefully acknowledged. The late Professor Thomas C. Porter contributed much to the first edition by suggestion, specimens, and the examination of proof sheets. Mr. Eugene P. Bicknell has contributed specimens studied for both editions and read the proof sheets of the first. Dr. John K. Small has assisted in the preparation of both editions, contributing the entire text of several families, and has read the proof sheets of the second. The Pteridophyte text was contributed to the first edition by the late Professor Lucien M. Underwood, and to the second edition by Mr. William R. Maxon. The text of the Grass Family has been written by Mr. George V.'Nash for both editions; many of the drawings of grasses made by Mr. Theodore Holm for the first edition were supervised by Professor F. Lamson Scribner. The late Mr. Charles E. Smith critically examined the final proof sheets of the first edition. Mr. Frederick V. Coville has contributed the text of Juncaceae to both editions. The late Dr. Thomas Morong wrote the text of several families for the first edition. The text of the Carrot Family in both editions has been examined by Dr. J. N. Rose. Most of the drawings for the first edition were supervised by Dr. Arthur Hollick.

For the second edition Mr. Kenneth K. Mackenzie has contributed the text of Carex, and supplied many specimens for study; Mr. W. W. Eggleston has written the text of Crataegus; Dr. Ezra Brainerd has written the text of Viola; Dr. Per Axel Rydberg has aided in the determination of specimens; and many others have aided by specimens, notes and information.