ACKNOWLEDGMENTS
As I send this book to the press, I feel a great sense of gratitude for the great amount of aid that I have received.
I wish to express my feelings of deep appreciation to a group of Philadelphians, who, under the leadership of Mr. J. Levering Jones, formed the University of Pennsylvania Tree Crops Exploration Committee. This committee assisted in making possible some of the journeys incident to gathering the facts that have helped to make this book: Mr, and Mrs. J. Levering Jones. Morris L. Clothier. Eckley B. Coxe. Jr.. Cyrus H. K. Curtis. George W. Elkins. Charles C. Harrison. Charles E. Ingersoll. J. Bertram Lippincott. J. Franklin McFadden. Mr, and Mrs. Louis C. Madeira. Joseph G. Rosengarten. William Jay Turner. John Weaver. J. William White. G. Searing Wilson. Stuart Wood. Dr. George Woodward. Charlton Yarnall.
Professor Howard H. Martin of the University of Cincinnati spent many weeks searching for material.
Valuable letters have been sent to me by many private individuals; also by agricultural scientists in the employ of the United States Department of Agriculture, the state universities, the state experiment stations, and foreign experiment stations. From among these many I must make especial mention of David Fairchild. H. P. Gould. C. A. Reed. Walter T. Swingle. William A. Taylor, and Raphael Zon, all of the United States Department of Agriculture.
For the troublesome job of reading chapters of another man's manuscript, I am indebted to Dr. W. C. Deming, of Hartford. Connecticut; J. Ford Wilkinson, of Rockport, Indiana; J. M. Westgate. Director. Agricultural Experiment Station. Honolulu; Robert Forbes. Ex-Director. Agricultural
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