ACKNOWLEDGMENTS 



The authors are deeply indebted to their associates at The Ohio 

 State University, to Dr. O. F. Curtis of Cornell Universit) , to Dr. C. L. 

 Wilson of Dartmouth College, and to Dr. W. F. Loehwing of the State 

 University of Iowa for their sympathetic and helpful criticism of the 

 manuscript. 



The illustrations have been secured from many sources, and are 

 properly credited to those who have furnished the photographs, nega- 

 tives, and materials from which photographs were made. We are 

 particularly indebted to the World Book Company for pemiission to 

 use some of the drawings and illustrations in the General Botany pub- 

 lished by them. Through the courtesy of the U. S. Forest Service, the 

 U. S. Soil Conservation Service, the U. S. Department of Agriculture, 

 and the New York Botanical Garden, it has been possible to secure 

 photographs suitable for illustrating the various vegetation types of 

 North America and some of the featvues described in other chapters. 

 Miss Jane Roller, of Washington, D. C, has made many of the new 

 drawings. Mr. Eugene B. Wittlake has made some of the diagrams 

 and most of the photomicrographs. Mr. Gordon Growl, Mr. Fred 

 Norris, and Mr. Clyde Jones have made photographs of experiments, 

 cultures, and some landscapes especially for this book. 



The greatest debt of all, of course, is to the thousands of fellow 

 scientists who cannot be mentioned individually but whose scientific 

 efforts have supplied humanit\^ with a fund of valuable data free for 

 all to use. A relativeh' few references appear at the end of certain 

 chapters, but it should not be assumed that we have always made 

 the best possible selections. 



E. N. T. 



H. C. S. 



L. H. T. 

 Columbus, Ohio 

 Evanston, Illinois 

 September 1, 1940 



