PRE FA CE Vll 



to Dr. W. E. Joseph of the Montana Agricultural College, and 

 to Professor J. M. Evvard of the Iowa State College for many 

 valuable suggestions and criticisms, and he extends to them his 

 sincere thanks. He also thanks the Illinois Experiment Sta- 

 tion for the use of unpublished data ; and The Macmillan 

 Company, The Chicago Medical Book Company, The W. B. 

 Saunders Publishing Company, Bailliere, Tindall and Cox, 

 Professor H. W. Mumford, United States Department of 

 Agriculture, United States Bureau of the Census, Ohio Ex- 

 periment Station, Wisconsin Experiment Station, Kentucky 

 Experiment Station, Connecticut Experiment Station, Penn- 

 sylvania Experiment Station, Vermont Experiment Station, 

 Missouri Experiment Station, Iowa Experiment Station, Cor- 

 nell Experiment Station, and Illinois Experiment Station for 



the use of illustrations. 



SLEETER BULL. 



Urbana, Illinois 

 July, 1916. 



