Introduction. 



7 



for under the most favorable conditions, especially with extensive root sys- 

 tems, the photograph is always made at the expense of detail, many of the 

 finer branches and root-ends being obscured. 



The writers wish to acknowledge the faithful assistance of Miss Annie 

 Mogensen and Mrs. F. C. Jean in the execution of the drawings of the root 

 systems. Grateful acknowledgment is made to Professor J. C. Russel, of the 

 Department of Agronomy of the University of Nebraska, for many helpful 

 suggestions in connection with the analyses of soils and the experiments con- 

 concerning the absorption of nutrients, as well as for the generous use of his 

 laboratories in pursuing this phase of the work. To Mr. O. R. Clark we are 

 indebted for valuable field assistance during 1920, and to Mr. E. Y. Lipetz 

 for the translation of certain papers from the Russian. It is a pleasure to 

 acknowledge the helpful suggestions given by Dr. F. E. Clements and Dr. 

 R. J. Pool throughout the period of the work. To both Dr. Clements and 

 Professor T. J. Fitzpatrick the writers are indebted for the reading of the 

 manuscript and proof. 



