iv PREFACE 



Professors Washburn and Brunc^r for the loan of plates; 

 to the Iowa Experiment Station for use of figures, and the 

 Ohio Experiment Station for a number of photographs for 

 original use here. Professors Hine, Metcalf, Barrows, 

 Mr. Kostir and Mr. Drake have assisted in reading manu- 

 script and proof and have generously given the author 

 the use of photographs and drawings. 



H. O. 

 Columbus, Ohio, 191G. 



