PREFACE ix 



of Agriculture and at the experiment stations and agri- 

 cultural colleges* Special acknowledgment is due to 

 C. E. Leighty and H. B. Humphrey for help in pre- 

 paring certain chapters, and to C. W. Warburton for 

 excellent criticism of manuscripts. Drawings for many 

 of the illustrations have been made by A. B. Boettcher, 

 J. H. Stevenson, W. E. Chambers, J. Marion Schull, and 

 F. H. Hillman. The greater part of the mechanical 

 construction of the text has been performed by Amanda 

 Faught Carle ton, who also helped much with criticisms 

 and suggestions. Credit is given for each illustration, 

 not original, in the list of illustrations. 



M. A. CARLETON. 

 WASHINGTON, D.C., 

 January, 1916. 



