viii Preface 



The photographs from which most of the illustrations were 

 made are from the files of the Office of Horticultural and 

 Pomological Investigations, Bureau of Plant Industry, 

 U. S. Department of Agriculture, and are here used with the 

 approval of the Chief of that Office. Although most of 

 them were taken by the author, credit for their use belongs 

 to the Department. Further acknowledgment is made 

 in the List of Illustrations. Finally, the Editor and Pub- 

 lishers have made concessions that should not pass unre- 

 corded. 



H. P. Gould 

 Washington, D.C, 

 June 8, 1918 



