x PREFACE 



made available through the kindness of Professor Andrew Boss. 

 The many helpful suggestions made by Dr. C. V. Piper, Consult- 

 ing Editor of these publications have been of great value. 



Several illustrations have been supplied by investigators who 

 have made intensive studies of particular crops; credit for these 

 has been given in connection with the illustrations. Most 

 of the other figures are from photographs by T. J. Horton, 

 official photographer at University Farm, St. Paul. Figures 

 on flower structure are from drawings made by G. D. George, 

 illustrator. 



The papers of many investigators have been referred to in 

 the text, as the advanced student will frequently desire to 

 study the original publication. The possibilities of errors are 

 very great in a text which reviews the studies of numerous 

 investigators. The writers, therefore, earnestly invite the 

 criticism of the readers. 



THE AUTHORS. 



UNIVERSITY OF MINNESOTA, 

 June, 1921. 



