Vlll PREFACE 



of the topics than the text affords. In recognition of this fact, 

 I have added, chiefly as footnotes, many references. Most of the 

 references are bulletins on the special topics, and in addition to 

 giving further information on the special topics, these references 

 introduce the student to that vast source of information contained 

 in the bulletins published by the U. S. Department and the ex- 

 periment stations of the different states. 



Many of the illustrations have been taken from the publica- 

 tions of various authors whose names or the names of their pub- 

 lications appear in connection with the illustrations. To these 

 authors I am much indebted. Most of the original illustrations 

 have been made by Mrs. Edith Martin, who has also given me 

 valuable assistance in other ways in the preparation of the book. 

 Also much credit is due Mr. H. S. Doty and Mr. L. E. Yocum, 

 my assistants, who have given me valuable suggestions. To 

 Dr. L. H. Pammel, who read some of the topics on Fungi and 

 offered valuable suggestions, I am also much indebted. 



The book no doubt has many faults, but I hope it has some 

 particular value and that the criticisms which teachers offer will 

 make me a more efficient teacher. 



J. N. MARTIN. 

 AMES, IOWA 

 Oct. 7, 1918 



