IV PREFACE. 



acknowledgment is made of the indispensable help given by Dr. Edith Clem- 

 ents, who has assisted throughout the field work, made the larger number of 

 the photographs, and read the manuscript. The latter has also been read by 

 Dr. H. M. Hall and Dr. J. E. Weaver, to whom acknowledgment is given. 

 The author is indebted to Dr. Frances Long and Miss Rena Huber, who have 

 aided in many ways in the preparation of the manuscript and the illustrations. 

 I*rints for several of the forest illustrations have been furnished by Dr. G. E. 

 Nichols and Mr. C. G. Bates, and the graphs have been drawn by Professor 

 F. C. Kelton, to whom due acknowledgment is made. 



Frederic E. Clements. 

 Tucson, Arizona, April 1919. 



