iv PREFACE 



professional obligation to him, and I do so with the utmost 

 satisfaction. 



It is also a great pleasure to express my grateful apprecia- 

 tion of the help I have received from my associates in this 

 University, Dr. Anstruther A. Lawson (Botany), Professors 

 Herman De C. Stearns (Physics), Frank M. McFarland 

 (Histology), William A. Cooper (German), and especially 

 from Professor Douglas H. Campbell. I am indebted also 

 in various ways to my friends Professor William F. Ganong 

 of Smith College, and Dr. Hermann von Schrenck of Wash- 

 ington University, St. Louis. Professor Walter R. Shaw of 

 the University of Oklahoma generously allowed me to use 

 his excellent photograph of Postelsias reproduced on page 

 190. Dr. Daniel T. MacDougal of the New York Botanical 

 Garden and his publishers, Messrs. Longmans, Green & Co., 

 have given me permission to use the figures of Mimosa on 

 pages 248-9. Professor Goebel of Munich kindly sanctions 

 my use of his figure of Opuntia shown on page 210. To these 

 and all others who have assisted me I wish most heartily to 

 express my sincerest thanks. 



G. J. P. 



Leland Stanford Junior University, 

 California, December, 1902. 



