viii PREFACE 



suggestions. I wish to thank Mr and Mrs Clement 

 Reid, Prof. MacDougal of the Arizona Desert Labo- 

 ratory, Prof Campbell of Stanford Universit}, Prof 

 F. H. Knowlton of the United States National 

 Museum, Washington, Mr A. G. Tansley, Prof Yapp, 

 and Mr W. R. Welch for photographs which they 

 have allowed me to reproduce. As on many previous 

 occasions, I am indebted to my wife for contributing 

 drawings. 



A. C. SEWARD. 



Botany School, Cambridge. 

 July 1911. 



The niiinbers in brackets interspersed in the text refer to the 

 Bibliography at the end of the volume. 



