vl PREFACE 



The author is especially indebted to his colleague, Professor G. J. 

 Peirce, for valuable assistance in the preparation of Chapter XIII ; 

 to Professor W. R. Shaw for many microscopical slides, which were 

 of great service in making many drawings, as well as for the use of 

 several photographs. Other photographs were furnished by Dr. 

 J. C. Branner and Dr. F. M. MacFarland of Stanford University, 

 and Professor W. Trelease of St. Louis. To all these gentlemen 

 the author wishes to express his sincere thanks. 



DOUGLAS HOUGHTON CAMPBELL. 



STANFORD UNIVERSITY, 

 March, 1902. 



