vi PREFACE 



The substance of the following chapters was pre- 

 sented in the form of a course of lectures at Stanford 

 University during the past year. These lectures have 

 been carefully revised, and a number of drawings pre- 

 pared, which it is hoped will be helpful in elucidating 

 the text. 



Very little originality can be claimed for the matter 

 presented, beyond its arrangement. The writer has 

 availed himself freely of the materials accumulated 

 through the labors of botanists during the past fifty 

 years, which have made possible such a general presen- 

 tation of the subject as has been here attempted. It was 

 intended, at first, to prepare a bibliography of the more 

 important works bearing on the subject ; but it was 

 soon evident that the magnitude of a bibliography, 

 which would be in any way complete, rendered this 

 impracticable. 



Most of the drawings have been made by the author 

 from nature. Where these have been borrowed, dua 

 acknowledgment has been made. 



Special thanks are due my colleague, Dr. G. J. 

 Peirce, for most valuable assistance in the reading of 

 the proofs. 



DOUGLAS HOUGHTON CAMPBELL. 



STANFORD UNIVERSITY, 

 November, 1898. 



