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, due 
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. 
