x PREFACE 



more apparent. The volume will thus naturally fall into 

 two Parts, the first dealing with the purely physical aspect 

 of these phenomena, and the second attempting to present 

 in a more or less unified whole the physiological results 

 which have so far appeared in this connection, together with 

 their bearing upon each other and upon the vital problem as 

 a whole. Chapter IV of Part II was presented to the Faculty 

 of the Ogden Graduate School of Science of the University 

 of Chicago in candidacy for the doctor's degree in 1901. 



The author wishes here to express his thanks to Professor 

 C. K. Barnes, of this laboratory, and to Professor Jacques 

 Loeb, of the Hull Physiological Laboratory, for much valu- 

 able aid. Professor Barnes has kindly read the manuscript 

 and has made many suggestions. The author alone is, how- 

 ever, responsible for whatever new departures are to be 

 found in the book. 



B. E. L. 



THE HULL BOTANICAL LABORATORY, 



The University of Chicago, 



October 1, 1902. 



