PREFACE 
TO 
THE THIRD EDITION 
TEN years’ experience of the use of this book with students 
has led me to make many alterations in the details of 
the treatment of the subject. In a few places consider- 
able expansion has seemed called for, particularly in the 
presentation of the genefak physiology sketched in the 
second chapter. Hence certain sections of the book 
have been re-written. In other places re-arrangement 
has proved advantageous—particularly in the case of the 
section dealing with the energy of the plant. This has 
accordingly been re-cast, and the chapter on respiration 
has been incorporated with it, a change partly carried out 
in the second edition. 
The past ten years have seen many advances made in 
experimental work and in the suggestion of new theories. 
I have endeavoured to incorporate as much of this ag 
seems to me sound, and I have eliminated certain of the 
older statements which more recent work has shown to be 
certainly or probably erroneous. 
In what I have added I have dealt with the correlation 
of internal structure with physiological need to a greater 
extent than in the earlier editions, and have examined 
