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 general 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 as 

 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 



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