PREFACE TO THE FOURTH EDITION. 



The original intention of this work was to provide 

 students with an account of the chemistry and physio- 

 logical significance of some of the more important 

 substances occurring in the plant. The founding of 

 Chairs of Biochemistry during recent years, with the 

 consequent dissemination of biochemical knowledge, 

 would appear to give just cause for the discontinuance 

 of the present work, but it is not possible for all students 

 to avail themselves of the facilities offered, and it is 

 primarily for such students that the work is intended. 



The enormous output of papers and the recent 

 advances in knowledge have necessitated much revision, 

 and, in the main, the present edition has been rewritten. 

 In so doing we have borne in mind the requirements of 

 those approaching the subject from different angles and 

 have included a certain amount of somewhat elementary 

 information, both botanical and chemical, and also have 

 admitted certain rather more advanced aspects of the 

 subject even though they be matters of controversy. 



We fully recognize that this involves some dispro- 

 portion, some lack of balance, but this is inevitable. 



R H. 

 T. G. H. 



June, ig28. 



