xii Preface by the 'Editor 



discussions and demonstrations to a form suitable for 

 permanent record, together with certain other alterations 

 which have been duly submitted to the author. The 

 original phraseology has been preserved as far as pos- 

 sible. The editor wishes to acknowledge material as- 

 sistance in this work from Miss A. M. Vail, Librarian of 

 the New York Botanical Garden. 



D. T. MACDOUGAL. 



New York Botanical Garden, October, 1904. 



PREFACE TO THE SECOND EDITION. 



THE constantly increasing interest in all phases of evo- 

 lution has made necessary the preparation of a second 

 edition of this book within a few months after the first 

 appeared. The opportunity has been used to eliminate 

 typographical errors, and to make alterations in the form 

 of a few sentences for the sake of clearness and smooth- 

 ness. The subject matter remains practically unchanged. 

 An explanatory note has been added on page 575 in order 

 to avoid confusion as to the identity of some of the plants 

 which figure prominently in the experimental investiga- 

 tions in Amsterdam and New York. 



The portrait which forms the frontispiece is a repro- 

 duction of a photograph taken by Professor F. E. Lloyd 

 and Dr. W. A. Cannon during the visit of Professor de 

 Vries at the Desert Botanical Laboratory of the Carnegie 

 Institution, at Tucson, Arizona, in June, 1904. 



D. T. MACDOUGAL. 

 December 15, 1905. 



