vi PREFACE 



Aquatic Angiosperms are derived from terrestrial ancestors, 

 and have adopted the water habit at various times subsequent 

 to their first appearance as Flowering Plants. The hydrophytes 

 thus present the great advantage to the student, that they 

 form a group for whose history there is a generally accepted 

 foundation. Throughout the present study I have constantly 

 borne phylogenetic questions in mind, and the first three Parts 

 of this book may be regarded as a clearing of the ground for 

 the more theoretic considerations concerning the evolutionary 

 history of water plants to which the Fourth Part is mainly 

 devoted. In that section of the book, and sporadically in the 

 earlier chapters, I have set down such speculations as have 

 been borne in upon me in the course of a study of water plants 

 with which I have been occupied more or less continuously for 

 the last ten years. 



The literature relating to Aquatic Angiosperms has now 

 grown to such formidable proportions that I have felt the 

 necessity of trying to provide some clue to the labyrinth. With 

 this end in view I have given a bibliography of the principal 

 sources, which includes a brief indication of the nature and 

 scope of each work, with page numbers showing where it is 

 cited in the text. For the convenience of those seeking informa- 

 tion about any particular plant, I have indexed the families and 

 genera named in the titles enumerated, and in the notes regard- 

 ing the contents of each memoir. I found it impracticable to 

 compile a subject index to the bibliography, but the references 

 under the individual chapters to some extent serve this purpose. 



It is a pleasure to express my grateful appreciation of the 

 kindness of those botanists who have helped me in various 

 ways during the preparation of this book. I am particularly 

 indebted to Professor A. C. Seward, F.R.S. for valuable sug- 

 gestions and advice; to Dr H. B. Guppy, F.R.S. for reading 

 the pages in Part IV which treat of Distribution; to the 

 Hon. Mrs Huia Onslow (Miss M. Wheldale) for some helpful 

 criticism of the chapters dealing with physiological questions; 

 to Mr F. W. Lawfield, M.A. for aid in fenland botany; and 



