I approached the study of water plants with the hope that the consideration of this limited group might impart some degree of precision to my own misty ideas of evolutionary processes. Botanists seem to be universally agreed that the 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 information about any particular plant, I have indexed the families and genera named in the titles enumerated, and in the notes regarding 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.
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