Agnes Arber (1879-1960) was a prominent British botanist specialising in plant morphology and comparative anatomy. In 1946, she became the first female botanist to be elected a Fellow of the Royal Society. First published in 1920, this volume provides a detailed anatomical study of aquatic flowering plants, with a discussion of their evolutionary history. Arber describes the general anatomical and reproductive organs, life histories and physiological adaptations of aquatic plants in detail, with interpretations informed from her previous experimental work. The final section of this volume discusses the evolutionary history of aquatic plants in the light of affinities to terrestrial flowering plants. Arber's account of aquatic plants was the first general description of these plants published, and provides a classic example of the comparative anatomy studies which were central to botanical investigation during the early twentieth century. An extensive bibliography and over 170 illustrations are included in this volume.