This book provides an introduction to recent analytical and experimental studies of plant growth in the sea. The physiology and ecology of marine plants are, therefore, emphasized at the expense of a more traditional taxonomic or morphological treatment. The physics and chemistry of the marine environment are examined with specific reference to the requirements of marine plants, and much of the book concentrates on those aspects of their physiology which are unique to marine plants, or which help us to understand their ecology. Since over 90% of the species of marine plants are algae, most of the book is devoted to the marine representatives of this group, with examples from all oceans and coasts of the world where detailed work has been done. Phytoplankton and seaweeds are discussed together in chapters on photosynthesis, growth and productivity, and geographical distribution, in order to provide an integrated picture of the biology of marine plants in general. There is, however, a deliberate bias towards the seaweeds in certain chapters (e.g. morphogenesis, rocky shore ecology, economic utilization) since the ecology and physiology of these plants have received less attention in books at this level than has the ecology of phytoplankton. Marine angiosperms are also discussed alongside the autotrophic algae, and the ecological roles of bacteria and fungi in the sea are covered in a separate chapter.