The surfaces of aquatic plants are not metabolically inert, yet much evidence seems to suggest that they exert little influence on the composition of the periphytic algal communities associated with them. Other evidence indicates a specificity of the algal communities to particular macrophyte species. Evidence is given here, from a variety of studies, that the influence of host type in determining periphyton community composition is greatest in infertile lakes, but, that in progressively more fertile water, external environmental factors become more important.The structure provided by aquatic plant beds, compared with the open water, to an aquatic habitat has many implications for the functioning of the ecosystem as a whole (Gaudet, 1974;Marshall & Westlake, 1978). The architecture provided by the plants is the physical basis for the niches of a much greater diversity of animals than is supported in the open water plankton and, on a microscopic scale, a far greater variety of algae and other micro-organisms is to be found in the weed beds than in the phytoplankton. Aquatic macrophytes, unlike rocks or sand grains, are not inert surfaces for the micro-organisms which grow associated with them; they absorb and secrete substances from and into the water close to them (Hasler & Jones, 1949; Khailov & Burlakova, 1969) and because of the reduced mixing of water within the plant community, may markedly alter the local water chemistry (Howard-Williams & Lenton, 1975;Dvorak, 1970;Carter, 1955). They must alter it even more greatly in the thin layers close to their surfaces in which live the periphyton community with which most aquatic plants are clothed. This community has an additional complexity of its own and includes algae, bacteria, fungi, protozoa, and other small invertebrates. This paper is concerned with the algal component, which is divided into the epiphytes, which are firmly attached by pads of mucilage, raphes, or stalks and the metaphyton, which includes species loosely draped around the plant surfaces and those freely moving among the attached epiphytes. Though essentially free living, the metaphyton is closely associated with the plant surfaces.The periphyton is apparently disadvantageous to its macrophyte host for it absorbs much light that would otherwise reach the plant surface (Phillips,
