In the more ecology-conscious world that has developed since the late 1960s, it has become almost a cliché to observe that ‘all flesh is grass’ or — to put it more scientifically — that the whole of the biosphere depends on the conversion of radiant solar energy to chemical energy that occurs in the process of photosynthesis. What is sometimes overlooked is that, although photosynthesis provides chemical energy and carbon skeletons for biosynthesis (both in the form of carbohydrates), it does nothing to provide other essential requirements, such as nitrogen and sulphur for cell proteins, phosphorus for nucleic acids and membrane lipids, and the many other elements that are necessary for the final structure of enzymes, pigments and cell walls. Since the supply of these elements may not always match the supply of energy and organic carbon provided by photosynthesis, there is not always a direct relationship between photosynthesis and plant growth. This is particularly true of many marine plants growing in their natural habitat, and the separate discussion of photosynthesis in the present chapter and of growth in Chapter 4 emphasizes the differential effects of environmental factors on the two processes. The primary productivity of an ecosystem, whether it is defined in terms of energy fixation (i.e. kj m-2 yr-1) or change in biomass (i.e. kg C m-2 yr-1), depends on the rate of photosynthesis per unit area of surface, and this is equal to the photosynthesis per unit of biomass multiplied by the amount of biomass per unit area (or the ‘standing crop’).