Experiments were made to determine the effect both of shading and of the level of the water-table on the growth of S~hagnum. Growth in mass is reduced by shading but is little affected by water level. Growth in length is greatest with high water levels and reaches a peak when about 80% of the incident radiation is absorbed by shading. Etiolation (straggliness) increases with shade and with higher water levels. There is little interaction between the effects of shade and water level. Different species behave differently; there is some indication that growth is greatest in a species' 'natural' environment although on average, production is the same for all three species investigated. (2) Investigations were also made to ascertain how Sphagnum modifies the supply of light and water to its growing apex. Suitable instruments for measuring light flux in a Sphagnum canopy and moisture profiles in a core are described. The attenuation of light in a Sphagnum profile follows Beer's Law with depth (or more precisely with cumulative dry mass). Attenuation is affected by the previous environment. Shaded conditions and a high water-table both reduce absorbance, an inverse relationship with etiolation. Moisture profiles in Sphagnum cores with various water levels are shown. Profiles of the volume occupied by pores of different sizes are obtained. S. rubellum has a smaller average pore size than S. papillosum and is able to maintain a larger water supply to the apex. A hysteresis effect due to the previous history of the watertable level is investigated. (;) A computer model of a Sphagnum lawn is shown to be capable of simulating the growth of the moss and in particular to reproduce the observed increase in 'surface roughness' with high water levels and shading. In mixed lawns of two species, each is shown to 'out-grow' the other in its own 'natural' environment.