We investigated the anatomical and physiological characteristics of stenophyllous leaves of a rheophyte, Farfugium japonicum var. luchuence, and sun and shade leaves of a non-rheophyte, F. japonicum, comparing three different populations from coastal, forest floor, and riparian habitats. Light adaptation resulted in smaller leaves, and riparian adaptation resulted in narrower leaves (stenophylly). The light-saturated rate of photosynthesis (P max) per unit leaf area corresponded to the light availability of the habitat. Irrespective of leaf size, the P max per unit leaf mass was similar for sun and shade leaves. However, the P max per mass of stenophyllous leaves was significantly lower than that of sun and shade leaves. This was because the number and size of mesophyll cells were greater than that required for intercellular CO2 diffusion, which resulted in a larger leaf mass per unit leaf area. Higher cell density increases contact between mesophyll cells and enhances leaf toughness. Stenophyllous leaves of the rheophyte are frequently exposed to a strong water flow when the water level rises, suggesting a mechanical constraint caused by physical stress.