Heterophylly, the production of different leaf forms on the same plant, is a widespread phenomenon in terrestrial and aquatic plants and provides an opportunity to study how sessile organisms sense and respond to changes in environmental factors. Nymphaea odorata subsp. tuberosa (American White Water Lily) produces 2 distinct leaf forms: a floating surface leaf and an aerial form in which the lamina is held above the water. Previous research suggests that changes in the light environment may be a critical determinant of heterophylly in Nymphaea. In this study, we tested the hypothesis that changes in light quantity and light quality stimulate the production of aerial leaf forms in water lilies. Specifically, shade cloth was used to reduce light intensity (quantity), and varying plant densities were used to increase leaf cover (affecting light quality) in artificial ponds. Aerial leaf production was not stimulated by reduction in light quantity alone but was when leaf cover exceeded 30–40%. We suggest that as the surface of a pond becomes covered with a canopy of leaves, American White Water Lily responds with the production of aerial leaves that rise above the surface of the water to gain access to light. Interestingly, water lilies exhibit an atypical shade response in that aerial leaves have short, thick petioles that allow them to rise above the surface of the water, rather than displaying the elongated phenotype associated with etiolation, which is the typical shade response of other flowering plants.