To understand the association between leaf form and leaf optical properties, we examined light absorption variations in the leaves of Mentha aquatica L., an amphibious freshwater macrophyte. Specific absorption of leaves of M. aquatica showed a 7.5-fold variation, decreasing as pigment per unit area increased. This relationship indicates that dispersive samples, such as leaves, although efficient light traps, can also be affected by the "package effect." Mentha aquatica leaves, by expanding their biomass (increased specific leaf area [SLA]), improve their light absorption efficiency per unit of both pigment and leaf biomass. Changes in leaf biomass expansion were mainly a result of changes in leaf density, and as a consequence, leaf density appears to be a better descriptor of light absorption efficiency in M. aquatica leaves than does leaf thickness. Light absorption efficiency per unit of leaf biomass was also enhanced by increasing pigment content. Our results indicate that M. aquatica produces two types of leaves: (1) thin, less dense, and highly pigmented leaves and (2) thick, denser, and low pigmented leaves. The first type shows higher light absorption efficiency per unit of leaf biomass, which may allow the plant to achieve a better carbon balance under light limitation. The second type shows lower light absorption efficiency per unit of leaf biomass. Although it is unclear whether this reduction affects plant performance, a reduction in net carbon income per unit of absorbed photon may result in a reduction in the specific plant growth rate (RGR). Understanding the association between leaf form and the ability of leaf biomass to absorb light provides a mechanistic explanation for the empirical relationship found repeatedly in the literature between RGR and SLA. Our results offer a quantitative basis to explain part of the association between biomass expansion, pigment investment, and plant growth.