Why do Polytrichaceae have lamellae?

Light-response curves of chlorophyll-fluorescence parameters from 12 species of Polytrichaceae show photosynthetic electron flow giving a good fit to negative-exponential saturation curves up to high irradiances; 95%-saturation irradiances for the species of unshaded habitats approached or exceeded noon summer sunlight. The species showed little or no indication of the non-saturating electron flow at high irradiances, or the very high values of non-photochemical quenching, seen in many other mosses of dry, sun-exposed habitats. Calculation of CO2 diffusion rates from first principles on a cell-area basis indicates that photosynthesis of mosses with unistratose leaves is likely to be limited by CO2 diffusion resistance at high irradiances. In Polytrichaceae the area for CO2 uptake is often >6 times the projected leaf area. By lowering the whole-leaf resistance to CO2 uptake, the lamellae remove the CO2-diffusion constraint that limits mosses with unistratose leaves and allow Polytrichaceae to utilize efficiently the high irradiance of direct sunlight.