A comparison of photosynthesis in two thalloid liverworts

Standard infra-red gas analysis techniques were used to compare the photosynthesis of the liverworts Marchantia foliacea Mitt. and Monoclea forsteri Hook. Parameters measured include net photosynthetic rates, light response curves, quantum efficiencies, diffusive resistances to CO₂ and water, apparent photorespiration and chlorophyll content. A series of morphological measurements were also made to determine resistance of pores and the 'mesophyll' to dorsal surface ratio, A _mes/A. Marchantia has a cuticularised thallus with the photosynthetic tissues arranged in air chambers giving an A _mes/A of 9 whilst Monoclea has a solid thallus, A _mes/A of 1. Both species are shade adapted and it was found that whilst the air chambers were advantageous for water relations they increased maximum photosynthesis only slightly. Calculations showed that the solid thallus would be photosynthetically superior in very moist environments. The results are discussed with reference to existing ideas on the evolution of the structure of land plants.