The retention of pyrenoid CCM on land may be useful in the response of these polykiohydric organisms to variations in environmental stresses, such as high light, but also more importantly at fluctuating thallus water contents. Smith et al 1998, conducted a investigation on the effect of the uptake and release pools of CO2 in the lichen Peltigera membranaecea at varying thallus water contents using a method termed light dark transients. It can be inferred from the data that a 10 fold drop in CO2 up take and release pool sizes occur when thallus water contents decrease from optimal (5.1- 6.2 mg g-1 d.wt) to (2.3 mg g-1 d.wt) a 63% reduction in water content.
Additionally, calculations by Green & Snelger (1982), show that when Monoclea spp. (a liverwort with a solid thallus, with no air pores) is compared to Marchantia spp.(with water proof cuticle penetrated with air pores), maximum photosynthesis is only slightly greater in Marchantia, but air spaces giving greater advantage over Monoclea for water relations. However, the solid thallus of Monclea showed superior photosynthetic ability in very moist environments, possibly like the hornwort (with CCM).