Early Silurian land plants (plants living 450 Million years ago), inferably similar to the living hornwort (my experimental plant), could have made use of a pre-existing pyrenoid CO2 Concentrating Mechaism (CCM) - inherited from an algal ancestor - not because CO2 was in short supply in the aerial environment; (7000ppm CO2; 20 times greater than today's CO2 conc.), but in response to higher temperature and more intense light conditions compared to the water column. The land CCM, in the Silurian atmosphere could have responded to high Photosynthetically Active Radiation (PAR) as my experimental hornwort CCM’s did; by increasing efficiency for CO2 in the photosyntheic process at the cost of a reduced light use efficiency; the CCM acting as - a ''light use efficiency reducer'' (based on results) - and at high temperatures the CCM up regulates, also increasing efficiency for CO2, acting as - an ''oxygenase reaction reducer'' - (based on results). The hornwort CCM thus can be likened to a primitive stomata, regulating CO2 uptake, in response to light, temperature and thallus water content variations. The eventual loss of the pyrenoid CCM, and the move towards a more advanced morphology, (as seen in the C3 liverworts), meant CO2 was no longer being pumped around the active site of Rubisco, thus early C3 liverwort Rubisco kinetics would have improved efficiency, moreover, early C3 liverworts evolved new chloroplast architecture to deal with high PAR in a more efficient way. Also, the evolution of pores, to reduce water loss, made the early C3 liverworts less dependant on water to photosynthesise, than the desiccation prone solid thallus of the hornwort.
