The importance of plants in regulating and defining Earth’s greenhouse gas and water vapor composition has been previously demonstrated. This study addresses the relationship between the morphological and physiological response of paleo-plants to changing atmospheric gas compositions, which in turn lead to changes in atmospheric pressures. Higher atmospheric pressures are here suggested to alter plant gas exchange dynamics and Photosystem II activation. These effects increases plant bulk carbon dioxide, an important greenhouse gas, and water vapor transport leading to changes in Earth’s climate through alterations in the carbon cycle and hydrological balance. To elucidate this relationship, the response of two extant lycopod species, Selaginella kraussiana and Lycopodium lucidulum, was measured in response to an atmospheric pressure of 5kPa over current conditions. Results show that L. lucidiulum changed leaf shape, decreasing in stomatal density but increasing in stomatal index, in response to higher pressures and harbors a closer correlation with stomatal conductance values in response to stomatal index over maximal stomatal aperture values. S. kraussiana, exhibited an increase in stomatal density and index values in response to increased pressures and that its stomatal conductance values are more dependent on maximal stomatal aperture values than stomatal index This research demonstrates that paleo-plant stomatal indices are by themselves not accurate measures of atmospheric carbon dioxide or water vapor values as two extant paleo-plants of closely related phyla exhibit confounding results. These results suggest a reexamination of geological atmospheric conditions by showing that paleo-plant gas exchange can be influenced by atmospheric conditions other than carbon dioxide composition.