Abstract
In order to be successful in a given environment a plant should invest in a vein network and stomatal distribution that ensures balance between both water supply and demand. Vein density (D(v)) and stomatal density (SD) have been shown to be strongly positively correlated in response to a range of environmental variables in more recently evolved plant species, but the extent of this relationship has not been confirmed in earlier diverging plant lineages. In order to examine the effect of a changing atmosphere on the relationship between D(v) and SD, five early-diverging plant species representing two different reproductive plant grades were grown for 7 months in a palaeo-treatment comprising an O(2):CO(2) ratio that has occurred multiple times throughout plant evolutionary history. Results show a range of species-specific D(v) and SD responses to the palaeo-treatment, however, we show that the strong relationship between D(v) and SD under modern ambient atmospheric composition is maintained following exposure to the palaeo-treatment. This suggests strong inter-specific co-ordination between vein and stomatal traits for our study species even under relatively extreme environmental change. This co-ordination supports existing plant function proxies that use the distance between vein endings and stomata (D(m)) to infer plant palaeo-physiology.