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As the most basal monilophytes, eusporangiate ferns can provide key insights into the origins of plant physiological adaptations. The genus Equisetum, the most morphologically and physiologically unusual genus of eusporangiate ferns, has a stomatal apparatus that is unique among all plants. Patterns of stomatal diffusive conductance (g(w)) were measured in the giant horsetail, Equisetum giganteum L. in southern South America. Maximum g(w) values (<200 mmol.m(-2).s(-1)) were low in comparison with typical angiosperm leaves, but were in the range measured in other pteridophytes. The range of measured g(w) was similar in contrasting environments of the Atacama Desert and northwestern Argentina. Stems in shade had a significantly lower g(w) than those in light. Developing stems had a higher average g(w) than mature stems. Stomatal conductance was higher for upper stem internodes than for middle internodes. Late-morning g(w) was primarily related to stem diameter, stem surface temperature, and interactions among these factors and vapor pressure deficit (VPD), light, elevation, and groundwater salinity. Equisetum giganteum likely has a passive system of stomatal regulation depending on overall stem turgor and red light. The stomatal conductance of patterns of this species exhibited a diurnal pattern typical of other pteridophytes, despite its unusual structure.