The exudation of sap from the decapitated stem by solution-cultured root systems of tobacco was measured continuously under constant conditions. The nicotine content of the sap was analyzed as an indicator of solute originating in the root cells. The typical, persistent endogenous rhythm of exudative activity was observed in both volume and nicotine content of sap. The rhythmic control was independent of respiration and of ionic changes designed to influence permeability, but it was reversibly affected by actinomycin D, which implicates DNA function. The removal of potassium or oxygen from the culture solution or the addition of cyanide to it caused a brief transient stimulatory response in exudative rate. Continued anaerobiosis reversibly stimulated exudation in a massive final cycle. These observations support the concept that exudation is a passive phenomenon, of solute leakage and osmosis, dependent on active salt accumulation.
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