Endogenous Stomatal Movements in Tradescantia Virginiana

S ummary The stomata of Tradescantia virginiana undergo rhythmic changes in aperture in either continuous light or darkness. Whilst these changes persist for several days in continuous light, in darkness the rhythm damps after one cycle. In continuous light the rhythm was characterized by susceptibility to phase‐shift and by a period of approximately 24 hours. In continuous darkness the single opening cycle was also susceptible to phase‐shift depending upon the timing of the previous light treatment. Stomatal opening and closing in response to short light‐dark treatments could be superimposed upon the endogenous rhythm.