In continued darkness following a standard pretreatment of 12 hours light and 12 hours dark, the stomata of Tradescantia virginiana did not reveal a truly repeating rhythm, but only a single opening cycle which matched the opening phase of the rhythm in light. An investigation of the phase response of this dark opening cycle to light and dark treatments revealed that the phase of the dark opening cycle could be manipulated as follows. When light commenced during that phase characteristic of darkness in nature, the dark opening cycle showed phase-shift, but little or no phase-shift occurred when light commenced during that phase characteristic of light in nature. The fact that the rhythms can be manipulated in this manner may indicate the plant's ability to adapt to different day lengths.
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