The Effects of Decapitation, Lack of Oxygen, and Low Temperature on the Endogenous 24-hour Rhythm in the Growth-rate of theAvenaColeoptile

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Abstract
Further work on the endogenous rhythm in the growth-rate of the Avena coleoptile was carried out by the methods previously described. Decapitation of the coleoptile does not prevent induction of the rhythm, nor does it affect the phase of a rhythm previously established. When seedlings are transferred to darkness and are simultaneously deprived of oxygen by replacing the surrounding air with nitrogen, a rhythm is induced, but the first peak is delayed for a period approximately equal to that of the nitrogen treatment. Provided the period in nitrogen is not excessively prolonged, the retardation of the second peak is less than that of the first. This suggests that the rate of the time-keeping mechanism has become accelerated. Since only a part of the initial retardation in the rhythm is thus eliminated, a change in phase persists. The younger the seedlings at the time of treatment, the smaller is this persistent change. When the nitrogen treatment is commenced some hours after the transfer to darkness, the retardation of the second peak is greater than when treatment and transfer are simultaneous. In each series the retardation increases exponentially as the length of the period in nitrogen is extended. Retardation followed by acceleration was also observed after the seedlings had been exposed to low temperature. It seems probable that induction of the rhythm involves the synchronization of independent time-keeping mechanisms already in existence, rather than the establishment of an entirely new system.

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