Excised segments of corn (Zea maysL., Bear Hybrid WF 9×38) coleoptiles show a strong“spontaneous”increase in growth rate about 4.0 hr after excision. The response can be delayed about 2 hr using a brief (10 min) exposure to IAA during the latent period. An established spontaneous growth response can be suppressed by a 30 to 60 min exposure to auxin and does not reappear until about 2.5 hr after withdrawal of the hormone. During the 3 hr period following withdrawal of exogenous auxin there is a two-fold increase in magnitude and a three-fold decrease in latent period of a growth response to a sub-optimal level of auxin. The data are consistent with the hypothesis that the spontaneous growth response is caused by a time-dependent change in sensitivity of isolated tissue to auxin and/or a change in the endogenous level of auxin.Apical sections ofZeacoleoptiles with the tip intact do not grow at the rapid rate one might expect of tissue with an endogenous auxin supply. Instead they grow very poorly and exhibit both a weak spontaneous growth response and a poor response to exogenously supplied auxin. Indirect evidence suggests that this is due to the production of a growth inhibitor by the
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