Stress-relaxation parameters were compared under different experimental conditions using 5th internode segments of light-grown pea seedlings and coleoptile segments of dark-grownAvenaseedlings. The following results were obtained.1. In a short incubation period at 25°C, IAA caused a decrease in the minimum relaxation time, To, of the epidermal cell wall of pea internodes when it induced elongation; the optimum concentration of IAA for decreasing To was 10 mg/liter.2. At all concentrations of IAA used, 0.1–1000 mg/liter, the relationship between the To value of the epidermal cell wall peeled from segments incubated for 2 hr and the subsequent elongation rate in 2–3 hr incubation was linear, indicating that the To value of the cell wall at a certain time regulates the rate of the following elongation.3. When segments of pea epicotyls orAvenacoleoptiles were incubated in mannitol solution of various concentrations in the presence and absence of IAA and then allowed to grow in the absence of both mannitol and IAA, the segments extended differently depending upon the mannitol concentration, which was less than 0.3 M, given during preincubation.4. The Toandb(relaxation rate,ΔS/Δlog t) values were smaller in the cell wall of segments which extended more, than in those which extended less. In this case, 0.2 M mannitol solution was most effective, since it inhibited IAA-induced elongation during pre-incubation and the segments thus incubated extended the most afterward.5. Extensibility, mm/gr, seemed to parallel the elongation which had occurred during pre-incubation, indicating that this value, contrary to To, represented at least partly the result of elongation.From these results we concluded that the growth rate to follow is regulated by the minimum stress relaxation time, To, and possibly by the relaxation rate,b, of the cell wall before extension, and these parameters may represent certain biochemical modifications of the cell wall components needed for cell ext
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