1. Sections of auxin-starved hypocotyls of Helianthus annuus do not show any significant growth rate in water of buffers of pH\>-6. However, in buffers with pH-values of approximately 4, elongation growth is observed; its rate is similar to the rate of auxin-stimulated growth (after 6 h incubation). \3- This phenomenon of acid-induced growth occurs also under anaerobic conditions in contrast to auxin-induced growth (Hager 1962). 2. Intact cell wall aggregates of Helianthus hypocotyls were obtained by complete plasmolysis of hypocotyls in 50% glycerol; cell wall associated enzymes were still active after this treatment, at least in part. While cell walls in solutions of pH\>-6 show only a small plastic extension during the first minute in response to a 50 g stretching force, a constant rate of elongation over longer periods of time (measured up to 1 h) is observed in weakly acid buffers. The highest rate of elongation is observed at about pH 4. This acid-induced plastic extension is completely inhibited by Cu(2+)-ions (5mM); the elongation of cell walls is apparently the consequence of an enzyme-catalysed increase in plasticity having a pH optimum of about 4. The pH optimum of acid-induced cell wall extension observed during stretching of plasmolysed hypocotyls coincides with the pH optimum of acid-induced growth of intact hypocotyl sections (around pH 4). 3. Under anaerobic conditions the growth rate of intact coleoptiles stays unchanged (at a low value) if the sections are incubated in a buffer of pH 5.0. Higher proton concentrations, however, stimulate growth immediately, whereas low proton concentrations are inhibitory (Fig. 7 and 8). The strongest initial growth response is elicited by buffers or acids of pH 3.9 (Fig. 9). Acid-induced growth of coleoptiles with a similar pH optimum is also found under anaerobic conditions. The growth of coleoptile cylinders can be switched on or off by repeatedly changing to acid or basic medium, respectively (under conditions of anaerobiosis) (Fig. 10). IAA-induced growth (aerobic conditions, pH≥5) can also be inhibited immediately by basic buffers or NaOH-solutions, and resumes after the pH value is lowered (Fig. 11). This pH-dependency may be taken as an indication that auxin affects the same reaction which is stimulated by high proton concentrations and which may be the last step in the process of cell elongation. CCCP, known to make membranes permeable for protons, rapidly inhibits the auxin-induced elongation growth (pH 6,5) when applied at a concentration which does not influence respiration; removal of CCCP shows that the growth inhibition by CCCp is partly reversible (Fig. 12). In contrast, acid-induced elongation growth (pH 4) shows inhibition by CCCP not before 10 min after application.-These findings suggest that auxin induces a proton accumulation in a cell wall compartment and, as a consequence, enzymatic cell wall softening. Such an accumulation could be brought about by an auxin-activated, membrane-bound, anisotropic ATPase or ion pump. The notion that ATPases...