1. A new method is described which gives a continuous record of the absolute rate of protoplasmic streaming in epidermal cells of the Avena coleoptile.

2. With this method a study was made of the influence of malate and iodoacetate on streaming velocity, in order to make correlations with the previously established effects of these substances on growth and respiration.

3. In the presence of optimum concentrations of indole-3-acetic acid in freshly cut sections, malate had no effect on streaming. In the presence of very low concentrations of the auxin, however, malate increased the range of response, so that the threshold of auxin sensitivity was lowered some ten times by the malate. Malate alone had no effect on streaming.

4. In coleoptile sections, soaked overnight in sugar solution or in water, the acceleration of streaming normally caused by auxin almost disappears, but the presence of malate causes large accelerations of streaming by the auxin.

5. Similarly, in sections from old coleoptiles, which no longer show acceleration of streaming by auxin, the acceleration is restored when malate is added together with the auxin.

6. Malate does not enter the cell as rapidly as does auxin, but easily detectable amounts penetrate within 30 minutes.

7. Iodoacetate in the concentration which inhibits growth (5 x 10–5 M) completely inhibits the acceleration of streaming by auxin. In still lower concentrations iodoacetate slightly accelerates streaming. Higher concentrations, up to 2 x 10–4 M, did not reduce the rate of streaming below that of controls without auxin. The effect of iodoacetate is therefore to inhibit the acceleration caused by auxin and not to affect the basal streaming rate.

8. It is concluded that, just as for growth and respiration, malate is necessary for the response to auxin shown by acceleration of streaming. This further strengthens the triple parallel between the effects of auxin on streaming, growth, and respiration, all of which are apparently mediated by the 4-carbon acid system.

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