The main results presented in this article may be summarized as follows:

1. A test method with deseeded Avena seedlings for small concentrations of auxin and precursors of auxin has been described.

2. This method makes possible quantitative determinations of about ten times as low concentrations of hormone as can be obtained with the standard method, (a) Through an increase in the time of the test, so that nearly all the hormone applied can be utilized. (b) Through an increase in sensitivity of deseeded plants to unilaterally applied small concentrations of hormone.

3. The effect of deseeding in relation to curvature growth is primarily the prevention of auxin regeneration through the removal of the material for auxin synthesis, and in addition the prevention of physiological aging.

4. The mechanism of auxin synthesis in the tip of the coleoptile and the mechanism of auxin regeneration in the new physiological tip have been shown to be identical.

5. The application of the deseeded method is illustrated by determinations of auxin in primary leaves and coleoptile sections of Avena seedlings.

6. The deseeded method has been used as a test method for precursors of auxin obtainable from the coleoptile and from other sources. The method further makes possible a distinction between auxins and these substances which may become activated by the plant.

7. Evidence for the existence of a precursor of auxin in the plant is given (a) indirectly by determinations of the decrease in auxin synthesis in deseeded plants. (b) Directly by its isolation from the plant.

8. Precursors of hetero-auxin are demonstrated; their chemical nature and activation are briefly considered.

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