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The flower-stem elongation of Arabidopsis thaliana, race $quot;Estland$quot;, a Cruciferae plant, was promoted by incandescent light, but was inhibited by fluorescent light. The length of the flower stem increased with the period of incandescent illumination; consequently earlier flowering was obtained. The response of the plants to these lights was essentially due to the difference in the spectral emission of red and far-red radiation. The difference in these light effects was modified by application of certain growth regulators. The inhibitory effect of fluorescent, light on the flower-stem elongation was reversed, and elongation was promoted, by gibberellic acid. On the other hand, the promotive effect of incandescent light was reversed and elongation was inhibited by maleic hydrazide treatment, whereas indole acetic acid appeared to be either not effective or independent of the light action.
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