THE PHOTOTROPIC RESPONSES OF AVENA IN RELATION TO INTENSITY AND WAVE-LENGTH

¹ From the Laboratory of Biophysics, Columbia University, New York City

The reaction time for positive phototropic responses of Avena sativa for short exposures to white light decreases with increasing intensity up to about 1,000 meter-candles, and then increases. The response curve shows two parts, indicating a double photoreceptor process. The two response curves are rectangular hyperbolas, from which it is deduced that the speed of the initial reaction time process is a logarithmic function of intensity.

When only the first 1.5 mm. of coleoptile tip is stimulated, only one response curve is obtained. When all but the upper 3 mm. of coleoptile is exposed, the other single curve is obtained. Hence the two photoreceptor systems have separate loci.

The relation between reaction time and relative energy found when homogeneous lights of various wave-lengths fall on naked and partly shielded coleoptiles yields two distinctly different spectral sensibility functions, a tip and a base response. While Bergann seems to have observed only base responses, the single functions reported by four other investigators appear to represent tip responses in spectral regions of greater tip sensibility and base responses in regions of greater base sensibility.

The bearing of these findings on the theory of phototropism is discussed.

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