Differential Growth and Phototropic Bending in Phycomyces

Using present knowledge of the cell's optical and growth mechanisms, a theoretical bending speed of about 5° min.^-1 is calculated for unilateral irradiation by a single beam of normally incident visible light; this figure is of the magnitude found experimentally. Between beams of light opposed at 180°, the resultant bending speed is given by the difference-to-sum ratio of the light intensities of the two beams. Valid comparisons between cells differing in size, growth speed, or optical properties are made by expressing bending speed as a fraction of each cell's bending response to unilateral irradiation. With multiple beams differing in intensity and azimuth, the resultant bending speed follows from vector addition of phototropic components proportional to the flux fraction of each beam. The bending speed in Oehlkers' experiment where a luminous area is the light source also appears compatible with this rule. In such experiments, the bending speed quantitatively matches the scaled asymmetry of the pattern of flux incident upon the cell. Resolution experiments support the assumption that light intensity enters into steady state phototropic formulations as the first power of I.