Studies on Phototropic Equilibrium and Phototropic-Geotropic Equilibrium in Phycomyces

Phototropic equilibrium was studied in Stage IV sporangiophores of Phycomyces blakesleeanus by illuminating specimens simultaneously from various directions with two beams of light. The dependence of the equilibrium position upon the angle between the light beams and upon their intensities was investigated and shown to be given by a simple empirical law. If a sporangiaphore is illuminated by a single light beam, an equilibrium position is reached between the direction of the beam and the direction of gravity. The dependence of this position upon the angle between the beam and the vertical was investigated and found to be given by another simple empirical law. The equilibrium position was found to be unaffected by changes in the intensity of the light over the entire range of intensities to which the sporangiophore gives a normal phototropic response. Geotropism does not occur in the dark however. Under certain specific conditions a sporangiophore in stable equilibrium shows regular oscillations around its equilibrium position. These oscillations are sinusoidal in form and often persist for 30 cycles, having a period of about 45 minutes and an amplitude of about 30 degrees. It is also possible for a sporangiophore, not necessarily in equilibrium, to show oscillations with a five minute period and a two degree amplitude. These two modes of oscillation appear to be independent of each other