The Change of Tropism in Polyporus brumalis Stipes and the Effect of Directional Stimuli on Pileus Differentiation

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Abstract
The directional influences of light and gravity upon the development of pileate and epileate sporophores of Polyporus brumalis, produced in pure culture, have been investigated. Growth in the dimitic fruit-body has a considerable subapical component which in the stipe is responsible for tropistic curvature. The stipe is competent to react negatively to unilateral gravitational and positively to unilateral photic stimulation throughout development, but when both stimulioperate phototropism masks geotropism. If illuminated from one side the growing epileate stipe is strongly positively phototropic but as the pileus reaches a characteristic diameter the stipe becomes negatively geotropic. Experiments with changing direction of illumination and with artificial pilei of black paper suggest that the change of tropism is explicable by the shading action of the expanding pileus on the sub-pilcal photoperceptive and photoreactive region of the stipe. Developing sporophores continuously rotated with reference to fixed directions of gravitational and light stimulation and others with stipes inverted as a result of illumination from below during development have in common that the morphologically upper surface of the pileus always develops towards and approximately at right angles to the direction from which the maximum light intensity is received. Normal but inverted stipes and dissepiments show no tendency to geotropic reorientation.