| Fig. 142. | Fig. 143. |
Fig. 142.—Response of leaflet of Mimosa to light applied below: transient positive followed by pronounced negative curvature.
Fig. 143.—Response of leaflet of Averrhoa, to light applied above: transient positive followed by pronounced negative curvature.
Up-curve represents up-movement, and down-curve, down-movement.
NEGATIVE PARA-HELIOTROPISM.
Response of leaflet of Averrhoa carambola: Experiment 144.—The leaflets of this plant, and also those of Biophytum sensitivum fold downwards under action of strong light, applied above or below. In these leaflets diffuse electric stimulation induce a fall of the leaflets demonstrating the greater excitability of the lower half of the pulvinule. The analysis of reaction under light is rendered possible from the record of response of leaflet of Averrhoa, given in Fig. 143. Light of moderate intensity from an incandescent electric lamp acted from above: the result was a feeble and short-lived positive response, quickly reversed to strong negative by transmission of excitation to the more excitable lower side. Illumination from below gave rise only to strong positive response. Thus in Averrhoa the effect of continuous light applied above or below is a downward movement; in Mimosa the movement is upwards. The explanation of this difference lies in the fact, that in Mimosa leaflet it is the upper half of the pulvinule that is more excitable; while in Averrhoa and in Biophytum the lower is the more excitable half of the organ.
Fig. 144.—Diagrammatic representation of different types of phototropic response. (See text.)
As a summary of the tropic action of light I shall give diagrammatic representations of various types of phototropic response, including the photonastic (Fig. 144). The direction of the arrow indicates the direction of incident light. Dotted specimens are those which possess transverse conductivity. Thick lines represent the more excitable side of an anisotropic or dorsiventral organ. The size of the circles, with positive and negative signs, represents the amplitude and sign of curvature.