286 Scientific Proceedings. Royal Dublin Society. 



Summary. 



The petiole of Oxalis macra is strongly phototropic, even after the lamina 

 has been removed. In this ease, however, the power of response is lost after 

 one or two days. 



"When a leaf of Sparmannia africana is shaded in such a way that portion of 

 the lamina is exposed to a reduced light intensity, as compared with that 

 falling on the remainder of the surface, a slow bending of the petiole takes 

 place. The direction of motion, is away from the shade and is parallel to 

 the surface of the lamina. 



The region where the greatest bending takes place is near the base of the 

 petiole, and the stimulus is transmitted through a distance of about 15 cm. 



The petiole itself is also phototropic, and this fact enables the leaf to 

 adjust itself correctly to the light when exposed to oblique illumination. The 

 lamina can only be expected to respond to oblique illumination when, owing 

 to curvature of its surface, the intensity of the light acting on it is not 

 uniform. 



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