284 Scientific Proceedings, Royal Dublin Societij. 



of the lamina took place in such a direction that, if the shade had not been 

 attached, the lamina would have moved away from it. The mean horizontal 

 movement of the three leaf-tips after 7 days was 5-5 cm. The tinfoil shades 

 were then removed and placed over the opposite edges of the leaves, with 

 the result that the three leaves moved in a direction contrary to the previous 

 movement, the mean movement of the leaf -tips after 7 days being 41 cm. and 

 after 14 days 8-4 cm. During the experiment the stem of the plant was tied 

 to a stake, and the shades were arranged so that the three leaves should not 

 all move in the same direction. 



The fact that shading the lamina does influence the movements of 

 the petiole appears to be definitely proved. The transmission of the stimulus 

 from the edge of the lamina to the basal region of the petiole, a distance in 

 this case of abo^^t 15 cm., raises some interesting problems which it is hoped to 

 investigate in the near future. ^ 



These experiments afford an explanation of the method by which the mosaic 

 arrangement of the leaves of a plant is attained. Shoulcl one leaf overlap 

 part of another and thereby shade it from the light, the shaded leaf will tend 

 to move until its edge is v/ithdrawn from the shade of its neighbour. It is 

 difficult to understand how this result could be achieved if the petiole were 

 the sole percipient organ. For, if this were the case, after the petiole had come 

 into position where it was freely illuminated, further movement of the leaf 

 would cease, even if portion of the lamina were still in shade. 



Furthermore, in the experiments described above, it is a difference in the 

 light intensity in the shaded and unshaded parts of the leaf which causes 

 the movement. That phototropie movements result as a response to the 

 direction of the light rays ancl not merely to differences in their intensity 

 has been the opinion of many plant physiologists. This view mainly seems 

 to have been responsible for the different conclusions at which they have arrived 

 as to the directing influence of the lamina on the petiole. For example, in 

 the experiments on Eranthis hiemalis described by Wager (6), leaves were 

 exposed to oblique illumination, but in such a way that the intensity of the 

 light over the whole of the lamina was the same. Under these circumstances 

 the petioles which were kept in the dark showed no phototropie curvature. 

 "When, however, the blades of the leaves were darkened and the petioles 

 exposed to lateral illumination the conditions were different, since the two 

 sides of the petioles were exposed to different light intensities, with the result 

 that curvature took place. Such experiments, while proving that the lamina 

 is not responsive to change in the direction of the incident light, leave unaffected 

 the question as to its power of responding to differences in light intensity on its 

 various parts. 



If a leaf in which the lamina is quite flat is exposed to oblique illumination 

 the whole surface will still be subjected to a uniform light intensity, and 

 no directive influence can be exerted on the movements of the petiole. If, 

 however, the lamina is somewhat curved the light intensity will not be unifenn 

 on all parts of the surface, and movements of the petiole may be expected. 



Although under appropriate conditions a certain directive influence can 

 be exei-t-ed by the lamina on the nnovements of the petiole, the main cause 

 of these movements normally appears to lie in the perception of differences 

 in light intensity by the petiole itself. That the petioles of leaves are positively 

 phototropie has often been demonstrated, and that Sparmannia is not an 

 exception in this respect is shown by the following experiments. 



A plant was exposed to lateral illumination, the blade of one leaf, in 



