Ball — Phototropic Movements of Leaves. 285 



which the petiole was at right angles to the incident light, being enclosed in 

 a bag of black paper in such a way that the petiole was quite uncovered. 

 The petiole bent towards the light as much as that of a similar uncovered leaf 

 on the opposite side of the stem, the region where the greatest bending took 

 place being close to the base of the petiole. Similar results were obtained 

 when the uppermost 3 cm. of the petiole (as well as the lamina) was covered 

 with black paper. 



In Sparmannia the motor regions of the petiole lie close to the upper and 

 lower extremities. If the petiole is split longitudinally into several parts, 

 which are then placed in water, the upper and lower ends of these become 

 sharply curved owing to the swelling of the cortical cells while the middle 

 portions remain approximately straight. 



The uppermost 5-10 mm. of the petiole is usually nearly at right angles to 

 the lamina, curvature taking place just below this region. It seems probable 

 that the orthotropic sensitiveness of this part assists in the final adjustment 

 of the lamina at right angles to the incident light. By illuminating the 

 uppermost part of the petiole by direct light and the remainder from the 

 opposite direction by means of a mirror it was found possible to cause the 

 upper and lower motor regions to bend in opposite directions. 



The view that phototropic curvatures are due to differences in light intensity, 

 and not merely to the direction of the light rays, is very strongly supported 

 by the work of Blaauw (9) and Buder (10 and 11), and the experiments 

 described above show that this is also the cause of the movements of the 

 leaves of Sparmannia. Such a view appears to admit of a simple explanation 

 of the method by which leaves respond to phototropic stimuli. 



It seems to have been conclusively shown (7 and 8), in the case of the 

 coleoptiles of Avena and other similar plants, that the phototropic stimulus is 

 conducted by means of a diffusible .substance or hormone. Provisionally, we 

 may regard such a hormone as being produced in any part of the lamina or 

 petiole, the amount produced depending directly on the intensity of the light. 

 This hormone will pass along the petiole in a basipetal direction. If the two 

 sides of the petiole are unequally illuminated, inequalities in the concentration 

 of the hormone on the two sides wHl result, and the petiole will tend to bend 

 until it points towards the light. Owing to mechanical reasons the response 

 of the petiole may be confined to certain regions where the tissues are more 

 pliable and the magnitude of the response may be limited largely by epinasty 

 or geotropism. 



Should the lamina be uniformly illuminated, even though the light be 

 directed obliquely on to its surface, the hormone produced in it will be distributed 

 evenly to both sides of the petiole and no bending of the latter will result due 

 to this cause. Shading of portion of the lamina will, however, disturb the 

 equilibrium, more of the hormone being distributed to one side of the petiole 

 than to the other. The petiole will therefore tend to curve in a direction parallel 

 to the surface of the lamina. 



It would appear, therefore, that both lamina and petiole have similar 

 potentialities with regard to the perception of phototropic stimuli, that is 

 the perception of differences in light intensity. Owing, however, to the 

 morphological arrangement of the parts of a leaf, the petiole will be exposed 

 more often than the lamina to differences in light intensity. The movements 

 by which a leaf is turned towards the light will therefore normally be 

 initiated by the perception of the stimulus by the petiole itself, but, under 

 certain conditions, the bending of the latter may partially or entirely be 

 controlled by the lamina. 



SCIENT. PROC. R.D.S., VOL. XVII, NO. 35. 8 H 



