INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



77 



all these cases the upper surface of the leaf-blade becomes so oriented that it is 

 perpendicular to the direction of the impinging rays. Even if such a plant is in- 



P- 



Fig. 127. — Diagrams showing phototropic movements of leaves, with reference to the direc- 

 tion of impinging light, this direction shown by the arrows. 



Fig. 128. — Inverted Phaseolus plant. Two petioles are fastened with wire so as to hold 

 them in their normal position. Leaf-blade b is represented as in its normal position, while 

 a has become re-oriented after the plant was inverted. Leaf с has responded by a torsion 

 of the petiole as well by a bending. {After Pfeffer.) 



verted (Fig. 128) the leaves bend in such a way as to direct their normally 

 upper surfaces toward the source of strongest illumination, 1 the movement being 



iVöchting, Hermann, Uehcr die Lichtstellung der Laubblätter. Bot. Zeitg. 46: 501-514, 517-527, 

 533-541. 540-560. 1888. 



