INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



279 



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- 



«+ 



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 c 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 



1 Vochting, Hermann, Ueber die Lichstellung der Laubblatter. Bot. Zeitg. 46: 501-514. 5I7-S27, 533~ 

 541, 549-S60. 1888. 



