INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 309 



illuminated side; with negative phototropism it bends toward the more weakly illumin- 

 ated side. Positive phototropism is common in stems (including flower scapes) and 

 frequent in leaves. It is pronounced in the sporangiophores of the fungus Pilobolus. 

 Negative phototropism occurs in many tendrils and in some roots, especially aerial 

 ones. In many leaves the phototropic response is such as to bend or twist the petiole 

 so as to place the upper surface of the blade perpendicular to the axis of strongest 

 illumination. Leaf mosaics arise from phototropic bendings. Some leaves (of "com- 

 pass" plants) respond to strong, direct sunlight by bendings and twistings of the 

 petioles or leaf bases in such manner that the surface of the leaf-blade comes to lie 

 parallel to the direction of the direct solar rays. 



Since phototropic bending results from unequal enlargement on the two opposite 

 sides of the organ, the response occurs only in the region of enlargement, where the 

 tissues are in the second phase of growth. The sensitive region, in which the one-sided 

 illumination sets up the primary protoplasmic disturbance that leads to the response, 

 is usually distinct and separated by a considerable distance from the bending region. 



Phototropic bending, like the retardation of enlargement by light,_is most markedly 

 promoted by light of short wave-lengths (violet region of the sunlight spectrum). 

 Wave lengths corresponding to yellow light are without phototropic effect, but the 

 longest wave-lengths (red region of the spectrum) are somewhat effective. 



6. Influence of Gravitation on Growth and Configuration. — Gravitation influences 

 the form of plants especially through geotropic bendings. Like other bendings due to 

 tropisms, these are produced by more rapid enlargement on one side of the bending 

 organ. Not only roots and stems, but also leaves, flowers, and floral parts, are influ- 

 enced in their positions and forms by gravitation. 



Gravitation acts continuously on all objects, and the pull is always toward the 

 center of the earth, but a plant organ may have any direction as related to the axis 

 and direction of gravitation. Different organs that exhibit geotropism differ with 

 respect to their equilibrium positions — that is, the positions in which they continue 

 to grow equally on all sides in spite of the one-sided pull of gravitation. Positively 

 geotropic organs (like primary roots) bend in such a way as to direct the tips 

 toward the center of the earth, while negatively geotropic organs (like the primary shoots 

 of many plants) bend so as to direct the tips away from the center of the earth. Primary 

 branches of roots and shoots are generally in the equilibrium position with respect to 

 gravitation when the tips point in a direction forming an angle with the axis of gravi- 

 tation; these are said to be apogeotropic or plagiolropic. Root branches usually bend 

 so as to direct the tips outward from the main root and downwar d, while shoot branches 

 usually bend so as to direct the tips outward from the main shoot and xipward. The 

 efect of gravitation may be avoided with organs that are not in their equilibrium posi- 

 tions (and bending may thus be prevented) if the plant is slowly rotated upon a clin- 

 ostat with horizontal axis. The clinostat may be rotated rapidly, so as to act as a 

 centrifuge, in which case the effect of gravitation is removed and the effect of centri- 

 fugal force is made manifest. Knight's experiment demonstrates that centrifugal 

 force acts like gravitation in its influence on plant bending. When thus treated, 

 positively geotropic organs bend so as to direct the tips toward the circumference 

 of rotation, with the centrifugal force, while negatively geotropic organs bend so as 

 to direct the tips toward the center of rotation, against the centrifugal force. 



In order that geotropic bending may occur, the organ in question must exhibit 

 geotropism, its sensitive region (tip of root or stem) must be in a position other than 

 that of its geotropic equilibrium, this region must remain in that position for a certain 



