6l2 PLANT RESPONSE 



The crucial test would thus lie in the observation of the 

 responsive movement under moderate unilateral stimulation. 



I placed a tendril in a dark room, and subjected it to light 

 of moderate intensity from a sixteen-candle-power electric 

 lamp, placed at a distance of 15 cm. This induced an active 

 movement of the tendril towards the light, or positive helio- 

 tropic response. I then brought the lamp nearer, to a distance 

 of 5 cm., thus increasing the intensity of light. The active 

 positive movement was now quickly reversed into a move- 

 ment away, or negative heliotropic response. This experiment 

 once more demonstrates the fact that positive and negative 

 heliotropic responses are not due to two specific sensibilities of 

 opposite sign. 



The negative heliotropic curvature in an organ originally 

 radial, which we have just studied, was due to the physio- 

 logical anisotropy induced by the stimulus itself; but there 

 are organs in which anisotropy is already developed in 

 various degrees of perfection, and in them we shall be able to 

 observe varying intensities of this negative heliotropic effect. 

 This will be discussed in detail in succeeding chapters. 



Summary 



Negative curvature is induced under the action of light in 

 two different ways : (i) by the indirect effect of the moderate 

 stimulation of the tip of root or shoot ; and (2) by the 

 transversely transmitted effect of strong or long-continued 

 stimulus acting on the distal side of an organ, when the 

 proximal has become fatigued. The parallelism between 

 geotropic and heliotropic effects is thus incomplete. 



The direct effect of stimulus, unilaterally applied, may be 

 longitudinally transmitted, and cause responsive movement 

 in the distant growing region. 



The unilateral effect of stimulating light on the growing 

 region varies with its intensity, as follows : 



I. Moderate intensity induces the normal positive re- 

 sponsive movement. 



