154 



CHAP. XIV. THE PHOTOTROPIC CURVE 



i8 minutes and remained constant for a short time, as seen 

 in the more or less horizontal record at the top of the curve. 

 After this, transverse conduction of excitation from the 



Fig. 85. Complete phototropic curve of a pulvinated organ. 

 Positive curvature above, and negative curvature below, 

 the horizontal zero line. Preliminary negative phase of 

 response due to subminimal stimulus. The positive in- 

 creases, attains a maximum, and undergoes a reversal. 

 Successive dots at intervals of 30 seconds. Abscissa repre- 

 sents duration of exposure and quantity of incident light. 

 (Terminal leaflet of Desmodium gyrans.) 



proximal to the distal side began to take place, inducing 

 neutralisation which was completed in the course of 

 24 minutes. Subsequently to this, there was a reversal of 

 phototropic curvature into a pronounced negative. 



Complete Phototropic Curve of a Growing Organ 



Experiment 92. — ^A seedling of Zea Mays was sub- 

 jected continuously to unilateral light from a small arc- 

 lamp for 2 hours. The characteristics of this curve are 

 very similar to those of Desmodium. The subminimal 

 stimulation induced, at the first stage, a negative curvature ; 

 this was transformed into positive after an exposure of 



