CURVE OF PULVINATED ORGAN 153 



into normal positive curvature towards the light, which 

 will reach a maximum value. 



The organ has been assumed to possess the capacity for 

 transverse conduction. After the attainment of maximum 

 positive curvature, the conduction of excitation across the 

 organ from the proximal to the distal side will bring 

 about a complete neutralisation, to be succeeded by an 

 actual reversal into negative curvature. These theoretical 

 considerations will now be subjected to experimental 

 tests. 



The reversal of positive into negative phototropic 

 curvature under strong unilateral illumination usually 

 takes place under an exposure so prolonged that it is difficult 

 to represent the different transformations in a single curve 

 that can be reproduced on a page. This can only be done 

 wdth a thin specimen, so that the transverse conduction 

 of excitation which induces reversal may take place within 

 a reasonable time. A complete phototropic curve was thus 

 obtained in an identical specimen, which exhibited all the 

 characteristic transformations. For a pulvinated organ, 

 I employed the thin pulvinus of the terminal leaflet of 

 Desmodimn gyrans. For growing organs, young seedlings 

 of Zea Mays were found suitable. In both the above cases 

 the specimens were in a slightly subtonic condition. 



Complete Phototropic Curve of a Pulvinated 



Organ 



Experiment 91. — A continuous record was taken (fig. 85) 

 of the action of light from a 50-candle-power incandescent 

 electric lamp incident on the upper half of the pulvinus of 

 the terminal leaflet of Desmodium. This gave rise first 

 to the abnormal negative curvature induced by subminimal 

 stimulation. The curve then proceeded upwards in the 

 direction of positivity, at first slowly, then rapidly. The 

 maximal positive curvature was attained in the course of 



