156 CHAP. XIV. THE PHOTOTROPIC CURVE 



in the region of subminimal stimulation, where the physio- 

 logical reaction is qualitatively different, being expansion 

 instead of contraction. 



Summary 



A simple phototropic curve exhibits, in the first part, 

 a slow ascent ; in the second part the gradient is steep, 

 indicative of a rapid increase of positive curvature ; in 

 the third part the rate of increase is uniform ; and in the 

 last part the curve tends to become horizontal, indicative 

 of maximal positive curvature. 



The susceptibility to stimulation is not the same, but 

 varies in different points of the curve. It is feeble at the 

 beginning, it increases rapidly at the second stage, and 

 reaches uniformity at the median range. Further on, the 

 susceptibility undergoes a rapid decline. 



Complicating factors are introduced by a subtonic 

 condition of the tissue, and by the conduction of excitation 

 across the organ from the proximal to the distal side. The 

 complete phototropic curve exhibits, in such circumstances, 

 a negative curvature in the first part, due to physiological 

 expansion under subminimal stimulation. The curve then 

 crosses the abscissa upwards, after which the curvature 

 attains its positive maximum. Subsequent to this is the 

 stage of neutralisation brought about by fatigue of the 

 directly stimulated proximal side, and by percolation of 

 excitation across the organ to the distal side. Further 

 stimulation causes the curve to cross the zero line in the 

 downward direction, the phototropic curvature being reversed 

 into negative. 



