172 CHAP. XV. PHOTONASTIC PHENOMENA 



introduce further complication, indirect stimulation in- 

 ducing an effect precisely the opposite to that of direct 

 stimulation. The response to unilateral stimulation is 

 further modified by transverse conduction of impulse, by the 

 intensity of stimulation, and by the differential excitability 

 of the organ. In an actual experiment the permutation and 

 combination of these different factors will give rise to effects 

 which, no doubt, appear at first sight to be highly capricious. 

 A given modification of response can, however, be traced to 

 a corresponding definite variation in the intensity and point 

 of application of stimulus, or in the tonic condition of the 

 reacting organ. 



The following is a diagrammatic representation of the 

 typical cases in which an arrow represents the direction of 

 incident light : 



a 



••" •. • • •• 



I • • If -r 





ft t ! 



+ +(+-)- +(+-)— + — 



Fig. 99. Diagrammatic representation of different types of 

 response to unilateral stimulation. Presence of dots 

 represents possibility of transverse conduction. 



+ represents positive curvature; + (H ) — represents sequence 



of positive, neutralisation, and negative. Thick -^ repre- 

 sents predominant negative. 



a, Radial thick organ. Transverse conduction absent. 



Response positive. 



b, Radial thin organ. Sequence of response : positive, neutral, 



and negative. 



c, Anisotropic thick organ. Thick line represents the more 



excitable distal side. Sequence of response : positive, 

 neutral, and pronounced negative. 



d, Anisotropic thin organ. High transverse conductivity. 



Sequence of response : positive, quickly masked by negative. 

 When light strikes on the opposite side, the sign of response in a 

 and b will remain unchanged. In c and d the effect will be 

 only positive. 



