314 LIFE MOVEMENTS IN PLANTS 



above or below. Finally, a radial organ is found to exhi- 

 bit under light of increasing intensity or duration, a posi- 

 tive, a dia-phototropic, or a negative phototropic curvature. 



In these circumstances the theory of specific positive 

 and negative irritabilities is untenable ; in any case, it 

 throws no light on the phenomenon of movement. The 

 difficulties of the problem are thus clearly stated by 

 Pfeffer : " When we say that an organ curves towards a 

 source of illumination, because of its heliotropic irritability 

 and we are simply expressing an ascertained fact in a 

 conveniently abbreviated form, without explaining why 

 such curvature is possible or how it is produced .... 

 Many observers have unfortunately devoted their attention 

 to artificially classifying the phenomenon observed, and 

 have entiri?ly neglected the explanation of causes under- 

 lying them."* 



COMPLEXITY OF PROBLEM OF PHOTOTROPIC REACTION. 



The complexity of phototropic reaction arises from the 

 summated effects of numerous factors ; for explanation of 

 the resultant response it is therefore necessary to take 

 full account of the individual effect of each of them. 



Among these operative factors in phototropic reaction 

 may be mentioned : — 



(1) The difference of effects induced by light at the 



proximal and distal sides of the organ. 



(2) The modification of the latent period with the 



intensity of stimulus. 



(3) The after-effect of stimulus. 



(4) The modifying influence of tonic condition on 



response. 



(5) The eft'ect of direction of light. 



* Pfeffer -/6id, Vol. II, p. 74 



