TROPIC MOVEMENTS 269 



kind of irritability, and a different mode of response. So 

 very anomaious are these various effects tliat Pfeff'er, after 

 showing the inadequacy of different theories that have been 

 advanced, came to the conclusion that "the precise character 

 of the stimulatory action of. light has yet to be deter- 

 mined. When we say that an organ curves towards a source 

 of illumination because of its heliotropic irritability, we are 

 simply expr3«<sing an ascertaine:! fact in a conveniently 

 abbreviated form, without explaining why such curvature 

 is possible or how it is produced."* 



The contradictory nature of the various responses is 

 however not real ; the apparent anomaly had lain in the 

 fact that two definite fundamental reactions of opposite 

 signs induced by stimulus had no: hitherto been recognised 

 and distinguished from each other. The innumerable varia- 

 tions in the resultant response are due to the summation of 

 the effects of two fluctuating factors, with further complica- 

 tions arising from : (I) difference in the point of application of 

 stimulus, (2) the differential excitability of the different sides 

 of the responding organ, and (3) the effect of temperature 

 in modifying tropic curvature. It is therefore most import- 

 ant to have the means for automatic record of cotttinuouf^ 

 change in the response bi'ought about by various fac'tors, 

 which act sometimes in accord, and at other times in con- 

 flict. The autograph of the plant itself, giving a history of 

 the change in response and its time-relations, is alone 

 decisive in explanation of various difficalties in connection 

 with plant movements, as against the various tentative 

 theories that have b^^en put forward. The analysis of the 

 resulting effect, thus rendered possible, casts new light on 

 the phenomena of response, proving that the anomalies 

 which had so long perplexed us, are more apparent than 

 real. 



* Pfeffer -/6i<Z— Vol. II. p. 74. 



