212 TROPIC MOVEMENTS 



The time required for induction is naturally shorter than that necessary for the 

 commencement of a reaction to constant stimulation l , for, if the latter ceases before 

 the reaction begins, an after-effect resulting in a response is shown. Thus Czapek 

 found that the length of the geotropic induction period was twenty minutes at 25 C. 

 in the case of various radicles, whereas the time of reaction was thirty minutes 2 . 

 Czapek found no shorter geotropic induction period than fifteen minutes, whereas the 

 heliotropic induction period of sensitive seedlings lies between seven and twenty 

 minutes, and in the case of the epicotyl of Phaseolus is as long as fifty minutes. The 

 relative lengths of the induction and reaction periods probably vary somewhat even 

 in the same plant according to the external conditions. 



It is evident, therefore, that a sensory excitation begins the moment the stimulus 

 is applied, and reaches a maximal value in a longer or shorter time under continuous 

 stimulation. In addition, a curvature would be perceptible sooner were it not for the 

 delay in bringing the motor mechanism into play. It is mainly for this reason that 

 freely motile organisms are capable of rapid response, for here the excitation merely 

 modifies a pre-existent activity. 



After-effects. If the stimulus acts longer than the minimal induction period, the 

 after-effect is naturally increased. Sachs 3 found that if a negatively geotropic stem 

 was placed horizontally until a curvature just began it continued to curve strongly 

 when placed vertically, and the after-effect lasted from one to three hours. Similar 

 results were obtained by Miiller and Wiesner 4 by heliotropically stimulating seedlings 

 until curvature just began. An after-effect is probably never entirely absent, though 

 it is not always pronounced. Freely motile organisms, for instance, on the removal 

 of a phototactic stimulus progress for a moment in the original direction. It was 

 probably owing to the result of the mode of experimentation adopted that Sachs was 

 unable to obtain any after-effects in roots, for Czapek found that they showed after- 

 effects extremely well 5 . The amount of the after-effect is, however, not directly 

 proportional to the intensity and duration of the induction, although in general the 

 after-effect is increased by prolonged exposure in the case of objects showing marked 

 reactions 6 . Various other after-effects are known, both periodic and non-periodic ; 

 but these are discussed in connexion with growth, daily periodicity, and heredity. 



SECTION 48. The Relation between the Intensity of Stimulus 

 and the Resultant Excitation. 



In general an increase in the intensity of the stimulus produces 

 a greater excitation, enlarging the amplitude of movement, and at the 

 same time shortening the times of induction and reaction. The relationship, 

 however, is by no means a simple one, and cannot be represented by 



1 Cp. Czapek, Jahrb. f. wiss. Bot, 1898, Bd. xxxil, p. 183. 



2 Czapek, 1. c., p. 184 ; in regard to the nodes of Tradescantia cf. Kohl, Bot. Ztg., 1900, p. 19. 



3 Sachs, Flora, 1873, p. 325. 



4 Mullet, Flora, 1876, p. 89. 



5 Wiesner, Die heliotropischen Erscheinungen, 1878, Bd. I, p. 61, and 1880, Bd. II, p. 87. 



6 Sachs, Arb. d. hot. Inst. in Wiirzburg, 1873, Bd. I, p. 472. 



