CHANGES OF IRRITABLE TONE 209 



irritability, nor heliotropic induction the geotropic irritability, whereas Noll states that 

 heliotropic excitation inhibits the geotropic irritability. Feeble lateral illumination 

 produces a complete, or nearly complete, assumption of the position of heliotropic 

 equilibrium in many organs, the geotropic tendency being easily overcome ; but this 

 may be merely the result of a strong development of the heliotropic irritability, coupled 

 with an inherently feeble geotropic irritability. Possibly, however, the geotropic 

 irritability may be partially or entirely suppressed by strong heliotropic excitation in 

 those organs which are especially dependent upon the assumption of appropriate light 

 positions. In any case various tropic responses of roots and other organs, such as 

 those due to hydrotropic and rheotropic actions, appear to take place unaffected by 

 gravity, since the latter may exercise little or no effect upon the position assumed, and 

 is also unable to prevent a traumatropic stimulus producing a complete coil at the 

 growing apex. In addition, Klebs has shown that hydrotropic stimuli readily over- 

 come the heliotropic irritability of Sporodinia grandis l . 



SECTION 47. Minimal Stimuli and. the Latent Periods of Induction 



and Reaction, 



Owing to the varying degrees of irritability in different organs towards 

 the same and to different tropic stimuli, a feeble intensity may act as 

 an excitation in one case, whereas in others a response may be produced 

 only when the stimulus is intense. Zoospores afford instances of the 

 almost complete absence of any latent period, the response to stimuli 

 being shown almost instantaneously, whereas in the case of tropic curva- 

 tures the latent period is rarely less than a few minutes, and is often 

 from one-half to several hours in duration. 



Once the curvature has begun it continues for a longer or shorter 

 time after the stimulus has ceased to act, and an after-effect may be 

 shown if the stimulus is removed just before the curvature has begun. 

 It follows, therefore, that a perceptible interval of time elapses between 

 perception and response, although it remains an open question whether 

 the delay lies in the progress of the sensory excitation or in the awakening 

 of the motory reaction. When the perceptive and responsive zones are 

 separately localized, the slow transmission of tropic stimuli interposes an 

 additional delay. 



A response presupposes a sufficient intensity of excitation, and 

 naturally a stimulus of very short duration may fail to produce any 

 reaction. Since a summation of transient stimuli is possible when they 

 are repeated at definite intervals of time, it is evident that each is per- 

 ceived, and that its inductive action has not faded away before the next 

 stimulus comes. Wiesner 2 found, for instance, that the hypocotyl of 



1 Klebs, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 56. 



2 Wiesner, Die heliotropischen Erscheinungen, 1880, Bd. II, pp. 25, 87. 



PFEFFER. Ill p 



