THE RELATION BETWEEN STIMULUS AND EXCITATION 213 



any general formula. Apart from other considerations, this is bound to 

 result from the fact that increases of temperature, illumination, or of chemical 

 action may deaden or inhibit sensation and motility, and may finally 

 produce death. Even within moderate limits the intensity of the stimulus 

 may modify not only the sensitivity and power of reaction, but also the 

 time of reaction and the ultimate position of equilibrium. Thus intense 

 unilateral illumination causes the positive phototaxis of swarm-spores to 

 become negative, and varying intensities of light suffice to convert the 

 positively heliotropic reaction of many rooted plants, and even of their 

 radial organs, into a plagiotropic or negatively heliotropic one. Similar 

 changes of reaction are known in the case of thermotropic, chemotropic, 

 hydrotropic, and galvanotropic stimuli. In addition, increasing intensities 

 of centrifugal action produce a lessening of the geotropic angle of the 

 lateral roots l , and cause in diageotropic rhizomes an inward curvature, 

 so that if the mass of the earth were suddenly increased they would curve 

 downwards ' 2 . These responses are physiological in character, although 

 intense centrifugal action may produce purely mechanical curvatures. An 

 already stimulated organ is less responsive than an unstimulated one, 

 and hence, to produce a perceptible increase in the reaction, the stimulus 

 must be increased by a greater amount than suffices in the first instance 

 for the primary reaction. This applies not only to tropic but to other 

 forms of irritability, and to animals as well as to plants. Weber's law is, 

 in fact; of general application, for in plants also a definite relation exists 

 between the intensity of an existent stimulus and the additional intensity 

 required to produce a perceptible reaction a . For instance, man can detect 

 changes of illumination of not less than one-hundredth of the existing 

 intensity, while in the case of Phycomyces the change must be at least one- 

 fifth. Thus this fungus under diffuse illumination equivalent to five units will 

 show a heliotropic curvature when exposed to an increase of illumination 

 of one unit on one side, whereas in diffuse light of 100 units intensity an 

 increase on one side of twenty units will be necessary. 



That the excitation increases less rapidly than the stimulus producing it was 

 shown by Sachs, Elfving, and Schwarz, in regard to geotropic, and by Wiesner 

 in regard to heliotropic stimuli 4 . Pfeffer's 5 researches on chemotactic irritability then 



1 Cf. Sachs, Arb. d. hot. Inst. in Wiirzburg, 1874, Bd. I, p. 607. Cf. also Pfeffer, Pflanzen- 

 physiologie, 1881, Bd. II, p. 334 ; Elfving, Beitrag zur Kenntniss d. Einwirkung der Schwerkraft auf 

 die Pflanzen, 1880, p. 33 (reprint from Acta Soc. Scient. Fennic., Bd. XII); Schwarz, Unters. a. d. 

 bot. Inst. zu Tubingen, 1881, Bd. I, p. 80. ' 



2 Czapek, Sitzungsb. d. Wien. Akad., 1895, Bd. CIV, p. 1233. 



3 For details see Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd. I, p. 395. 



4 Wiesner, Die heliotropischen Erscheinungen im Pflanzenreiche, 1878, Bd. I, and 1880, Bd. II. 



5 Pfeffer, Ber. d. bot. Ges., 1883, p. 524; Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd. I, 

 p. 395 ; 1888, Bd. n, p. 633. 



