THE RELATION BETWEEN STIMULUS AND EXCITATION 215 



been made in this direction, and it is often difficult to raise the intensity of a tropic 

 stimulus without increasing its diffuse action. 



Similar relationships hold good for non-tropic stimuli, although in many cases 

 the diminished excitability under increasing intensity of stimulation is accompanied by 

 special peculiarities l . Growth and other functions, under rising temperatures, or increas- 

 ing aeration or nutrition, increase at first rapidly and then more slowly as the optimum 

 is approached. Precisely similar curves are given by the action of unnecessary or 

 poisonous substances, and, in fact, every agency when sufficiently intense produces 

 a lessened response or excitation. Similarly, movements which alter with increasing 

 stimulation may be represented by angular curves. The apex of the curve does not 

 correspond to the optimum point on a growth-temperature curve, since beyond 

 it the response is reversed instead of continuing of like kind but lessened quantity 2 . 



Considering the complicated nature of the reactions involved, it is hardly 

 surprising to find that the relationship between the intensity of the stimulus and the 

 degree of excitation should show many divergences 3 from Weber's law, according 

 to which the stimulus must increase in geometric procession to produce an arith- 

 metical progression of the excitation, or, in other words, that the excitation is 

 proportional to the logarithm of the stimulus 4 . In accordance with the logarithmic 

 curve, the excitation at first increases rapidly when the minimal intensity of stimulation 

 is passed, but subsequently more slowly with equal increases of intensity. In regard 

 to plants, there can be no doubt that the phenomenon is a physiological one, 

 although Fechrier considered it to be of psychic origin in the case of man. It is, 

 therefore, inadvisable to use the term ' psycho-physical law ' as was done by Fechner. 

 In spite of this, however, the comparative effects of the receipt of a shilling upon 

 a pauper and upon a millionaire may be used as an explanatory illustration. 



As in other cases, the change of tone with increasing intensity of stimulation 

 is undoubtedly the result of a modification of the power of sensation, and if this has 

 no effect upon another stimulatory reaction, it is evident that the two stimuli act 

 upon different sensory mechanisms B . This applies more especially to chemotropic 

 excitations, and Rothert 6 has, in fact, shown that the attractive actions exercised 

 upon Amylolacter by meat-extract and by ether involves different powers of sensation, 

 for the attractive action of meat-extract is unaffected by the presence of 1-6 per cent, 

 of ether inside and outside the capillary. 



A change of tone in a particular irritability may, however, also arise from 

 stimulation involving an entirely dissimilar sensory perception, and hence direct 

 conclusions can only be made with caution from changes of tone. The chemotropic 

 action of malic acid upon the sperms of Ferns is weakened in solutions already 



1 See Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. I, pp. 406, 506, 521 ; Correns, 

 Flora, 1892, pp. 107, 150. 



2 On Phobophototaxis cf. Rothert, Flora, 1901, p. 401. 



3 According to Mendelssohn (Centralbl. f. Physiol., 1903, Bd. XVII, p. n), the thermotropic 

 excitation is proportional to the temperature. 



* Cf. Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1884, Bd. I, p. 401 seq., and 1888, Bd. II, 



p. 638. 



5 Cf. Pfeffer, 1. c., 1888, Bd. II, p. 648. 

 s Rothert, Flora, 1901, p. 387. 



