360 LIFE MOVEMENTS IN PLANTS 



minimal, the simple tropic curve is corfined to the second 

 stage (see Fig. 132). 



WEBER'S LAW. 



If we neglect the preliminary negative portion under 

 sub-minimal stimulus, the curve of excitation under increas- 

 ing photic stimulation obeys what is known as Weber's law. 

 This is equally true of modes of stimulation other than 

 that of light as is seen in figure 130 of the contractile 

 effect of continued electric stimulus on growth ; the excita- 

 tory effect is also seen to reach a limit. 



"Weber's law is applicable for a limited range of stimu- 

 lation. For the quantitative relation fails in the region of 

 sub-minimal stimulus, where the physiological reaction is 

 qualitatively different, namely expansion instead of contrac- 

 tion. This holds good even in the case of animal tissues, 

 for here also my recent experiments show that two opposite 

 reactions— expansion and contraction — ta,ke place under 

 stimulus, and that a very feeble stimulus tends to induce 

 expansion instead of contraction. The responsive reaction 

 of a kitten under gentle caressing strokes must be qualita- 

 tively different from that of a blow. The psychological 

 effects under the two treatments evidently differ qualita- 

 tively rather than quantitatively.* 



SUMMARY. 



The excitation curve exhibits a slow ascent in the 

 first part ; in the second part the gradient is steep, indi- 

 cating rapid rise in excitation ; in the third part it is 



■* " It has been argued by James that the feeling Hoes not cause, but is 

 caused by the bodily expression .... Miinslerberg concludes that the feeling of 

 agieeableness is the mental accompaniment and outcome of reflexly produced 

 movements of extension, and disagreeableness of the movement of flexion." 

 Schafer— Text Book of Physiology, Vol. 11, p. 'JTo (H'OO). 



