REVIEW OF RESPONSE OF ANISOTROPIC ORGANS 713 



understand the possibility of certain modifications occurring 

 in the response. Thus, in highly excitable and conducting 

 tissues, the responses will be by galvanometric negativity, 

 and the state of excitation will be conducted to a certain 

 distance. But we have seen that in tissues which are not 

 highly excitable, stimulus, falling below the excitatory value, 

 gives rise to positive response. Thus, under the action of 

 light, we obtain in plants two types of response, negative 

 and positive. Moreover, under continuous stimulation of 

 light, these may undergo phasic alternations ( + + ) or 

 (+ + ). As an example of negative response to direct 

 or transmitted stimulation of light may be seen the response 

 of Bryophyllum (fig. 238), the positive response being exem- 

 plified in the record obtained with a petiole of cauliflower 

 (fig. 240). 



It has been explained how these alternating phasic 

 responses lead us to the phenomenon of multiple and 

 autonomous response. 



A leaflet of Biophytum, or a Desmodium leaflet in a state 

 of standstill, under the continuous action of strong light, will 

 exhibit multiple mechanical responses. The corresponding 

 multiple electrical responses are seen in the response of the 

 lamina of Bryophyllum under the action of continuous light 

 (fig. 242). It has also been shown that these phasic alterna- 

 tions are brought about by the fact that the antagonistic 

 elements in the response become effectively predominant by 

 turns. Either of these antagonistic factors may be unmasked 

 more effectively by the arrest of external stimulus at a 

 particular phasic maximum. Thus, in the case where the 

 normal alternation is ( -t- +), if the stimulus be sud- 

 denly withdrawn at the end of the second phase, or positive 

 maximum, the response overshoots in the positive direction 

 (figs. 243, 244). The characteristic direct and after-effects 

 in this Type I., then, during the application of light and its 



removal are ( 1- -f). In specimens whose characteristic 



response under continuous stimulation is (+ + )> if 

 stimulus be again withdrawn at the end of the second 



