2IO PLANT RESPONSE 



fifty volts, whereas normally in Mimosa the critical value is con- 

 siderably above a hundred volts. These tissue-modifications 

 sometimes proceed so far that I have occasionally observed 

 reversal in the case of this plant even with a moderate 

 E.M.F. 



I was next desirous of determining whether these different 

 types of polar effects — normal, transitional, and reversed — 

 could not be demonstrated in some novel and striking manner, 

 in the case of animal tissues. It occurred to me that the 

 intermittent flashes of light emitted by the firefly might 

 be simple expressions of rhythmic excitation, a subject 

 which will be dealt with in detail in Chapter XXIII. The 

 emission of light, or an increased intensity of emission on 

 the part of the insect, would in that case be indicative of 

 the state of excitation, and this mode of excitatory expression 

 I shall designate as glow-response. 



Investigation of polar excitation by glow- response — 

 I may here state in anticipation that I have succeeded in 

 demonstrating, by means of this glow-response, all the prin- 

 cipal characteristic effects of {a) normal response, due to 

 moderate electromotive force ; {b) the reversed effect due to 

 high electromotive force ; and {c) the reversed effect due to a 

 modified condition of the tissue. It may be pointed out 

 further, that some specimens gave the normal, and others, 

 owing to a modified condition of the tissue, the reversed 

 effect ; but that the results obtained from any given in- 

 dividual were always consistent and characteristic. 



I shall first describe certain results which were frequently 

 observed, and which are entirely analogous to those described 

 in a previous chapter as given by a nerve-and-muscle pre- 

 paration, and highly excitable tissue of Mimosa (p. 197). We 

 there saw that while the current was ascending, the excira- 

 tion exhibited by the terminal organ at make was due to 

 direct action of the proximal kathode. Excitation was also 

 produced at break, and this was due to the transmission of 

 the distal anode-break effect. Again, when the current was 

 reversed, excitation was exhibited in a corresponding manner, 



