RESPONSE OF ISOLATED VEGETAL NERVE 471 



and the other with the unkilled portions of the specimen 

 higher up. In order to ensure that the electrical indication 

 be a true responsive reaction, it is well to use a non-electrical 

 form of stimulus. One of the most perfect forms as we 

 have seen in the previous chapter, on excitation of animal 

 nerve is the thermal, and this may be applied in precisely 

 the same manner, that is to say, by means of a platinum 

 wire, surrounding, but not necessarily in contact with, the 

 given area of the specimen, this wire being heated periodically 

 in the manner previously described, by means of a metro- 

 nome closing an electric circuit. With a good specimen, 

 a single thermal shock, lasting for less than a second, will be 

 found sufficient to induce a considerable electrical response, 

 or a response of still greater amplitude may be obtained by 

 the summated effects of several such stimuli. One of the 

 most noticeable differences between this plant nerve and 

 other vegetable tissues lies in its greater excitability. For 

 example, while a single thermal shock of less than one second's 

 duration is sufficient, as has been said, to evoke immediate 

 and considerable response from the isolated nerve, we find that, 

 in order to evoke similar response from the petiole of the fern 

 as a whole, it is necessary to submit it to the same stimulus 

 some twenty times in succession, the response even after this 

 taking place with relative sluggishness. 



A still further characteristic is its indefatigability. A long 

 series of responses to uniform stimuli, such as would in the 

 case of ordinary tissues bring about marked fatigue, will in 

 that of nerve induce little or none. Rapidly succeeding 

 tetanising shocks, moreover, such as in other tissues induce 

 rapid decline, induce, generally speaking, but little of such 

 an effect on the response of nerve. In the case of this vege- 

 table nerve also the same statements hold good. A long 

 continued series of responses shows little fatigue. After 

 tetanisation, moreover, we find that the responses of nerve, 

 whether animal or vegetable, become enhanced. 



In the matter of the effects induced by chemical re- 

 agents on animal and vegetable nerves, a further remarkable 



