472 REPORTS ON THE STATE OF SCIENCE. 



conductivity of the tetanised portion is increased by about tenfold, 

 repeated stimuli on the further side of it produce no electromotive 

 variation at the leading-off electrodes, but become immediately effectual 

 on their application between the latter and the tetanised tissue. 



That the excitatory change undergoes a considerable decrement 

 during its transmission is suggested by the facts that a stimulus of given 

 intensity and duration, ineffectual at a certain distance, may be effectual 

 if that distance is diminished ; that a stimulus repeated at points 

 successively neai'er to the leading-off electrodes elicits from a tissue in 

 optimum condition responses of successively higher voltage ; finally, that 

 a first diphasic effect may on repetition of the stimulus at the same or a 

 nearer point be followed, perhaps owing to fatigue of the tissue, by a 

 simple monophasic response, showing that the wave has not in this case 

 reached the distal electrode. 



The present data are insufficient to determine the true nature of the 

 change, by transmission of which an electrical disturbance is set up at 

 points distant from that of the exciting stimulus. The question arises as 

 to whether it is physiological in nature or a purely physical process 

 independent of any activity of the living tissue as such. From the 

 succulent nature of the seedlings used in the greater number of the 

 expei'iments, as also from the fact that a thermal stimulus is almost 

 invariably effective where a mechanical stimulus generally fails to elicit 

 a response, it appears likely that the electrical variations observed may 

 be due to a movement of water through the tissue set up by the sudden 

 change of temperature. On the other liand, the remarkable response 

 obtained from the maidenhair petiole, which is a particularly dry and 

 juiceless tissue, can hardly be so explained. Again, that the transmission 

 is essentially a physiological pi'ocess appears from the following con- 

 siderations : — 



First, that in character and extent the propagation varies in accord- 

 ance with the general character of the tissue tested. In a comparison 

 of roots and shoots in the pea seedling a correspondence was found 

 of general sluggishness of response with limited propagation in the root 

 as compared with general excitability and extensive propagation in the 

 shoot. 



Again, that a very slight etiolation or unhealthiness of a plant 

 destroys its power of propagating a stimulus, as seen with seedlings 

 grown in the laboratory — i.e., under bad conditions of nourishment and 

 light — with these tissues, the results obtained, though at first good, were 

 after continued growth consistently negative. 



Again, that the tissue quickly fatigues unless in optimum condition, 

 as appears from the fact, noted above, that a first diaphasic may be 

 followed by a monophasic response, or in many cases by complete 

 inexcitability. 



Finally, that on altering the state of the tissue in such a way as to 

 annul its power of response while at the same time greatly increasing its 

 electrical conductivity— e.p'., by boiling or tetanising. No propagation of 

 an excitatory state can be obtained with strong and repeated stimuli. 

 Plate lu. records such an experiment. 



The undifferentiated nature of some of the tissues used in these ex- 

 periments, together with the great sensitivity of the obsei-ved phenomena 

 to external conditions, would bring these phenomena under the head 

 of propagation by activity of the ' ground tissue,' following Fitting's 



