go PHYSIOLOGY OF STREAMING MOVEMENTS 



That the time factor should be of importance when the electrical 

 stimulus is nearly minimal might be expected ; a momentary weak current 

 producing no perceptible effect. In the case of induction currents, it is 

 only natural that the stronger secondary current at ' break ' may act as 

 a minimal stimulus, when the weaker ' make ' current affords a sub- 

 minimal one. 



Using a Lippmann's capillary electrometer, Hormann was able to 

 show that a stoppage, whether produced by induction shocks or by a 

 sudden local fall of temperature, is accompanied or preceded by an 

 electrical disturbance which travels around the cell in the form of a wave, 

 and corresponds to the ' negative variation ' of animal physiology. Since 

 the wave may precede the stoppage it cannot be the direct result of it, 

 but is probably due to a chemical disturbance which is propagated through 

 the cell, and acts as the stimulus inhibiting streaming. 



From these facts Hormann concludes that a stoppage of streaming 

 in Nitella is fundamentally the precise counterpart of a contraction in 

 a muscle-fibre, and that the different response is due to some essential 

 difference of protoplasmic structure. This last deduction is by no means 

 a correct one, for a difference in the character of the response may be 

 due to a difference in the manner in which the stimulus is received by 

 the percipient organ. Thus given a slight difference in the construction 

 of the steam-cock (percipient organ), the same movement of the hand 

 (stimulus) may cause one steam-engine to be suddenly arrested, and the 

 other to move rapidly forwards, although both have otherwise precisely 

 similar constructions (motor-mechanisms). Moreover there are several 

 fundamental differences between a stoppage of streaming and a muscular 

 contraction. Thus in the first case less energy is consumed and less work 

 done than before, while the shape of the protoplasmic contents remains 

 under minimal stimulation the same as before. In the second case more 

 energy is consumed and work done than in the resting condition, and the 

 external shape of the protoplasmic contents of the muscle-fibre or muscle- 

 cell always undergoes a pronounced change. This latter difference is 

 partly due to the presence of a relatively rigid cell-wall in plants, for 

 under maximal stimulation plant-protoplasts also exhibit a change of shape. 



It must always be remembered that the negative variation is the 

 result of differences of electrical potential, produced probably by chemical 

 changes propagated in the form of a wave, and it does not necessarily 

 follow that these chemical changes are always precisely similar in cha- 

 racter, and always act in a similar manner upon the irritable protoplasm. 

 The very fact that the rate of propagation varies enormously in nerves, 

 muscle-fibres, and plant-cells or tissues is almost conclusive proof that 

 dissimilar { explosive' or specifically differentiated conducting substances 

 are concerned in the different cases. 



