ELECTRICAL STIMULI 91 



In spite of these numerous and exhaustive researches many points 

 of interest still remain to be solved not only as regards the direct and 

 the after-effects of electrical excitations, but also concerning the conjoint 

 effects of electrical and other stimuli. For purposes of experimentation 

 cells of Chara and Nitella and strips of leaf-cells of Elodea and Vallisncria 

 were mainly employed. In estimating the sensitivity to electric currents 

 account must always be taken of the sectional area the current traverses, 

 even when the passage of the whole of it through the experimental object 

 is ensured by vaseline insulations. When this is done it can easily be seen 

 that Nitella is more sensitive than Chara, Chara than Vallisneria^ and 

 Vallisneria than Elodea ; from six to twelve times the intensity of current 

 necessary to produce an immediate stoppage of streaming in Nitella being 

 required to produce the same effect in Elodea^ if applied with corresponding 

 suddenness. 



Jurgensen 1 states that the current from one Grove cell produces no 

 perceptible effect on streaming in Vallisneria^ that from two to four cells 

 a retardation and ultimately a permanent stoppage, and that from thirty 

 Groves an immediate permanent cessation. These figures are, however, 

 much too high if strips of leaf-cells are used, and the whole of the current 

 passes through them. The true shock-stoppage is, however, usually merely 

 temporary, and its non-observance by Jurgensen was probably due to 

 the fact that the currents used by him did not gain their full strength 

 instantaneously. Occasionally, it is true, when streaming is very active 

 in cells of Vallisneria and Elodea, comparatively strong currents may only 

 cause a temporary retardation, and still stronger ones may cause either 

 a permanent stoppage, or a temporary one followed after a period of slow 

 streaming by a permanent cessation. 



In producing a shock-stoppage the rapidity with which the current 

 attains its maximal strength is of great importance. This can easily be 

 shown by interposing in the circuit long insulated coils of wire immersed 

 in water, and increasing the voltage of the battery so as to give the same 

 strength of current as before. Under such circumstances a certain strength 

 of current may act as a sub-minimal stimulus, although when the same 

 strength of current is suddenly applied in a free circuit of low resistance 

 a shock-stoppage may be produced. The current must also act for an 

 appreciable fraction of time such as a quarter or a half of a second, and 

 under such conditions T V tn to sSth of the current from a standard Daniell's 

 cell may suffice to cause a shock-stoppage in Nitella. If, however, a very 

 weak current is slowly increased in strength by moving the slider of 

 a rheochord, the stoppage usually only occurs with currents of distinctly 

 greater intensity, provided that the increased excitability resulting from 



1 Stud. cl. phys. Inst. zu Breslau, 1861, Heft I, p. 38. 



