92 PHYSIOLOGY OF STREAMING MOVEMENTS 



the previous sudden excitation is allowed to pass away before the slowly 

 increasing current is applied. These results therefore confirm those 

 obtained by Hermann in a somewhat different manner (1. c., p. 63). 



Weak currents frequently accelerate streaming both as a direct effect 

 and as a transitory after-effect. The acceleration is especially noticeable 

 when streaming was previously slow. After such currents have been 

 passing for one or more hours, a secondary retardation usually becomes 

 manifest, and this leads ultimately to a permanent stoppage and death. 

 Frequently, however, temporary spasmodic accelerations of streaming are 

 shown until just before the final stoppage and death. 



After a weak constant current has been passed through a cell for some 

 time, it becomes less sensitive owing to accommodation or fatigue, and 

 may now not show any direct response to a current of treble the intensity 

 of one which previously caused a temporary shock-stoppage. The sudden 

 cessation of a constant current may also cause a shock-stoppage, and this, 

 according to Hermann, is owing to the disappearance of the anelectrotonic 

 condition at the anode constituting an excitation. 



SECTION 40. Constant Currents at varying Temperatures. 



The plant-cells were placed in a hot chamber on the under side of 

 a cover-slip in a minimal quantity of water, and were laid across two 

 greased lines on the cover-slip. At these points the water does not adhere, 

 and hence the whole current used passed through the cell examined, which 

 lay between the electrodes but not actually upon them. The platinum 

 electrodes were sealed to the wires within small glass tubes entering the 

 hot chamber through the metal tubes provided for the entry and exit of 

 gases. The appended figure represents a simple arrangement suitable 

 for demonstration if provided with a rheochord or coils of resistance wire 1 . 

 For accurate experiments a galvanometer and shunt are required. 



An electrical stimulus which is sub-minimal at a low temperature 

 may suffice to produce a shock-stoppage at a higher one. This applies 

 in general to temperatures lying between 10 C. and 40 C. ; above and 

 below these limits a stronger current being usually required, the cell being 

 less readily excitable. The latent period of recovery on the other hand 

 steadily decreases as the temperature is raised from o C. to from 36 C. 

 to 45 C., above which temperatures it rapidly increases to infinity. This 

 effect of temperature is best shown when a sub-maximal stimulus is used 

 and allowed to act for a regulated period of time (two to ten seconds). 



1 In accordance with the general law concerning the influence of temperature on the electrical 

 conductivity of electrolytes, the resistance of the filament decreases slightly as the temperature rises, 

 and hence the current traversing it increases correspondingly, but if care is taken that the external 

 resistance is at least 100 times that of the heated portion the error becomes insignificant. 



