THE INFLUENCE OF LIGHT 69 



With regard to the optimal and maximal points, their determination 

 is somewhat difficult, for successive observations frequently fail, either 

 owing to the non-recovery of the cell or a permanent alteration of its 

 tempo. By frequent repetitions, however, the interesting fact was established 

 that with short periods of exposure the maximum point for streaming is 

 raised during anaerobic respiration from 3 C. to 5 C., and the optimum 

 by from about 5 C. to 8 C., these two cardinal points hence being nearer 

 together than is normally the case. 



In the absence of one stimulus (oxygen) the other (temperature) must 

 apparently be intensified to produce the maximal velocity. 



With prolonged exposures, however, the maximal point is lowered 

 by as much as 5C. or ioC. below the normal, whereas even after two 

 or three hours in pure hydrogen the optimum point either approximates 

 closely to that for aerobic respiration or may still be slightly above it. 

 In such cases a rise of only a few degrees above the optimum temperature 

 may cause the velocity of streaming to fall abruptly to nil. In all cases 

 the recovery from the effects of exposure to high temperatures is much 

 retarded and often permanently so, in an atmosphere of hydrogen, especially 

 if no internal supply of oxygen by GO 2 -assimilation is possible. From 

 the above facts it may safely be concluded that the stoppage produced 

 by prolonged exposure to temperatures approaching 45 C. in the presence 

 of oxygen is not due to a partial coagulation of the protoplasm, but 

 mainly to some disturbance in the motor-mechanism. A partial coagulation 

 and consequent increase of viscosity may take part in the retardation and 

 ultimate stoppage produced between 45 C. to 55 C., and above 55 C. 

 is probably mainly responsible for the stoppage. Complete coagulation 

 involving irremediable injury usually takes place subsequently to the 

 stoppage, whereas as a general rule recovery is possible from a stoppage 

 caused by slight partial coagulation, or by a temporary disturbance of the 

 motor-mechanism. 



When the temperature is lowered, streaming ceases sooner in the 

 absence of oxygen than in its presence, i.e. the minimal point is raised 

 3 to 5 C. during anaerobic respiration. This is probably the result of the 

 decreased liberation of energy acting conjointly with the increased viscosity 

 at low temperatures. 



SECTION 28. The Influence of Light. 



A slight retardation of streaming in the sensitive plasmodia of 

 Myxomycetes was noticed by Hofmeister and by Baranetzsky l when they 



1 Hofmeister, Pflanzenzelle, 1867, p. 21 ; Baranetzsky, Mem. de la Soc. des sci. nat. de Cher- 

 bourg, 1876, xix. 



