62 PHYSIOLOGY OF STREAMING MOVEMENTS 



The following detailed examples will illustrate these points :- 



On lowering the temperature to 20 C. streaming slows and ultimately 

 ceases, whereas the Rotifers and a few Vorticellae recover and again 

 move actively. If the temperature is raised 5 C. every two or three 

 minutes, streaming may not entirely cease until from 55 C. to 60 C. 

 is reached, whereas if suddenly plunged into water at from 40 C. to 

 50 C. it immediately and usually permanently ceases, and the same 

 ultimately occurs after prolonged exposure to 40 C. to 50 C. There 

 is, therefore, a particular rate of rise of temperature at which stream- 

 ing persists longest. A very sudden rise does not give any time for 

 accommodation, and completely deranges the motor-mechanism at rela- 

 tively low temperatures. A very slow rise extending over hours or 

 days exercises a cumulative effect, and hence again lowers the maximal 

 temperature. 



After an exposure of ten minutes to 50 C. irregular streaming may 

 still be present in Nitella, and may become normal again at 20 C. Similarly, 

 if the moment streaming has ceased at 69 C. the preparations are cooled, 

 in a few cases the cells remain living, though streaming is frequently slower 

 than at first. Cells killed by sudden heat-rigor do not contract, or only 

 slightly after several hours. This is obviously due to the rapid coagula- 

 tion of the protoplasm, and the fluctuations in rapidity often shown at high 



