64 PHYSIOLOGY OF STREAMING MOVEMENTS 



a withdrawal of water from it, or by chemical or physical changes in its 

 substance. The net result is a decrease in the velocity of streaming. At 

 temperatures above 40 C. a continuous retardation is shown, leading ulti- 

 mately to a complete cessation accompanied by an irremediable overthrow 

 of the vital equilibrium. Death is here the result of heat pyrexia. The 

 almost immediate stoppage at very high temperatures is the result of the 

 partial or complete coagulation of the protoplasm. 



At temperatures lying between 10 C. and 30 C. the immediate stream- 

 ing tempo is usually also the permanent one, provided no other factors come 

 into play, such as an inadequate supply of oxygen, or an insufficiency of 

 food. Under either of these latter circumstances a rise of temperature 

 brings about a retardation or cessation of streaming sooner than would 

 otherwise have been the case. 



Below ioC. the acceleration due to a rise of temperature is usually 

 not as great as it subsequently becomes. This is probably because 

 the semi-dormant vital activity is not immediately awakened to the 

 full extent possible at the particular temperature, and the same thing 

 commonly occurs as regards growth, photosynthesis, &c., when plants 

 that have been kept at a low temperature for some time are brought 

 into a warm room. 



In the case of plants in which streaming is usually secondary in 

 character (Elodea, Vallisneria), and less intimately connected with the 

 vitality of the cell than it is in Chara and Nitella, there is a greater ten- 

 dency to internal changes which modify the natural response to a change 

 of temperature. Thus, if leaves of Elodea are suddenly raised from i8C. 

 to 35 C., streaming commences in one to two minutes, and becomes active 

 in half to one hour. A similar sudden rise to 40 C. produces a more 

 rapid response, although after two to three hours at 40 C. streaming is 

 slower than at 35 C. If now raised to 50 C. the velocity increases slightly 

 in the first one or two minutes, but in fifteen minutes is from \ to -| the 

 velocity at 40 C. If now allowed to fall slowly to 30 C. the velocity 

 steadily increases until greater than at any previous temperature. The 

 character of the material, therefore, changed during experimentation. 

 Similarly, in cells of Vallisneria showing slow streaming the latter may 

 steadily decrease from 35 C. onwards, ceasing after one minute at 50 C., 

 although on lowering to 30 C. it soon becomes rapid. 



Hence adult cells exhibiting active and well-established streaming 

 should be used for experimentation. Leaves of Elodea may be treated 

 with dilute glycerine, and then washed in water, while section-cutting 

 usually affords a sufficiently powerful stimulus to prolonged streaming 

 in Vallisneria. 



