74 PHYSIOLOGY OF STREAMING MOVEMENTS 



the entire cell, the mechanical disturbance being propagated with great 

 rapidity in the cell-sap and directly stimulating each portion of the 

 endoplasm, which latter responds either at once or after a latent period 

 common to the entire cell. 



The latent period of recovery varies from a few seconds to several 

 minutes, according to the strength of the stimulus applied, the previous 

 condition of the cell, the temperature and the supply of oxygen. After 

 a strong shock, the recovery takes longer than after a weak one, but an 

 increasing stimulus produces rapidly decreasing increments to the time of 

 recovery, and ultimately no recovery at all. When streaming was previously 

 slow, it is usually a longer time before it is active again, and the new 

 velocity may be increased, whereas recovery is more rapid in cells which 

 were originally actively streaming. Similarly a low temperature or a 

 deficient supply of oxygen prolongs the latent period of recovery, which 

 takes place, for example, more rapidly at 25 C. to 30 C. than at 10 C. 

 to 15 C. 



After several repetitions of the same shock-stoppage accommodation 

 ensues, and the latent period of recovery may decrease from one to two 

 minutes to 20 or 60 seconds, while if a nearly minimal stimulus is used, after 

 ten to twenty repetitions a slightly stronger shock is required to produce 

 a complete stoppage. A very severe shock may inhibit streaming for 

 as long as five or rarely ten minutes, although a stoppage lasting more 

 than five minutes is usually permanent in the case of large adult cells and 

 indicates fatal injury. 



Similar phenomena are exhibited by Elodea and Vallisneria, in which 

 the streaming is usually secondary in origin. The cells of Vallisneria are 

 readily killed by too violent a shock, while the stoppage is shorter and less 

 easily produced when streaming has only recently become active than 

 when it has been active some hours or days. Elodea is less sensitive 

 than Vallisneria, and although many of the cells are killed by a shock 

 sufficient to produce stoppage throughout the entire field, streaming may 

 easily be stopped temporarily in single cells by locally applied mechanical 

 stimuli. At I5C. streaming recommences in from one to five or ten 

 minutes, but at 25 C. in from twenty seconds to two or three minutes. 

 In some cases the shock awakens streaming in previously quiescent cells. 



A mechanical disturbance may be rapidly propagated to a neigh- 

 bouring cell, and may act as an inhibitory stimulus there also, but the 

 transference is a physical phenomenon, and hence affords no proof of 

 the existence of plasmatic connexions between the cells, as Hermann 

 supposes, for in these connexions the transmission of stimuli takes place 

 relatively slowly. 



If a cell of Nitella or Chara is snipped in two, the neighbouring cells 

 may suddenly cease to show rotation for one or two minutes. Probably 



