MECHANICAL STIMULI 



73 



pressure tubing. The stoppage may take place almost instantaneously, 

 or may occur after a latent period of a few seconds, and similarly streaming 

 may recommence either almost immediately or after a few seconds to 

 a minute or more. 



The simplest mode of producing a shock-stoppage is to lay a small 

 cover-slip over the object, and then to allow a thin metal rod to fall through 

 glass tubes of various lengths, which are held over the cover-slip but do 

 not touch it. Or rods of different weight may be used and allowed to fall 

 from the same height, the force of impact being then directly proportional 

 to the mass. The magnitude of the minimal impact to cause a complete 

 stoppage depends upon a variety of factors, such as (i) the temperature, 

 (2) the velocity of streaming, (3) the age of the cell, (4) its previous 

 treatment, (5) the time during which the compressing force acts. 



At low temperatures a response is less readily induced than at high 

 ones, provided these are not above 40 C. On the other hand, at the same 

 temperature a smaller stimulus is required when streaming is fairly slow 

 than when it has become rapid. Streaming stops more readily in young 

 cells than in adult ones, and the stoppage lasts longer in the former case 

 than in the latter. Previous sub-maximal stimulation of almost any kind 

 renders the cell less responsive to mechanical stimuli. The more suddenly 

 the force is applied, the more powerful is its action, a smart shock being 

 much more effective than one applied more gradually. Thus if bodies 

 of different weight are allowed to fall on the cover-slip from such heights 

 that they have the same momentum on impact, the smaller body with the 

 higher velocity exercises the greater shock-effect, and when it just affords 

 a minimal stimulus, the heavier body may afford a sub-minimal one. 

 None of these factors have previously been taken into account, and hence 

 it is hardly surprising that widely divergent results should be obtained 

 by different investigators or even by the same one. 



A shock which is not sufficiently powerful to cause a complete 

 stoppage may produce a more or less marked retardation, usually of short 

 duration. A stronger minimal stimulus is required in the case of large 

 axial cells of Nitella than in the smaller branch cells, provided these are 

 of equal ages or nearly so ; but if the intensity of the stimulus is made 

 proportionate to the surface-area of the cell the reverse is the case. The 

 same applies still more markedly to comparisons made between the 

 end-cells and exposed medullary cells of Chara. 



The stoppage usually almost immediately follows the application of 

 the stimulus, but a latent period amounting occasionally to eight or even* 

 ten seconds may intervene between the two. The latter usually occurs 

 when a nearly minimal stimulus is applied to a slowly streaming cell, but 

 sometimes also in large cells in which streaming is fairly active. The 

 stoppage always occurs simultaneously, or almost simultaneously, over 



