MECHANICAL STIMULI 



75 



the sudden removal of the osmotic pressure on one side of each partition- 

 wall operates as a sufficiently powerful mechanical disturbance, for as 

 a matter of fact a suddenly removed hydrostatic or mechanical pressure 

 does act as a temporary inhibitory stimulus to streaming. If streaming 

 was slow in the cells adjacent to the injured one, it may become in ten 

 to thirty minutes one and a half to twice as rapid as it was previously. 

 This may possibly be due to the transmission of a vital stimulus by 

 plasmatic connexions between the injured dying cell and the living ones, 

 and the length of the latent period is such as to permit this interpretation. 



The mechanical shock-stoppage is the result of an internal pulsation 

 of the cell-sap which acts directly upon the particles of endoplasm, and 

 either temporarily disturbs the arrangement essential for streaming, or 

 sets up vibrations which prevent the one-sided liberation of energy 

 necessary for its maintenance. A similar stoppage can be produced 

 in Chara and Nitella by vibrations reaching the cell through a dense 

 external medium (water), but not through a less dense one (air). 



As regards the distinction between primary and secondary streaming 

 in cells, Hauptfleisch 1 gives the following critical test for the latter : the 

 first section cut exhibits streaming only after some time has elapsed, but 

 now a second one cut from the same surface exhibits streaming im- 

 mediately on examination. Suddenly applied pressure does, however, 

 cause a temporary cessation of streaming, although Hauptfleisch denies 

 this. Hence, the first section might show no streaming owing to the 

 shock of section-cutting, while in the case of the second section, as the 

 result of the first shock and recovery, a stronger stimulus might now 

 be required to produce a stoppage. This difficulty may be partially 

 obviated by allowing as long a time as possible to elapse between the 

 preparation of the first and second section, and by noting the time 

 streaming takes to commence in the first case. This latter is usually 

 considerably longer than the latent period of recovery from a shock- 

 stoppage, but in some cases, as Hauptfleisch himself shows, streaming 

 begins in directly stimulated quiescent cells within five minutes (Elodea, 

 Vallisneria, &c.), although in others not until after fifteen minutes to an 

 hour. Even in the first case, however, temporarily inhibited primary 

 streaming will recommence almost simultaneously over the entire pre- 

 paration, whereas secondarily induced streaming will start first in the 

 directly stimulated cells and spread slowly in regular sequence to sur- 

 rounding cells, if the stimulus is sufficiently powerful and prolonged, 

 viz. mechanical injury involving the death of certain cells. In the absence 

 of a mechanical shock, cells bordering an injured one may be stimulated 

 by exuded chemical products, or by the death of the connecting plasmatic 



Jahrb. f. wiss. Bot, 1892, xxiv, pp. 190-200. 



