362 PLANT RESPONSE 



We have seen in Chapter XXI. that when a strong 

 stimulus is applied to the base of any organ, say a stem, 

 a multiple series of excitatory waves is propagated onwards, 

 such multiple responses being detected by electrotactile or 

 electromotive pulsations. It was also shown in Chapter 

 XXVI. that such multiple, passing into automatic, response, 

 may be induced by the action of a constant stimulus. It 

 was further demonstrated in Chapter XXI. that these excita- 

 tory waves, and the concomitant cell-to-cell contraction, would 

 produce a movement of water forwards, along the direction 

 of propagation. This series of excitatory waves, proceeding 

 from the base of the organ, and propelling water forwards, 

 must then cause a deficit of water behind. If, however, the 

 base of the organ be kept supplied with water, this deficit will 

 be made up by suction. 



It is thus seen that by such rhythmic activity a one- 

 directioned movement of water may be produced. Just as 

 the various effects produced by multiple or autonomous 

 response — in, for example, the electromotive and electro- 

 tactile responses, and in the multiple mechanical responses of 

 Desmodium — give us an indication of the degree of rhythmic 

 activity exhibited by the tissue, so, in the rate of this water- 

 movement also, we have an additional means of measure- 

 ment. This would be analogous to the measurement of 

 the rhythmic activity of the heart by a determination of the 

 rate of flow of the circulating blood. In the case of the 

 plant this rate of movement may be measured, either by 

 means of the propulsion of water forwards or by the suction 

 exerted behind. 



The ascent of sap in the plant, then, may be brought 

 about by the rhythmic activity of the tissue. How this 

 activity is initiated will be discussed later. Meanwhile it 

 is clear that if the movement of sap be really an expression 

 of protoplasmic activity, then any physiological modifica- 

 tion which tends to increase that activity will also tend to 

 increase the rate of movement ; and pari passu any physio- 

 logical condition which tends to depress the activity, will 



