EXCITATORY CHARACTER OF SUCTION AL RESPONSE 383 



activity was again renewed, after a latent period of two 

 minutes, and found to persist for a considerable time, even 

 on the cessation of stimulation (fig. 226). We have here, 

 then, an exact parallel to the renewal of the so-called 

 autonomous response of Desmodium leaflet referred to 

 above. 1 



We have thus studied the phenomenon of the variation of 

 suction by renewal when found at zero. We shall next con- 



FIG. 226. Renewal of Suction, Pre- 

 viously at Standstill, by Action of 

 Stimulus 



The half-shaded portion of figure repre- 

 sents time of application of stimulus. 

 Suctional response is seen to be 

 initiated after a latent period of one 

 minute, and to persist after the cessa- 

 tion of stimulus. 



FIG. 227. Photographic 

 Record of Effect of 

 Stimulus in Enhancing 

 Rate of Suction 



This record was taken 

 under balanced con- 

 ditions. Vertical line 

 represents moment of 

 application of stimulus 

 for 30 seconds. 



sider the case of a variation induced in the existing rate by 

 the action of stimulus. The normal rate is exactly balanced, 

 a condition which is represented in the photographic record by 

 the straight line which results from the stationary position of 

 the mercury index. Stimulus of 5 seconds' duration was now 

 applied, and the responsive acceleration is seen as a steep rise 

 in the record (fig. 227). This responsive acceleration persists 



1 If a cut branch of any plant be kept in water for several days, its suctional 

 activity, as is well known, disappears. This is commonly attributed to the 

 blocking of the cut end by mucilage and bacterial growths, since the making of 

 a fresh section is found to renew the activity. This making of a fresh section, 



