MODIFICATION OF SUCTIONAL RESPONSE 385 



which the excitatory effect is shown by the very great 

 increase in the rate of suction induced after the application 

 of the reagent. We have here a very great enhancement of 

 the ascensional movement of water in spite of the osmotic 

 attraction of the solution, which alone would have retarded 

 the normal rate. 



The action of poisonous reagents. — In studying the 

 effect of poison on the rhythmic activity of Desmodium, we 

 found that this was modified by the tonic condition of the 

 plant. Thus a vigorous specimen was shown to be much 

 less affected by poison than one which was weakly. Certain 

 poisons, again, act more quickly than others in inducing 

 the death of the plant. We have also seen, in that experi- 

 ment in which the root was killed with hot water, that 

 upper and unkilled portions of a specimen will continue 

 to exhibit suctional activity when lower parts are killed ; and 

 also that, in general, the killed area offers no barrier to the 

 passage of water through its dead tissues. 



That a poison can easily pass through killed tissues, 

 owing to the suctional activity of cells higher up, we have 

 seen in our experiments on Desmodium, when the cut end 

 of the petiole was placed in copper sulphate solution (p. 326). 

 It is fortunate that in this case, during the ascent of poison, 

 we have areas, the activity of which is indicated visibly 

 by the rhythmic motile indications of the pulvini of the 

 inserted lateral leaflets. That copper sulphate solution 

 arrests rhythmic activity and induces death, is seen by 

 the rapid stoppage of pulsation, when we apply it directly 

 on the pulvini of the pulsating leaflets. When it is applied, 

 however, at the cut end of the petiole, the arrest of 

 pulsation takes place much later, this delay being due to 

 the time taken for the poison to ascend through the 

 intervening distance. This shows clearly that successive 

 zones are killed one after another, and that the death of a 

 point below does not stop the suction above. From this 

 experiment it is evident that the application of poison at 

 the root, or the cut end of a stem, does not in general arrest 



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