364 COMPARATIVE ELECTRO-PHYSIOLOGY 



must be distributed throughout the plant. Death of a given 

 zone, then, would arrest the activity of that particular zone 

 but not that of another zone higher up. Thus a poisonous 

 solution would only abolish activity in those cells which it 

 had already reached. The activity of cells above would 

 remain unaffected. That the death of cells below offers no 

 resistance to the passage of water, when suctional activity is 

 maintained above I have demonstrated elsewhere, and shall 

 again show in the course of the next chapter, in cases in 

 which the lower part of the plant was killed by boiling 

 water. Under the action of poisons, similarly, I have been 

 able to show that a poison can pass easily through killed 

 tissues owing to the suctional activity of cells higher up. 

 This was demonstrated by means of experiments on Des- 

 modium gyrans, where the cut end of the petiole was placed 

 in copper sulphate solution. It is fortunate that in this case, 

 during the ascent of poison, we have areas whose activity 

 is manifested 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 only takes place after sufficient time has 

 elapsed 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 evi- 

 dent that the application of poison, at the root, or the cut end 

 of a stem, need not be expected to arrest suction until the 

 whole plant has been killed, and from Strasburger's account of 

 his experiment it is evident that the movement of water did 

 come to a stop when the poison reached the top of the tree. 



We thus see that the objections which have been raised, 

 with regard to the physiological nature of the ascent of sap, 

 are not valid. I shall therefore proceed in the next chapter 

 to describe crucial experiments in demonstration of the fun- 

 damentally excitatory character of this process, 



