EXCITATORY CHARACTER OF SUCTIONAL RESPONSE 373 



at an even greater rate than ordinary. This result would 

 at first appear to show that protoplasmic activity had 

 nothing to do with the ascent of sap, and the objection 

 would have been fatal if the activity which produces suction 

 had been confined to the roots alone. But in reality, as we 

 have already seen, such activity is present, to a greater or 

 less extent, throughout every zone of the plant. The only 

 part which is killed, however, in the experiment just 

 described, is that which is actually immersed in, or in 

 immediate contiguity with, the boiling water ; and the 

 unkilled tissues above continue their suctional activity 

 unabated. The increase in the rate of suction is to be ex- 

 plained by the fact that the entrance of water, instead of 

 being effected through the extremely attenuated channels 

 of the root-hairs, now takes place through the whole mass of 

 the root, acting virtually as a wet rag tied round the base 

 of the living stem. The mass of water which it is thus 

 possible to suck up directly through the broad-sectioned 

 stem is evidently much greater than could have been taken 

 in through the resistant, organically-conducting channels of 

 the rootlets. 



The fact thus demonstrated, that the local death of a 

 given zone does not/^r se arrest the suctional activity of the 

 tissues above it, explains why a poison may be carried to the 

 top of a tree. It is evident that only when it has thus been 

 conveyed, and when all the tissues have thus been killed, 

 could a permanent arrest take place. And Strasburger him- 

 self admits that arrest under these conditions does occur. 

 , With reference to the effect of poison, again, it is im- 

 portant to bear in mind that its toxic effect depends, to a 

 certain extent, on the tonic condition of the tissue. Thus a 

 Desmodium leaflet which was moderately vigorous had its - 

 pulsatory movement arrested soon after the local application 

 of the copper sulphate solution. In more vigorous leaflets, 

 however, the arrest did not take place till after a considerable 

 length of time. Indeed, in some cases, after a preliminary 

 arrest, the leaflet is able to shake off the effects of the poison 



