382 PLANT RESPONSE 



than the original rate. This was due to the fact that, instead 

 of the extremely attenuated channels of the root-hairs, 

 through which suction normally takes place, there was now 

 substituted the whole mass of the root, acting virtually as a 

 wet rag, tied round the base of the living stem ; and indeed 

 it was found that, whereas the stem outside the vessel was 

 turgid, the portion within was limp and soft. The mass of 

 water which it was thus possible to suck up directly, by 

 means of the broad-sectioned stem, was evidently much 

 greater than could have been the case through the interven- 

 tion of the resistant organically conducting channels of the 

 rootlets. 



We must not forget the obvious fact that a plant is a 

 colony of more or less independent living cells, each of which 

 maintains its physiological activity as an individual. The 

 death of one group does not necessarily, therefore, arrest 

 the physiological activity of its neighbours. The plant is 

 finally killed only when everyone of its cellular elements has 

 undergone death. 



Further proof that suction is an excitatory response.— 

 We have seen in the case of rhythmic Desmodium, when it is 

 kept for a long time under unfavourable circumstances, that its 

 activity comes to a stop owing to the run-down of stored-up 

 energy. We also saw how the application of thermal stimulus 

 would re-initiate this activity. Again, if we keep a cut branch 

 of any plant in water, after a few days its suctional activity, 

 as is well known, disappears. This abolition of suction is 

 attributed to the blocking of the cut end by mucilage and 

 bacterial growths, and the making of a fresh section is found 

 to renew the activity. 



But, though the blocking of the cut end of the stem by 

 outgrowths does, no doubt, obstruct the passage of water, 

 yet the total abolition of suction may not be due to this 

 cause alone. It may be induced, in part at least, by depres- 

 sion of the rhythmic activity of the tissue, owing to the run- 

 down of its latent energy. The making of a fresh section 

 does not decide this question, for, in doing this, we apply 



