540 PLANT RESPONSE 



from forty-five to sixty minutes. It is thus unlikely that 

 amputation would permanently abolish the power of renewed 

 response. 



With regard, next, to the contention that amputation 

 would be liable to arrest that growth of the root on which 

 responsive curvature is supposed to depend, it is to be 

 borne in mind that though growth-activity is a sign of 

 excitability, yet we may have excitability without growth. 

 Hence it is quite possible that responsive curvature might 

 be initiated in a tissue which was not beforehand in a con- 

 dition of growth by the stimulus of gravity, as indeed was 

 found to be the case with grass haulms. 



Investigation on supposed abolition of excitability by 

 amputation of root-tip. — The question as to whether am- 

 putation of the tip abolishes the general excitability of the 

 growing region of the root, can only be decided finally by 

 the direct observation of the variation, if any, in the mechani- 

 cal response of the root to stimulus after amputation. Should 

 there be no such variation, then —the researches of Ciesielski 

 and Darwin having proved that a root deprived of its tip 

 does not respond to gravity — it will follow that the tip of the 

 root alone is geotropically sensitive. That this is so — that is 

 to say, that amputation does not permanently modify the 

 general sensibility of the root — I shall next proceed to 

 demonstrate. 



The experiments that have been carried out by different 

 observers, for the purpose of determining whether amputation 

 of the root-tip produces any modification of the rate of 

 growth, have led to very various results. Some have found 

 that it has the effect of inducing an accelerated rate of growth ; 

 others, again, state that it induces no change whatever in the 

 rate ; and still others find that it causes retardation of 

 growth. All these discrepancies are reconciled, however, 

 when we remember, as I have demonstrated, that indirect 

 stimulus causes increase, and direct stimulation retardation, 

 of growth. Moderate stimulation, therefore, by section of the 

 extreme tip, giving rise to the transmitted effect of indirect 



