374 EFFECT OF MOLAR AGENTS [Ch. XIV 



evident. Thus, in general, the more rapid the growth, the less 

 is the retardation provoked by pulling. 



The direction in which we must look for an explanation of 

 these facts is indicated by the circumstance that, in the most 

 rapid period, growth is chiefly due to imbibition of water, while 

 in the preceding and succeeding periods it is due more to an 

 increase in the plasma. Thus not all kinds of growth are 

 equally affected by the irritation of pulling, but principally that 

 growth which is due to assimilation. Further insight into this 

 matter is gained from the circumstance that despite the fact 

 that growth is slower in the plant under tension the tur- 

 gescence in the stretching zone is greater in such a plant than 

 in a normal one. This indicates again that it is not the imbib- 

 itory process which is interfered with but rather the assimila- 

 tive one. All these facts thus lead to one conclusion, that, 

 under tension, the plasma, especially that of the cell-wall, 

 grows in length less rapidly than under normal circumstances. 

 This diminution of growth can hardly be explained in a direct 

 mechanical way ; we must consider it a response to the 

 stimulus of pulling. 



A confirmation of this conclusion is found in the fact that 

 the effect of the pulling gradually wears off. Thus when one 

 of a pair of seedlings of Cannabis sativa is subjected to a pull 

 of 20 grammes, there is a retardation during the first day of 

 61% in the stretched plant as compared with the control plant; 

 during the second day of 51% ; during the third day of only 

 9%. (Hegler, '93, p. 389.) In order that the retardation 

 should continue, additional weight must be imposed ; then an 

 increased retardation occurs. Thus in one case Heglee sub- 

 jected one of two Helianthus seedlings to a pull of 50 grammes. 

 During the first day the retardation of the pulled plant was 

 20% ; during the second there was an excess of growth over 

 the control of 17%; then 150 grammes were added; on the 

 third day the retardation was 18% ; on the fourth there was 

 an acceleration of 2%. This series of phenomena is clearly 

 like that which we have observed in locomotion — there is an 

 accommodation of the growing protoplasm to the stimulus. 



There can be little doubt that in the cases of diminished 

 growth in length there is a thickening of the cell-walls and 



