INTERNAL ENERGY AND EXTERNAL STIMULUS 



435 



hitherto horizontal base-line. This bifurcation causes the 

 first contractile response of the now growing organ — that is 

 the sixth response of the record — to be smaller than usual. 

 But as the tonic condition is established, and the molecular 

 mobility of the responding organ is increased, the contractile 

 response becomes larger, and growth goes on at a certain 

 steady rate. From this intermediate link we pass on to the 

 case of response to stimulus 

 of a style of Datura which is 

 in a state of uniform growth 

 (fig. 179). Here also we find 

 that stimulus produces the 

 normal contractile effect. All 

 these clearly demonstrate that 

 the response of growing 

 organs is in no way different 

 from that of stationary organs. 

 In a stationary organ, sti- 

 mulation produces negative 

 turgidity-variation, resulting 

 in contraction. The same 

 contraction is seen in growing 

 organs, causing a temporary 

 retardation of the rate of 

 growth. 



The retardation of growth 

 which is caused in a growing 

 organ by external stimulus 

 may be exhibited in a some- 

 what different way. We may use the Method of Balanced 

 Record, by which the normal rate of growth is made to appear as 

 a neutral horizontal line. For the purpose of this experiment 

 I took the growing peduncle of a Eucharis Lily. The balanced 

 record (fig. 180) is here seen to be horizontal, save for 

 minute autonomous oscillations about the neutral line. Sti- 

 mulation, in this case, was produced by tetanic electric shocks 

 from an induction coil, which were applied on the growing 



F F 2 



Fig. 178. Photographic Record of 

 Responses of Style of Daiura alba 

 in which Growth had come to a 

 Temporary Slop 



The up curve shows contraction. As 

 long as the base-line is horizontal, 

 growth is seen to be at standstill. 

 Renewal of growth at sixth re- 

 sponse, after which growth-elon- 

 gation is shown by the trend of 

 the base-line downwards. 



