INTERNAL ENERGY AND EXTERNAL STIMULUS 425 



responses of (a) this order is reversed, the amplitude being at 

 first great, and undergoing a steady decrease to the end. 



(2) Effect of external stimulus on growth when at 

 standstill. — It will be remembered that the rhythmic excita- 

 tion of Desmodium comes to a standstill under unfavourable 

 circumstances — that is to say, when the sum total of internal 

 energy has fallen below par ; and when this has happened, 

 the application of fresh external stimulus is found to renew 

 the activity. Growth-response, similarly, comes to a stop 

 when the plant is in an unfavourable > 



condition with regard to light, tem- 

 perature, or moisture. I shall now 

 show that under such circumstances 

 the application of external stimulus 

 is found to be competent to renew 

 growth. Taking a specimen of the 

 hypocotyl of Tamarindus indicus, in 

 which growth had come to standstill, 

 I stimulated the plant by thermal 

 means, and this was found to renew 

 the multiple response of growth, as 

 is clearly seen in fig. 171. The pulses 

 are here characterised by interesting 

 periodic variations. The period of 

 each is relatively long, the average 

 value being about six minutes. In 

 some other cases the renewed pulsa- 

 tions were so rapid as almost to 

 appear continuous. When the tonic condition of the plant 

 was very low, the energy supplied by brief stimulation was 

 only sufficient to maintain the rhythmic growth-activity 

 for a short time, and after this the plant would again return 

 to the state of standstill. It will thus be seen that when the 

 tonic condition of the plant is below par, the applied external 

 stimulus is absorbed, and, becoming latent, serves as internal 

 energy for the production of growth-response (cf. p. 462). I 

 shall adduce other instances of this in the course of the present 

 chapter. 



FlG. 171. Renewal of 



Growth-pulsation by Ther- 

 mal Stimulus in Tamarin- 

 dus indicus originally at 

 Standstill 



