REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 709 



india-rubber tubing, the lower half of which is thinner, and 



therefore more expanding or responsive, than the upper. 



The upper end of the tube, representing the end of the plant, 



is closed over with a thin elastic strip of india-rubber. When 



now there is contractile action at the base, the pulse of 



increased hydraulic pressure thus induced is found to bring 



about a practically simultaneous erection of the artificial 



lateral leaf L, and an expansion or bulging outwards of the 



terminal yielding body G. The ..^ 



analogy in the case of the latter 



would be still more perfect if 



we imagined it covered, instead 



of with india-rubber, with some 



plastic substance which would set 



quickly on elongation or expansion, 



thus representing the permanent 



growth-effect. From this we see 



the connection between growth and 



those other responses with which 



we are already familiar. The only 



difference between the responsive 



expansion of pulvinated organs and 



this responsive expansion of growth 



lies in the fact that in the former 



recovery is perfect, and in the latter 



imperfect. It must also be borne 



in mind that growth may be 



initiated locally at the growing region if the sum total 



of energy, directly or indirectly supplied to it, be above 



par. 



We further see in the case given that growth-response is not, 

 strictly speaking, ' of its own accord ' or spontaneous ; for the 

 energy of a definite stimulus, causing contractile movement 

 at the base, is transmitted hydraulically, and performs the 

 work of growth. It would be as accurate to describe the 

 work done by hydraulic machinery as spontaneous, ignoring 

 the energy that had set the pump in action, as to call growth 



277. Model of an Arti- 

 ficial Plant 

 Sudden compression of india- 

 rubber bull), R, causes 

 hydraulic wave upwards, 

 downward movement being 

 prevented by valve V. 

 This wave of increased 

 pressure causes erection of 

 the leaf, l., and the expan- 

 sion of the terminal se[)lum 

 into c. 



