726 PLANT RESPONSE 



the plastic material of that zone, repeated hydrostatic blows. 

 The consequent expansive response was thus the indirect 

 effect of stimulation. 



It is thus the internal energy, ultimately derived from 

 external stimulus, that gives rise to those rhythmic activities 

 by which the pulsations of growth are maintained. When 

 the sum total of the latent stimulating factors that determine 

 the tonic condition is below par, there is an arrest of the 

 multiple response of growth, corresponding to the similar 

 arrest of multiple motile response in Desmodium. In a plant 

 in which growth is at standstill, it may be renewed by a fresh 

 supply of energy. Thus, if hot water be applied to the root 

 of such a plant, energy is hydraulically transmitted to the 

 growing region, and there re-initiates growth. 



If moderate stimulus be thus imparted, the responsive 

 growth-movement persists for a short time, and then comes 

 to a standstill, to be again renewed by a fresh supply. 

 Again, the movement of growth being due to the indirect 

 effect of stimulus, we might renew or accelerate it by apply- 

 ing stimulus, say, on the stem or its top, at such a distance 

 from the growing region that the direct excitatory effect 

 would not be transmitted to it. Stimulus applied directly on 

 the growing region would, however, by its true excitatory 

 effect, induce contraction and retardation of growth. 



The longitudinal growth thus described takes place in a 

 strictly radial organ. If the organ, however, be bilateral, 

 instead of radial, it will exhibit lateral oscillation, owing to 

 the alternate growth of the two sides. Or growth may pro- 

 ceed in a spiral line, giving rise to circular or elliptical move- 

 ments. A very good example of the last is afforded by the 

 torsional growth-movements of climbing plants. These 

 various circummutating autonomous movements of growth, 

 passing from regular movements in a circle, through ellipses, 

 to a straight line, are exactly paralleled by different 

 examples of autonomous mechanical responses in Desmo- 

 dium y where also we find circular, elliptical, and rectilinear 

 movements. 



