3*^ LIFE MOVEMENTS IN PLANTS 



of the 'excitation curve' (Fig. 180) exhil:)ils increasing in- 

 cipient contraction (retardation of growth) ciilminatin<» in an 



Fig. 130. — Effects of continuous (a) electric, and (6) photic stimulation on rate of 

 growth. Abscissa represents duration of application of stimulus. Note induced 

 retardation, and arrest of growth. 



arrest of growth ; the abscissa represents increasing stimu- 

 lus consequent on increased duration of application. The 

 record shows that the incipient contraction is slight at 

 the first stage ; it increases rapidly in the second stage ; 

 finally, it declines and reaches a limit. The excita- 

 tory reaction is thus not constant throughout the entire 

 curve of excitation, but undergoes very definite and 

 characteristic changes. We shall find similar characteristics 

 in the phototropic carves under unilateral stimulus which 

 will be given presently. The explanation of the similarity 

 is found in the fact that the tropic curvature is also due 

 to incipient contraction or retardation of the rate of 

 growth, which remains confined to the directly stimulated 

 proximal side of the organ. 



For facility of explanation of what follows, I shall 

 have to use a new and necessary term, siiscejjtibility, to 

 indicate the relation of cause and efi"ect, of stimulus and 



u- •. 4.- cr 4 1 Tt ■ ^u Excitation 



resultmg excitation. Susceptibthty is thus = -rrr. v 



° x- c/ btimulus 



