468 PLANT RESPONSE 



after-effect. Thus a plant under natural conditions, acted 

 upon by different stimuli, gives to each stimulus direct re- 

 sponse, direct after-response, and indirect response. But as 

 all these individual stimuli do not act, or cease to act, 

 simultaneously, we can easily understand the infinite com- 

 plexity of the combinations which take place between the 

 direct and indirect effects of stimuli of unlike forms, whose 

 maxima, instead of being coincident, are superposed on 

 each other, with various differences of phase. 



Factors which determine periodic after-effects. — We 

 shall now proceed to enumerate some of the most important 

 stimulating factors instrumental in modifying the response of 

 growth. 



(i) Stimulus of light. — If we take the average rate of 

 growth during the twenty-four hours as the normal, then the 

 direct effect of this stimulus will appear as a retardation of that 

 normal rate of growth, and, after the long-continued action 

 of the whole day's illumination, this retardation may persist 

 for a time as the positive after-effect. Later, however, on 

 account of the stimulus which has been absorbed and held 

 latent, we shall observe an acceleration of growth above the 

 normal, or a negative after-effect. The persistence, again, of 

 this negative after-effect will depend on the amount of the 

 energy held latent by the tissue. The diurnal sequence of 

 light and darkness will, after long repetition, impress itself 

 upon the organism, and, other factors remaining constant, 

 will find expression as periodic retardation and acceleration 

 of growth during day and night ; such periodicity continuing 

 to show itself, for some time, even when the plant is kept in 

 continuous darkness. 



(2) Temperature. — The effect of temperature up to the 

 optimum point will, by increasing the internal energy, prove 

 favourable to growth. Thus, the temperature during day- 

 light will usually be favourable, with the exception of the 

 tropical noon, when it may be excessive. A certain amount 

 of heat, again, may be stored up in the plant to give the after- 

 effect. At night, if the fall of temperature be very great, 

 there will be, relatively to this factor, a retardation of growth. 



