RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 577 



light on the growing region will be a responsive con- 

 cavity. 



2. The effect of such sttmukis of light, acting on the tip 

 of either shoot or root, will be a convexity of the same side of 

 the responding region. 



3. Strong or long-continued stimulation of light acting 

 on the growing region will induce a neutral or reversed 

 effect. 



4. The response, under certain conditions of continuous 

 stimulation, may be characterised by pulsations. 



It has been shown in previous chapters that the responsive 

 action of anisotropic or dorsi-ventral organs is not funda- 

 mentally different from that of radial organs, the seeming 

 differences being accounted for by differential action. In 

 explaining the various effects of stimulus of light, then, the 

 assumption of various sensibilities in the plant is unjustified. 



I shall attempt, therefore, to trace out the manner in 

 which one fundamental effect of responsive contraction is 

 made to find diverse expressions, owing to the anatomico- 

 physiological differentiation of the responding organ. And as 

 the supposed different specific sensibilities do not exist, I 

 shall designate all movements and curvatures induced. by 

 light as heliotropic, the signs positive, negative, and dia- being 

 used only for descriptive purposes. An investigation into the 

 action of stimulus of light, then, must apply itself to the 

 following points : 



1. The effect of unilateral stimulus, of varying intensity 

 and duration, on the tips and growing regions respectively of 

 radial organs. 



2. The induction of autonomous movement by the absorp- 

 tion of energy of light. 



3. The action of stimulus of light on molecularly aniso- 

 tropic and on dorsi-ventral organs. 



4. The direct and after-effects of light, in inducing move- 

 ments of daily periodicity. 



These are the questions which will be specially dealt with 

 in the course of the following chapters. 



V 1' 



