TRANSMITTED EFFECT OF LIGHT 371 



action of unilateral light, the positive curvature after 

 attaining a maximum in the course of lo minutes, under- 

 went a diminution and final neutralisation (Fig. 137). On 

 account of this neutralisation the seedling became erect after 

 an exposure of 30 minutes ; in contrast with this is the 

 increasing positive curvature under unilateral illumination of 

 the tip (Fig. 336) which continues for several hours. The 

 explanation of this neutralisation under direct stimulation 

 of the growing region is found iti the fact that transverse 

 conduction of excitation induces contraction at the tlistal side 

 of the organ and thus nullifies the positive curvature. The 

 seeming absence of tropic effect under direct stimulation 

 is thus not due to want of perception, but to balanced 

 antagonistic reactions on opposite sides of the organ. 



EFFECT OF SIMULTANEOUS STIMULATION OF THE TIP 

 AND THE HYPOCOTYL. 



Though stimulation of the hypocotyl results in neutra- 

 lisation, yet the illumination of one side of the organ 

 including the tip and hypocotyl is found to give rise to 

 positive curvature. This will be understood from the 

 following experiment. 



After the neutralisation in the last experiment light was 

 also applied to the tip from the right side at the dotted 

 arrow (Fig. 137). The record shows that this gave rise 

 at first to a negative curvature (away from light) ; under 

 the continued action of light, however, the negative was 

 subsequently reversed to a positive curvature, towards light. 

 Inspection of the curve shows another interesting fact. 

 The positive curvature induced by direct stimulation is very 

 much less than that brought out by indirect stimulation. 

 This is due to two reasons: (1) the sensitiveness of the 

 tip of the organ is, as is well known, greater than that 



