336 LIFE MOVEMENTS tH PLANTS 



continuation of light induced a pronounced negative cur- 

 vature (Fig. 121). 



I shall in the next chapter give other instances which 

 will show that all organs (pulvinated and growing) possessed 

 of power of transverse conduction, exhibit a transformation 

 of response from positive to negative under continued action 

 of strong light. 



Thus an identical oi-gan, under different conditions of 

 intensity and duration of stimulus, exhibits positive photo- 

 tropic, rff'«-phototropic, and negative phototropic curvatures, 

 proving conclusively that the three effects are not due 

 to three distinct irritabilities. The responsive movements 

 are, on the other hand, traced to a fundamental excita- 

 tory reaction, remaining either localised or iiicreasingl)'^ 

 transmitted to the distal side. 



NEGATIVE PHOTOTROPISM OP ROOTS. 



From the analogy of opposite responses of shoot and 

 root to stimulus of gravity', it was surmised that the 

 root would respond to light by a negative curvature. This 

 was apparently confirmed by the negat-ve phototropic 

 curvature of the root of Sinapis. The supposed analogy is 

 however false ; for while the stimulus of gravity acts, in 

 the case of root, only on a restricted area of t'le tip, 

 the stimulus of light is not necessarily restricted in the 

 area of its action. That there is no true analogy be- 

 tween the action of light and gravitation is seen from 

 the fact that while gravitation induces in the root a 

 movement opposite to that in the stem, in the case of 

 light, this is not always so ; for though a few roots turn 

 away from light, others move towards the light. 



As regards negative phototropic response of the root of 

 iSinapisy it will be shown (p. 376) to be brought about by 



