xxxii.-thp: phototropio curve and its 

 characteristics 



By 

 Sir J. C. BosE. 



When h plant organ is subjected to the continued 

 action of unilaterial stimulus of light, it exhibits in- 

 creasing tropic curvature, which in certain cases reaches 

 a limit ; in other insiances a reversal takes place, seen in 

 neutralisation, or in the conversion of the positive into 

 negative curvature. I shall in this chapter enter into a 

 detailed study of the phototropic curve, and determine its 

 characteristics. 



As the vague terminalogy at present in use has been 

 the source of much confusion, it is necessary here to 

 define clearly the various terms which will be employed 

 in this investigation. It is first of all necessary to distin- 

 guish between cause and effect, between external stimulus 

 and the excitation induced by it. As regards stimulus 

 itself I have shown elsewhere* that its effective intensity 

 becomes summated by repetition. This was demonstrated 

 by the two following typical experiments carried out with 

 the pulvinus of Mimosa. 



(1) The intensity of a single electric shock of intensity 

 of 0'5 unit was found to be ineffective in inducing 



♦ " Irritability of Plants " — p. 54, 



