376 LIFE MOVEMENTS IN PLANTS 



Type B is exemplified by the seedling of Setaria where 

 the transmission of excitatory impulse from the tip upsets 

 the neutral balance and induces the final positive curvature. 



Example of type A is found in the negative phototro- 

 pisni of the root of Sinajjis. 



Negative photntropism of rant of Sinapis : Expe^nment 

 140.— For investigation of the negative phototropism of 

 the root of Swapis nigra I took record of its movement 

 under unilateral action of light by means of a Recording 

 Microscope, devised for the purpose.* When the root-tip 

 alone was stimulated by unilateral light, the root moved 

 away from the source of light. This was due to the 

 longitudinal transmission of positive impulse to the grow- 

 ing region at some distance from the tip. The intervening 

 distance between the tip and the growing region is practi- 

 cally non-conducting, hence the excitatory impulse could 

 not be conducted from the tip. After a period of rest in 

 darkness, I next took record of its movement under 

 direct unilateral illumination of the growing region ; the 

 result was at first a positive movement ; but this, on 

 account of transverse conduction of excitation under 

 continued stimulation, underwent a neutralisation and 

 slight reversal. In taking a third record, in which both 

 the tip and growing region were simultaneously subjected 

 to unilateral stimulation of light, I found that a result- 

 ant responsive movement was induced which was away 

 from light. 



Thus in the root of Sinapis., the expansive eH'ect of 

 indirect stimulation of the tip is superposed on that of 

 direct stimulation of the growing region (neutral or slightly 



* •• Plant KesiiDiise" — p. lioi. 



