PHOTOTROPIC TORSION 405 



For obtaining an explanation of these complex move- 

 ments under cliflFerent intensities of light, we have first to 

 discover the particular disposition of the two halves of 

 the pulvinule which are unequally excitable ; we have next 

 to explain the responsive movements under the directive 

 action of moderate and of intense light. 



Determination of differential excitahilities of the organ : 

 Experiment 153. — In the leaflet of Cassia the movement 

 of opening under diffuse stimulation of light can only be 

 brought about by the contraction of the outer half, which 

 must therefore be the more excitable. This is indepen- 

 dently demonstrated by the reaction to an electric-shock. 

 On subjecting the half closed leaflets to diffuse electric 

 stimulation, they open outwards in a lateral direction. 

 The disposition of the unequally excitable halves of the 

 pulvinule is thus different from that of the main pulvinus 

 of Mimosa. In the latter, the plane that divides the two 

 halves is horizontal, the lower half being the more excitable. 

 Thus in the pulvinule of Cassia the plane that separates 

 the two unequally excitable halves is vertical, the outer 

 half being the more excitable than the inner. By inner 

 half is here meant that half which is inside when the 

 leaflets are closed. 



Effect of strong vertical light : Experiment 154. — 

 When the plant is placed in a moderately lighted room, 

 the leaflets open out laterally to the outmost. This is 

 brought about by the contraction of the more excitable 

 outer half of the organ. If strong light be thrown down 

 from above, a new movement is superposed, namely, of 

 torsion by which the leaflets undergo a twist and thus place 

 their inner surface at right angles to the vertical light. 

 In order to investigate this phenomenon in greater detail I 

 placed the plant in a well lighted room, the leaflets 

 being three quarters open un<ler the diffuse light. A very 



