PHOTONASTIC CURVATURES 387 



In a differentially excitable organ the effect of strong 

 unilateral stimulus becomes internally diffused, and causes 

 greater contraction of the more excitable side of the 

 organ. 



In the absence of transverse conduction, the positive 

 curvature reaches a maximum without neutralisation or 

 reversal. The leaflets of Ernjtliriiia indica and of 

 Clitoria tei'natea thus fold upwards, the apices of the 

 leaflets pointing towards the sun. 



Internally dift'used excitation under strong light induces 

 greater contraction of the more excitable half of the 

 pulvinule, causing upward folding of Mimosa leaflet, and 

 downward folding of the leaflets of Biophytum and 

 Averrhoa. 



