20 



with elimination of pith. Surrounding the vascular region is a 

 mueh-thiekened cortex of parenchyma which constitutes par 

 excellence the motile organ. The direct cause of motion has 

 been shoAvn to be a difference in turgescence, or osmotic pressure 

 in the cortical cells on opposite sides of the pulvinus. Osmotic 

 pressure is reduced under the influence of sunlight, for instance, 

 in those cells directly illuminated. That reduced pressure is 



