19 



those that induce the motion. Just which cells of the pulvinus 

 are the sensory ones is problematical, but, of course, they can not 

 be farther removed from the motion-producing cortex than the 

 epidernuis surrounding the cortex. How fast the stimulus travels 

 from the blade to the pulvinus cortex is also problematical, but 

 in any case it has a greater distance to go, and hence would likely 

 require more time than when all is included within the pulvinus 

 itself. This may or may not be the sole cause of the difference 

 in time of response, or it may be one of two or more factors 

 which, acting together, account for the difference. 



The final point to be considered is the structure of the 

 pulvinus. While this organ has not been studied in Erythrina, 

 there is no reason to believe that it differs in essential respect 

 from the pulvinus in other Leguminosae. All agree in having 

 an axial strand of vascular tissue, which, as has been shown by 

 Pfeffer, Sachs and others, is in the form of a solid rather than 

 a hollow cjdinder. This feature is to permit flexibility. The 

 hollow cylinder arrangement of the bundles with a central pith 

 prevails in the midrib of the leaf where rigidity is desirable, 

 but as they enter the pulvinus, they pass to the axial position 



