Chap, xi Sensitivity and Stimulation 493 



side, but that on the other hand there is an actual con- 

 traction of the tissues of the concave side, while the sub- 

 sequent coiling of the free portion is due to directly contrary 

 behaviour. The region of maximum growth of the tendril 

 of Passifiora lies between its middle and its tip, and never 

 coincides with the sensory zone. He found curvature 

 attended by an accumulation of protoplasm in the cells of 

 the concave side, and attributed the action of the tendril 

 to changes in those cells. These changes consist of an 

 increase in the permeability of the protoplasts with con- 

 sequent extrusion of water into the intercellular spaces, 

 causing thus a release of the stretching tension that has 

 been exerted on the walls of the vascular, cortical, and 

 epidermal tissues, which then recoil elastically. 



McDougal, during the next year, published his researches 

 on the curvature of roots under the influence of contact 

 or of a traumatic stimulus, in the course of which he made 

 very careful measurements of the dimensions of all the cells 

 of the curving area. He came to the conclusion that the 

 curvature of roots is due to excessive active elongation of 

 the internal layers of the cortex of the side which becomes 

 convex, an elongation brought about by the stretching of the 

 longer membranes. The latter process further causes the 

 walls to become thinner, while as a later effect of compression 

 on the growth of the concave side, the membranes there 

 increase in thickness. Converse changes in the radial 

 diameters of the cells of the two sides accompany those in 

 the longitudinal direction. 



Fitting's views, based on the study of tendrils already 

 alluded to, were that the increased growth-curvature of 

 the convex side far exceeds the contraction of the concave 

 side, and that the neutral zone is consequently near the 

 latter. The greater part of the change is thus due to 

 greater growth on the convex side. 



The mechanism of the rapid fall of the leaf of Mimosa 



