492 The Physiology of Plants BOOK in 



fications of the properties of the cell membranes ac- 

 cumulated, but the activity of the protoplasm in causing 

 them maintained its ground. In 1887 Wortmann stated 

 that in the curving of coenocytes there is clearly visible 

 an accumulation of protoplasm on the concave side, 

 leading to a thickening of the membrane there. The 

 unequal thickening of the membrane causes the structure 

 to yield unsymmetrically to the internal pressure and 

 so curvature results. The cause is thus rather a retarda- 

 tion of the rate of growth of the concave than an accelera- 

 tion of that of the convex side. Wortmann further claimed 

 that in multicellular curving organs there is an actual 

 migration of the protoplasm to the cells of the concave 

 side where it accumulates, so that the membranes of the 

 cells of that side are on the whole thicker than those of 

 the other. There is consequently practically a repetition 

 of the conditions found in the coenocyte. The curvature 

 is there also to be regarded as due to the retardation of 

 growth on the concave side. 



Wortmann's conclusions were opposed at once by Elfving, 

 who showed in 1888 that his views fail to explain geo- 

 tropic curvature, for the geotropic stimulus causes increased 

 growth whether allowed to cause curvature or not. He 

 made his measurements on plants rotating on a klinostat. 



Noll, in 1888, argued that the proximate cause of the 

 curvature is really an increase in the extensibility of the 

 tissues on the convex side rather than an interference with 

 the growth on the concave side. In his own experiments 

 he failed to confirm Wortmann's observation of an accumu- 

 lation of the protoplasm on the concave wall of the curving 

 coenocyte. Noll advanced further evidence to the same 

 effect in 1895. 



McDougal investigated the mechanism of the curvature 

 of tendrils in 1896. He found that the curvature round 

 a support does not accelerate the growth of the convex 



