70 BOTANICAL GAZETTE [july 



DeVries (5) studied the forces released in gravity stimulus, 

 the effect of these released forces upon curvature, and the release 

 of elasticity by gravity stimulus. He concludes that gravity 

 produces an increase of osmotically active material in the cells of 

 the convex-becoming side, causing an intake of water from the 

 adjacent tissue, the resulting increased turgor producing a longi- 

 tudinal extension of the elastic cell membranes, which, originally 

 plastic, become fixed through growth and lignification. 



Cielsielski (2) observed a difference of the cell sap on opposite 

 sides of geo tropically stimulated roots, the cells of the convex- 

 becoming flank exhibiting a thin watery protoplasm in contrast 

 with the denser, more opaque plasma of the side becoming concave. 

 Kohl (13) obtained analogous results in the sporangiophores of 

 PJiycomyces, in that in geotropic stimulation the plasma of the 

 concave side of the filament became much thicker, while that of 

 the convex side became thin and watery. He concluded that 

 there was a causal relation between this differentiation of cell 

 plasma and the curvature of the organ. Elfving (7), however, 

 according to his reviewers, produced a similar differentiation of 

 protoplasm in PJiycomyces sporangiophores by allowing them to 

 push against a glass obstruction, a purely mechanical stimulation. 



Hilberg (11), contrary to DeVries' results, found that in 

 geotropic stimulation the osmotic pressure of the concave side of 

 leaf joints and stem nodes of various plants is greater than that of 

 the convex side. Wortman (22) negatives both DeVries' and 

 Hilberg's conclusions, since he could find no difference in the 

 osmotic pressure of the two flanks of stimulated organs, and 

 holds DeVries' view of the causal relation between turgor and 

 curvature to be wholly untenable. On the other hand, he agrees 

 with Kohl in that he found in geotropically stimulated organs 

 the plasma " wandered" from the convex to the concave side, the 

 thickened plasma inducing the cell membranes of the concave 

 side to become thicker but less elastic and less extensible than 

 those of the convex side. These latter, stretching longitudinally, 

 force the concave side upward, thus producing curvature. Noll 

 (16) confirms Wortman's work, but refutes his argument of the 

 causal relation between the changed activity of the plasma and the 

 curvature of the organ. 



