382 MacDougal . — The Mechanism of 
convex side of the tendril has continued unchanged, except 
for the reaction to the slight traction exerted upon it, and the 
difference between the two sides in the course of a few hours 
becomes quite marked. 
Plasmolytic Investigations. 
The analysis of induced curves by means of plasmolyzing 
agents has been carried out at great length by De Vries upon 
tendrils of Sicyos, Eckinocystis, Bryonia , Cucurbit a, and Passi - 
flora , as well as upon stems and other organs (II, III). He 
was led to the conclusion that the cells of the convex side 
of a curved organ contained a greater amount of osmotic 
substance than those of the opposite side, and that curvatures 
are produced by the excessive lengthening of the cell-mem- 
branes of this side due to the consequent increased turgidity. 
His first conclusion has been signally disproved by Wortmann 
(XXIII-XXVII), by Noll (XV), and by Kraus (VIII) so far 
as stems are concerned. In further consideration of the 
conditions of heliotropic and geotropic curvatures of stems, 
Wortmann believed himself to have proved that in curvature 
a segregation of the protoplasm on the concave side of the 
organ occurred, and that the membranes on this side became 
thickened and more resistant. The action of the uniform 
turgor stretched the thinner and more elastic membranes of 
the convex side. Kohl ascribes the curvature of stems to 
changes in form of the cells of the concave side due to their 
different elasticity in different directions (VII). Such changes 
of form were not described as a contraction, but rather as 
accompanied by an increase in volume. Noll has shown that 
the segregation of protoplasm on the concave side is a result 
that has no causal relation to curvature. His conclusions, 
which are strengthened by his more recent work (XV), point 
to the variations in the plastic and elastic extensibility of the 
membranes of the convex side under the regulatory power 
of the protoplasm as the chief factor. These conclusions, 
however, relate especially to curvatures of stems, petioles, & c., 
