1900 ] MECHANISM OF ROOT CURVATURE 5 
grown in a moist chamber the partial growth in the zone of the 
traumatropic curve was greater than in the corresponding zcne 
of normal roots, though the total growth of those in the moist 
chamber was less in decapitated than in uninjured ones, 
In looking for an explanation of growth-curvatures which 
would apply to both unicellular and multicellular organs, Wort- 
mann thought he had discovered the cause in the aggregation of 
the protoplasm on the side of the cell that becomes concave in 
the case of unicellular plants and a similar aggregation in the 
cells of multicellular plants on the side that becomes concave. 
This accumulation of protoplasm on the concave side had been 
noticed by Ciesielski (5) and Sachs, who wrote (65, p. 221) that 
it was not the cause but the result of greater growth on the side 
becoming convex. Wortmann thought (80) the protoplasm 
accumulated in the cells of one side produced a thickening of 
the cell membranes on that side. This thickening retarded 
growth in the longitudinal direction, and as the growth on the 
opposite side was not retarded, the organ curved, the side with 
the thickened membranes becoming concave. 
Elfving (19) and Noll (48) have shown that the difference 
in the thickness of the membranes appears only after the curve 
is formed ; hence it cannot be the cause of the curve as Wort- 
mann supposed. Also Noll’s measurement of the membranes in 
curved grass nodes (48) shows that the membranes on the con- 
cave side are not at all thickened, or only slightly more than the 
normal, but those on the convex side are much thinner than 
normal. Noll found in unicellular plants, walls unequally thick- 
ened on opposite sides of the organ which did not curve as it 
must have done had Wortmann been correct. 
In partial agreement with Wiesner, Noll believes the curve is 
produced by a change in the quality of the membranes, those on 
the side which is later convex becoming more extensible. He 
seeks to prove this point by his bending experiments. Stems 
that were just beginning to curve were subjected to a pull at 
right angles to their long axes in the plane of the curve just 
beginning. This pull was exerted by a given weight for a 
a 
